Lecture 14: Cell Cycle & Mitosis

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Hi everyone.

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Welcome to your online lecture for today, where we're going to be discussing a very normal, very common, very important cellular behavior that uses up quite a bit of your energy budget, particularly as you develop, as you repair yourself, when you injure yourself, and as your body goes through normal processes by which some cells have to be regenerated.

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We're going to talk about the cell cycle, how a cell lives its life, and then the process of being able to pass on the genetic information in that cell to produce more cells.

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Now this is a really pivotal point in the semester where we're sort of connecting where we've been to where we're going in the semester.

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And the reason is we've spent quite a bit of time talking about what cells are made of, what's inside of them, the different compartments and what they do, and then bringing us to a point where we've talked about the energy, but energy budget that we have to regenerate and what we spend it on.

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And this is a good Nexus for thinking about us becoming or being multicellular organisms that have billions of cells that are grouped into different organs and tissues that function together in order to keep the entire Organism running long enough to maintain itself and its environment, to find a continued energy budget, and with the ultimate goal of being able to reproduce and help contribute to our species surviving.

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So thinking about this, is this normal cell behavior that really does tie together everything that's in a cell and what it's doing and relates also to the cell theory that we already talked about, right?

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We talked about the tenets of the cell theory that the smallest unit of life, of independent autonomous life is an individual cell, that cells are able to reproduce and pass on their genetic material and that they are autonomous, separate from the external environment and that cells come from pre-existing cells.

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And so this really is the crux of that third part, the hotly contested part that cells actually come from pre-existing cells.

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This is also going to be your first little dip of the toe into thinking about real genetics.

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We focused largely on actual genetics in the second complement semester of this particular course that's in Bio 2.

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And so we really talk about that as a mechanism for passing on your genes and your characteristics to your offspring.

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In the case of mitosis, what you're doing is you're passing on your genes to the rest of the cells in your body, right?

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If you think about reproduction, that you start out as a single cell after the complement of chromosomes you received from your mom and dad fused together, you are a zygote, one individual cell, and the rest of you is generated in utero through this process of cell division through mitosis.

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All of your other body cells except for your eggs and sperm, which are gametes, everything else is made by mitosis.

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And so this is a very important cell cycle or so important cell production process.

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And then, you know, you recognize we're soft squishy humans.

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We do dumb things like we run into stuff and we cut ourselves and we injure ourselves and we don't often stop to think about we're kind of like Wolverine, just slower, right?

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We have the ability to regenerate a lot of our tissues.

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We're just slower at it than he is.

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And we rely on this process of mitosis to be able to regenerate tissues that we may damage or normal tissues that we have to generate new of all the time, right?

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You think about your skin cells, your stomach cells.

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You're losing certain layers of cells regularly, as you should, because they become damaged by exposure to things in our external environment, like our skin, in our internal environment, like the lining of our intestines.

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And so we have to be good at doing mitosis in order to regenerate certain cells throughout our lifespan.

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I as always, I love the Amoeba Sisters GIFs here where we've got this one little cell who seems kind of confused, but looks like it's starting to go through maturation.

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And you can see that it's in the S phase of the cell cycle and it was knitting some DNA.

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And then by G2 it's made-up its mind and by M phase, it's starting to divide into two cells.

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And so as always, clever cartoons or memes are generally an easy way to remember some of the harder science and all the other parts kind of have to be filled in with actual studying.

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So if you can find a video or a meme or a cartoon or joke that helps you remember something about the cell cycle, you should do that.

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This picture on the bottom right is actually a really, really important picture, and this is going to relate to some of the activities that we will likely do in class.

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We will be thinking a little bit about how we were really able to start studying human cells by maintaining them in cell cultures in labs.

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And this particular type of cell referred to as a HeLa cell, which you might have heard of before, was pivotal for about the last 100 years or so, almost 100 years at this point for being able to study human cells and culture.

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But unfortunately has a sort of a sad story behind it that is important to think about from an ethical lens, which is something that we're probably going to do in our activity in class later.

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So thinking about eukaryotes like you, which are organisms that have a nuclear envelope bound set of chromosomes, many of which are multicellular, we think about the cell cycle as being very important and regulated by a molecular control system.

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So we have these series of proteins who are responsible for communicating within and without the cell and the environment and for receiving signals from elsewhere in the body, like growth signals, for example, that convey that message into the cell and control what it does.

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And so being able to do cell division is going to vary from cell type to cell type and also the different stages you are in development, right?

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Most of your organs tend to develop to a certain size as you mature.

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And at some point they stop, they stop getting larger.

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And so it's absolutely imperative for us to develop as fully grown mature humans to have a tightly controlled cell division system for making different types of tissues and proteins, gene expression principally.

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But the proteins that are produced from your genes are going to be regulating on the molecular level that control of going through different phases of life of a cell and producing new ones.

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And so we kind of have to think about what are the things that would cause this cell cycle to be regulated.

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Now you are lucky.

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We're not going to go into too much molecular detail about cyclins and cyclin dependent kinases and how every single phase of the cell cycle is very tightly regulated.

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But we will talk about these phases and what happens.

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And we're going to think about mitosis, where it is that you are going to produce 2 new daughter cells from this original parent cell.

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And it's important to understand how the system works because of the consequences, you know, not only growth and development, but the consequences of what can happen when the system is broken, when the system breaks for whatever reason, generally from some kind of mutation that may happen from an external exposure to something or an internal mistake that can happen.

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Cells that that defy the rules of cell division often can divide out of control and they can actually become cancerous.

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And that, you know, creates a wave of issues for an Organism, especially depending on the type of cancer that results.

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And so we're going to think a lot about this, this life of a cell, which is largely comprised of interphase, while a cell is sort of just just taking out its normal business on things and developing and a phase where we have DNA replication that occurs.

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And then a very short period of that life, which involves actually dividing into two new cells that each receive a full complement of chromosomes that the parent cell had.

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And so interphase is about 90% of the life of a cell and it's divided into 3 sub phases.

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So we have the first gap phase, which is G1, which is where the cell is just kind of doing whatever job it's supposed to, right?

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It depends on the tissue that it's in.

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If it's, you know, a liver cell, maybe it's working on helping to detoxify your body.

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If it's a a secretory cell where it's supposed to be releasing some kind of chemical or hormone, it's just kind of doing its job at this point and there are proteins that are controlling it that are telling it.

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It's fine for now, just keep doing your job.

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But there's going to be a checkpoint near the end of the G1 phase that will tell the cell, all right, we've received an external signal.

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We're going to gear up to get ready to do cell division, where we're going to make another cell just like us.

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We're going to make two cells from this first parent cell.

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And so it's time to start committing to that process.

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And so we have checkpoints and proteins that regulate the chains from D1 into S phase.

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S stands for synthesis, and this is where we're actually going to do DNA replication.

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We have a bunch of enzymes that we'll talk about in Bio 2 next semester that are responsible for copying all of your chromosomes, all of the DNA, all of your genes are in your DNA, all of the chromosomes, and doing it with very high fidelity, which is making as few mistakes as possible.

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And so that's going to create two full copies of all the chromosomes that you've inherited from your mother and your father that are then going to be sorted later.

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So notice that we're doing DNA copying in the middle of interphase.

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Don't forget that.

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That's going to be really important later when we think about mitosis and especially later when we think about the cell division that you use to produce your gametes.

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OK, So DNA is already being copied in the middle of the cell.

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And then there's going to be a second gap phase.

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And so other proteins allow the cell to shift from S phase into G2.

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They're regularly like, OK, everybody's finished copying chromosomes.

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Now we can go back into G2 phase where we might continue sort of whatever job we're supposed to be doing.

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But also, we're going to spend some time activating other proteins that can check over our work from S phase and make sure that we didn't incorporate a bunch of mistakes into our chromosomes.

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So we've copied all of our chromosomes.

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But every now and then the enzymes that do that can make errors.

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Or there are things in your environment that you're exposed to that might make errors.

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And what you don't want to do is keep those errors.

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You want to fix them before they are passed on to either of the two new daughter cells because the consequences of it can be severe.

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The daughter cells might not work right.

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They might make something they're not supposed to or make something that, or not make something that's supposed to be there, or they might just die outright.

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And so repairing mutations that could occur in South phase or even mutations that might have occurred earlier in the life of the cell need to be done in G2 phase.

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And so we're going to spend some time in G2 before we get another signal that's going to say, OK, this is AG2M regulation.

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It's time for you to go ahead and start dividing.

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And you're going to go into what's called the mitotic phase, which is only about 10% of the life of the cell.

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It tends to happen very quickly and doesn't happen to all cells all the time.

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And, and, and it's very tightly regulated as well.

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Once we've copied our DNA, we've pretty much committed to the fact that later we're going to go through cell division.

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We're going to have to separate our chromosomes and sort them into two new daughter cells and then follow up with dividing all of our cytoplasm.

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So you notice that M phase, which stands for mitotic, is divided into two parts.

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It's divided into mitosis itself, which is where you are dividing.

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Now.

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See, this is a little bit of a misnomer on this slide.

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The duplication and division of the genetic material, it's already duplicated at this point, but you're going to sort it into two halves of this cell.

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And then we're going to go into cytokinesis, where we're going to divide that cell in half.

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And we're going to sort some of the cytoplasmic components that you've already learned about to both daughter cells.

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Because in order for the daughter cells to be functional, they're going to need some nuclear envelope and they're going to need some ER and some ribosomes and some Golgi and some mitochondria, and they're going to need all these machinery.

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So we're going to have to pull some of them to both ends of the daughter cell so that each daughter lives with.

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So this is our goal, right?

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We don't want our daughter cells to only have one that survives.

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And so cells continue growing through these phases, but we only see duplication of chromosomes in South phase, and then we see the separation of them later when we commit to going through mitosis and cytokinesis, right?

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So these checkpoints that are controlling the cell cycle are very, very well timed and they're very similar to what you would see on a washing machine, right?

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Maybe at this point in your life, not everybody washes their own clothes.

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Maybe you're at home and mom still does you a solid and does your clothes for you.

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But at some point, that's absolutely going to change, and you're going to have to learn what the different types of settings actually mean.

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But they all have these sort of checkpoints around the way, depending on the cycle that you're in or the face that you're trying to use, that help control what happens inside the actual washing machine machine.

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And so the cell cycle itself is going to be regulated not only by internal factors, these genes that make proteins that are controlling what happens inside, but they're also going to be influenced by external factors.

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Our bodies make a lot of growth factors that control when, where and how we're supposed to replicate our cells.

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You have tons of growth factors that are expressed early in development because you're doing lots and lots of cell division after you become a one cell.

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And you're going to go to form a blast, a blastula, which is this ball of cells.

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And then you form a gastrula where your cells are starting to migrate around each other.

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And then you're going to start forming actual and actual embryo that starts to look like organisms you've probably seen in utero before.

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And all organisms are going to do mitosis when they are forming multicellular offspring.

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And so these checkpoints are very, very important and they tend to be signals that make the cell pause check that we've done everything we're supposed to.

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Now go ahead into the next phase, OK?

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And we're going to emphasize one of those very important before we finish doing mitosis in particular.

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And so there are three major checkpoints.

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There are others in here.

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There are checkpoints that ask you, Are you ready to go from G1 to S?

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We're getting ready to commit.

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Somebody told us we need to divide.

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Then there are signals that go from S to G2G2 to M and then exit you out from M back into G1.

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But really only emphasizing these three important ones right here.

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So the G1 checkpoint is the one that's actually telling the cell, OK, in addition to your normal jobs, you need to get ready to go ahead and go through cell division.

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And First things first, you're going to have to go ahead and copy all your chromosomes.

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And lots of, lots of, lots of proteins are involved in helping to copy all your chromosomes.

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Remember that you have 46 of them.

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You have 23 from your mom and 23 from your dad.

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You have to copy all of those in order to sort a full set of each, a full set of moms and dads to every daughter cell that you produce.

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A lot of these signals that after the cell receives information from the body to go ahead and divide.

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A lot of these signals are going to be internal, right?

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So we're going to talk about some of these individual checkpoints.

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So then we've got a checkpoint at G2 that says, hey, OK, we've copied our DNA.

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We've checked over all of our work.

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We've tried to fix as many mistakes as possible.

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Now it's time to start activating all the things in the background that will allow us to line up our chromosomes to get ready to do mitosis, shrink them down and line them up, but also we're going to activate everything moving in the background.

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Now you've learned about the cytoskeleton already.

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You've learned about microtubules.

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We have to activate microtubules and motor proteins to go pick up some organelles and some vesicles and package things and sort some of them to one daughter and to the other.

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So there's a lot of background things happening in moving the cytoplasm around, gearing it up to actually divide the cytoplasm in half, not just all the chromosomes.

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OK.

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And so with the go ahead signal here with a green light through G1, it's going to automatically take the cell into S phase.

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It's going to kick off enzymes and proteins that go find origins of replication your chromosomes and they're going to start there and start copying your chromosomes.

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Once you commit to that, you're absolutely committing to mitosis and going through to divide.

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But some cells, once they have reached the maximum capacity of cells that they're supposed to have in a particular tissue or organ, may actually be asked to exit cell division capability and go into what's called the G0 phase where a cell is non dividing.

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It just stays functional, right?

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There are some tissues that we have that once we make them, we don't really make more of those tissues, but provided that they stay alive and stay functioning and we don't need to regenerate them or G0 can be a temporary phase as well.

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But cells aren't constantly dividing all the time.

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If they were, you know, our livers would be getting bigger, our lungs would be getting bigger.

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You know, we'd constantly be making more cells if we weren't getting rid of some of them simultaneously.

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So some cells do go into what's called AG0, which is a non dividing state.

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I mentioned that there are checkpoints that come later.

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And so it's a very important checkpoint from G2 into M phase where it's like, OK, we're ready to start going and then we'll kind of talk a little bit about what happens in the middle of mitotic phase.

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We're not going to focus too much on G2 to M, but cell division really is important, not just for sorting half your chromosomes into each of the daughter cells, you know these two copied sets, but making sure also that a lot of other genetic material and cellular material is also copied.

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For example, you know you have to sort some of your mitochondria into each new daughter cell, and sometimes mitochondria will divide by fission to to produce new mitochondria.

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They divide like bacteria do, right?

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Well, if you're going to do that, you also need to copy the chromosomes that are inside of those.

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So you have machinery for copying your cytoplasmic genomes or your extra nuclear genomes as well that you inherited from your mom.

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If you were a plant, you would be worried about sorting chloroplasts to each of your two new daughter cells.

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But DNA is obviously what we focus largely on.

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We think about mitosis.

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Later, when you study cell biology, we'll also think about the sorting of all the other things that the daughter cells need to be able to survive.

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So we're moving literally half of all the cytoplasm, everything that's there, all those organelles, all those vesicles, half to one daughter, half to the other.

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And that's not by chance that actually requires energy.

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It requires work.

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Motor proteins are going to move along your microtubules and haul those things, literally pulling them to opposite poles of the cell.

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And so as I mentioned, this happens pretty with pretty high fidelity, which means being being honest or being true to the original.

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Because what we don't want to do is fix mutations, accidental changes that we didn't mean to happen in our daughter cells.

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And all the other cellular bits have to be sorted and and somewhat equitable just like the DNA molecules are.

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So when we are getting ready to prepare for a eukaryotic cell division, we think about the fact that DNA is replicated in South phase and we have these long chromosomes that are now copied into what are called sister chromatids, but they're still sort of loose and floating around in the cell and they can still express genes, but they are already paired up in the buddy system sister chromatids.

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When they are made, are actually looped together with each other in the buddy system all along their length.

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They have these fascinating protein lassos that are not only around the centromere like you see in this condensed pair of chromatids, but they're also lassos all the way around the length of the sisters to hold them together.

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Those get broken off later, but the two sisters that we see eventually in mitosis, they're already stuck together once they're made in South phase.

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And so we use these terms to talk about chromosomes that are getting ready to do cell division.

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We call them sister chromatids because they're joined together principally and most obviously at their centromere.

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Every chromosome in somewhere near the middle has a space that's full of centromeric DNA.

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And what that does is that forms a point where two sister chromatids are actually cinched together.

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It looks like they're wearing a belt at the middle.

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Keep that in mind as you go forward thinking about chromosomes, that every single chromosome has its own centromere.

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That when these two chromosomes separate later into the different daughter cells, they will still then have their own centromeres.

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That's a geographical marker and a chromosome that is always in the same place depending on the type of chromosome you're looking at.

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Barring any genetic problems, Chrome centromeres don't actually move.

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So we refer to these as sister chromatids, particularly when we can actually see them because they're so condensed down and shrunk into these traveling forms that are getting ready to be easily moved through mitosis.

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OK.

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And we'll, we'll see when we look at images of mitosis that when we start this process or we're an interphase, even though we've duplicated all of our chromosomes, they're stuck together as sisters, but they're still sort of very loose and not shrunk down yet.

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When we use the terminology sister chromatid, what we're talking about is a chromosome that is currently duplicated and attached to its identical sister because they're getting ready to travel.

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This is the easiest way to be able to separate them into new daughter cells.

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And So what we're going to see is that in mitosis, the chromosomes are going to start to shrink down.

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The sisters that are stuck together of each type are going to start to shrink down and form these highly condensed versions that we can actually see under a microscope.

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That's as the DNA is all jacked on top of each other and squish down really tight because this makes it travel easily.

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What it also does is it makes sure that every chromosome of a type is attached to its sister and then when they line up together in the buddy system, when you split them apart, one will go one way and one will go the other.

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That way it guarantees that every chromosome, a version of every single one, all of your 46 set, your whole 46 set, you're going to get copies to every new daughter cell.

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So every new daughter cell is going to end up with a whole set of 4623 of your chromosomes from mom and dad, 23 of your chromosomes from mom and dad to the other cell.

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All right.

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And so there's just some terminology here that you understand what the geographical markers are.

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These are called sister chromatids.

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Each one of these these lengths around the centromere, which is this middle part where you see cinching in each chromosome.

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Remember there's two centromeres in this image.

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Both chromosomes have their own.

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They have arms.

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We often refer to the ends of the chromosomes as arms.

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And remember that these are very, very long DNA molecules that are all wound around proteins that help them shrink down into this traveling form that you're familiar with seeing.

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We've been looking at chromosomes for a very long time, basically with the introduction of early microscopes.

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And early biologists really just wanted to look as much as possible and many possible things as they could underneath these early microscopes.

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One of the easiest cell types to actually look at because they don't tend to move around, are plant tissues.

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And so people would peel, you know, onions or they look at cork from a bark, from bark on a tree underneath a microscope.

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And they started to see these these little circles inside of a cell.

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And they had a bunch of squiggly lines in them.

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And they started to sort of figure out what these squiggly lines were behaving and doing, even though they didn't know yet what they were for.

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So if you think about 1882 and an anatomist, Walter Fleming, is learning how to sort of stain cells.

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We've got all these fancy green and red dyes and things that we can actually stain chromosomes with and watch in different cells these different positions or behaviors that the chromosomes are in.

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And now we're at a point in our life where we connect our point in our technology where we can actually watch cell division happening live, which is pretty fascinating.

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But he was able to sort of mark these different phases of mitosis.

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And then also seeing the separation of the cytoplasm that you start to see beginning in H of this image, but really advanced in I where you're splitting compartments.

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And you'll notice also these two cells that are produced in I are are a little small.

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They'll get a little bigger, they'll get some bulk as they get older.

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But back then we didn't even know what the squiggly lines were for.

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We just noticed that they looked like they shrunk down and they separated and they sorted into daughter cells and nobody knew what they were actually for.

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If you think about that time that he, that Fleming was a contemporary in a different region of Europe as Gregor Mendel was, Gregor Mendel was looking at how we inherit our characteristics from our parents, using plants as a model to do that.

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But even he didn't know anything about what these squiggly lines were or what they were doing or why they were there.

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It wasn't until the early 19 or early 20th century, the early 1900s, from about, you know, nineteen, 10/19/20, when other early geneticists started figuring out that our characteristics are traveling on these squiggly lines, that we inherit these in our sex cells.

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We passed these from our sex cells to our offspring and that the characteristics are actually traveling with them.

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And the thing that helped geneticists figure that out were actually sex chromosomes and studying fruit flies.

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Male fruit flies have X&Y just just like, you know, human animals do.

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And we knew something about the sex of organisms and female fruit plays have X chromosomes.

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And they started to notice that certain characteristics were traveling into one sex versus the other because of how they were inheriting sex chromosomes.

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And so this started, it'll be able to link the actual physical features you are getting with the chromosomal movements.

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Now.

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People still didn't know what the actual molecule was in our chromosomes.

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That was the heritable kind, right?

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What was actually controlling those characteristics?

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Chemists had been pulling out this long, snotty, stringy molecule from the nucleus for a very long time and trying to figure out what it was.

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And they'd figured out that nucleon, this molecule they're calling that's in the nucleus or the center of the cell, is made-up of lots of nucleic acids and also proteins.

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And so it took a lot of experimentation to figure out, well, are we inheriting then the the DNA, the nucleic, the nucleic acids, is that giving us our characteristic?

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Or is it the proteins that are there that are giving us our characteristic?

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Didn't, that wasn't really solved until the 1950s.

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So that took, you know, a good fifty more years to figure out which was the heritable material.

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And now we've arrived at a place where we can visualize chromosomes in very fancy ways.

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And quite commonly, in fact, it's very common for people to get karyotyping done, particularly if they're going to have a child, to understand what the geographical makeup of their chromosomes is.

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Right.

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So first and foremost is knowing do we have a normal set as humans?

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We have 22 different normal chromosomes that are called autosomes, and we inherit one pair of sex chromosomes.

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And in fact, you can see here that this individual is a male.

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It's got an X and AY chromosome.

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And so we may take fetal cells from floating inside the placenta and culture those and stop them at mitosis in mitosis so that we can see their condensed version.

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And you can stain them directly or you can use fluorescent markers like are shown here to stain specific regions of chromosomes to not only identify which are which, but to see if there are any mistakes or errors.

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For example, if this bottom portion that's read in chromosome 3 moved and broke off and attached to chromosome 10, a cytogeneticist would look at this and go, oh geez, there's an error.

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We've got a translocation where a portion of this chromosome is broken off and is actually attached to the other.

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And then we can tell usually what kind of consequence that might mean for the offspring.

28 minutes 52 seconds

And so looking at a set of chromosomes has come very far clearly since the end of the 19th century.

28 minutes 59 seconds

And we can do a lot, we can tell a lot of things by looking at pictures of chromosomes.

29 minutes 4 seconds

So when we're in S phase and the cell cycle, what's going to happen is a series of enzymes are going to go to the chromosome and they're going to start pulling apart the double stranded DNA molecules that are present in the chromosome and reading and copying them.

29 minutes 20 seconds

OK, We focus more heavily on DNA replication in Bio 2.

29 minutes 24 seconds

The nuts and bolts in the details here.

29 minutes 26 seconds

Really all you need to know is that you have these double stranded DNA molecules that are wrapped up into your chromosomes.

29 minutes 32 seconds

And what we're going to do is we're going to need to copy all of them with very high fidelity.

29 minutes 36 seconds

So that for every chromosome in your nucleus, you're going to need to copy it so that there are now two that they are attached to each other and they are sisters.

29 minutes 44 seconds

So let's say that this is chromosome 17 that you inherited from your mother.

29 minutes 49 seconds

You're going to go ahead and open up the DNA molecule and we have enzymes that come in and read and copy both DNA molecules in both directions so that we now have two full, hopefully identical, no mistakes, versions of chromosome 17 that are attached together at their centromeres.

30 minutes 6 seconds

Remember I mentioned they're looped all along their length, but at some point a lot of those loops are cut and only the centromere loop is going to stay holding them together.

30 minutes 14 seconds

But we refer to them as sisters.

30 minutes 16 seconds

They are genetically identical.

30 minutes 17 seconds

They are exact copies of each other because your goal is to shuffle up and deal exact copies of your chromosomes to every daughter cell when you do mitosis.

30 minutes 26 seconds

Later when you learn about meiosis, which is why we don't talk about it here, you will learn that that doesn't happen, that you make differences, that you want to incorporate differences in all the daughter cells that you make so that you can pass on some differences in to your offspring.

30 minutes 39 seconds

And so these two sister chromatids are stuck together.

30 minutes 42 seconds

And then when they go through mitosis, what's going to happen is it's going to pull them apart and sort them into two different daughter cells.

30 minutes 48 seconds

So imagine that mitosis and cytokinesis have now happened, and every daughter cell it's produced is going to look exactly like the original parent cell that we started with in cell division.

31 minutes

And so now we're going to think about the actual finer features of all those individual parts of mitosis.

31 minutes 6 seconds

And this might be a complete review for you.

31 minutes 8 seconds

I hope that it is that you learned about mitosis and the mechanism by how it happens when you were in high school or if you've taken another biology class and that it's stuck with you, or that you have a cool acronym for being able to remember them and the different phases.

31 minutes 23 seconds

You might have learned only four of the phases.

31 minutes 25 seconds

We actually separate them into five when we talk about this because there are sort of five obvious phases here.

31 minutes 31 seconds

We talked 1st about prophase.

31 minutes 34 seconds

So prophase is when you are just now starting to shrink your pairs of sister chromatids down.

31 minutes 39 seconds

They're all sort of floating in their own zip codes inside the nucleus.

31 minutes 43 seconds

And we're also going to simultaneously start preparing the cytoplasm to get ready to separate those chromosomes as well as all the other cytoplasmic constituents.

31 minutes 51 seconds

And then we have something called pro metaphase, which is clearly in between prophase, but not quite metaphase.

31 minutes 57 seconds

And so this is where your chromosomes are really condensed down and they're starting to get ready to get caught and lined up, but they're not quite lined up yet.

32 minutes 4 seconds

Metaphase is probably the easiest one everybody recognizes when the chromosomes are all sort of lining up in a row along this imaginary plane called the metaphase plate.

32 minutes 14 seconds

Anaphase is a lot of our favorite phases because this is where we start separating the sisters.

32 minutes 18 seconds

You can actually see their arms dragging behind them, which is how you know that they're really just attached at the middle and they're being pulled opposite directions.

32 minutes 25 seconds

And then we're going to start to see them really start to finish this.

32 minutes 28 seconds

So the chromosomes are now, you know, been relegated to their their different poles of this cell and they're starting to sort of loosen up.

32 minutes 35 seconds

We're going to see a nuclear envelope reform, but also cytokinesis overlaps with this last stage of mitosis.

32 minutes 43 seconds

So looking a little deeper at this, we've got an image of an actual cell going through division up here and a cartoon version on the bottom.

32 minutes 50 seconds

You will see in the actual cell up here, this region in blue is stating something called Dappy.

32 minutes 55 seconds

Dappy actually sticks to chromosomes and so you can actually see the activity of the chromosomes inside the nucleus.

33 minutes 3 seconds

And then the other things you're going to see here in green are microtubules and in red are going to be actin filaments.

33 minutes 10 seconds

So red is actually giving you the background of the rest of the cytoplasm and the green you really want to focus on because this is going to do the work of getting the chromosomes lined up and separating them.

33 minutes 20 seconds

So now you taking something you learned early in the semester about the cytoskeleton and applying it to this really important process.

33 minutes 26 seconds

So the beginning of of before we've committed to go through cell division, we've actually got, we're an interface here, right?

33 minutes 32 seconds

This is most of the life of the cell and every cell is going to have a structure called a centrosome.

33 minutes 38 seconds

OK, anytime you see this suffix in in a biology word with SOME so means body, all right, So chromosome, autosome, proteasome, ribosome, it means body.

33 minutes 48 seconds

It tends to be a large complex.

33 minutes 50 seconds

It's not always an organelle like lysosome or peroxisome.

33 minutes 54 seconds

It can something sometimes be something a bit smaller.

33 minutes 56 seconds

But a large protein complex like a ribosome is pretty big, and the centrosome itself has two structures inside of it called centrioles.

34 minutes 6 seconds

When you see IOLE on the suffix of a word, it means something small.

34 minutes 11 seconds

Later when we talk about arteries, you're gonna see arterioles.

34 minutes 14 seconds

They're smaller versions of arteries, right?

34 minutes 17 seconds

So when you see IOLE, you're seeing something that's a little smaller.

34 minutes 20 seconds

So centriole is a little thing inside of the centrosome.

34 minutes 23 seconds

Lots of C words happening in genetics that you have to remember.

34 minutes 26 seconds

You've got centromere, chromosomes, centriole, centrosome.

34 minutes 29 seconds

There's a lot of words to to make sense of.

34 minutes 32 seconds

OK, So early in interface, when you know you're going to start going into the cell cycle, you're going to actually duplicate each of these, the centrosome into two because we're going to migrate 1 to one end and one to the other.

34 minutes 43 seconds

And each daughter cell is going to get one of those.

34 minutes 45 seconds

But they're going to help us produce our fishing poles for reaching out and pulling our chromosomes apart.

34 minutes 51 seconds

The nuclear envelope is still in place.

34 minutes 53 seconds

We can see the nucleolist.

34 minutes 54 seconds

This is that dense area where you're doing gene expression to make all the parts for doing translation.

35 minutes 1 second

You can see that all of our chromosomes are sort of loose and hanging out.

35 minutes 4 seconds

It looks kind of like a ball of yarn.

35 minutes 5 seconds

This picture is a little bit misleading because it looks like all the chromosomes are just kind of tangled together and hanging out because all they're they're all the same color.

35 minutes 12 seconds

That's not really what happens when we do some staining of the nucleus.

35 minutes 16 seconds

We can see that your individual chromosomes sort of live in different zip codes, right?

35 minutes 21 seconds

You might have chromosome 4 of mom, one of them's living over there and one of them's living over here.

35 minutes 26 seconds

And they tend to be attached a little bit to this nuclear envelope on the inside.

35 minutes 30 seconds

So they actually have their own zip codes that they live in, in your nucleus.

35 minutes 33 seconds

They're not all tangled together like it kind of looks like in this image, but they are at this point on well, this is where we, we're in interface still, but we've passed S phase.

35 minutes 42 seconds

So this is that they're duplicated, but they're uncondensed, which means they haven't shrunk yet.

35 minutes 46 seconds

And the reason that they're going to shrink down is because it's much easier to line them up and separate them for traveling when they're shorter and more condensed.

35 minutes 55 seconds

And so the first phase here, prophase, we can actually start to see, if we look in the real cell, we can see the squiggles of the chromosomes.

36 minutes 1 second

We're starting to see them really start to condense down.

36 minutes 3 seconds

And it looks like we've got some activity in our microtubules here where we've duplicated our centrosomes.

36 minutes 9 seconds

And so it looks like there's going to be two.

36 minutes 11 seconds

We're going to start to see them migrate away from each other.

36 minutes 14 seconds

So here in the cartoon version, we've got the chromosomes, the sister chromatids that I've already been detached together, starting to shrink down into versions, but they're all still kind of tangly, like little blue noodles inside of the nucleus.

36 minutes 26 seconds

And we've got our centrosomes that are starting to migrate away from each other and start to produce an early mitotic spindle.

36 minutes 37 seconds

Now you probably heard of that word before.

36 minutes 38 seconds

Spindle.

36 minutes 39 seconds

We see spindles on things like your bike tires or we call them spokes on bike tires.

36 minutes 44 seconds

But they're similar to spindles.

36 minutes 45 seconds

If you have a front porch at your house and there's a railing and you actually see or you have a stairs in your house and there's a railing and you see these these wooden pieces holding the railing together.

36 minutes 55 seconds

Those are for do as spindles.

36 minutes 57 seconds

And so these resemble those as well.

36 minutes 58 seconds

These are going to be made out of these are microtubules.

37 minutes 1 second

So they're made out of the protein tubulin.

37 minutes 4 seconds

So those proteins are are are being generated from the centrosomes.

37 minutes 8 seconds

They're going to keep adding tubulin and making these longer and longer.

37 minutes 11 seconds

And you'll notice at this point that these guys are actually touching each other.

37 minutes 15 seconds

And so this is actually pretty fascinating and completely intentional.

37 minutes 18 seconds

Those little overlapping spindle microtubules in the middle, they are actually pushing off of each other.

37 minutes 24 seconds

So there's little motor proteins in between them that are kind of pushing them off to help push those centrosomes away from each other so that they go to opposite ends of the cell.

37 minutes 33 seconds

So prophes, we can already see some condensation here in pro metaphase.

37 minutes 37 seconds

We can see a lot more condensation, but they are not yet lining up on the metaphase plate.

37 minutes 43 seconds

And the other thing that's happening is you'll notice that there's not a defined barrier around the chromosomes that they're not really still stuck in what looks like the sort of egg of the nucleus that their start of.

37 minutes 54 seconds

You can kind of see their arms are sort of free.

37 minutes 57 seconds

That is because the nuclear envelope is being fragmented.

38 minutes 1 second

It's not being degraded, it's actually being fragmented into vesicles so that some of those vesicles are going to go to the new daughter cell on each side.

38 minutes 8 seconds

Then they're going to be assembled back together as a nuclear envelope.

38 minutes 11 seconds

So when that nuclear envelope starts to break down into little vesicles and and move out of the way, these spindle microtubules that are being generated from the centrosome, some of them are going to reach out and start grabbing a hold of the sides of these pairs of sister chromatids.

38 minutes 27 seconds

And what the goal here is, is to get spindle microtubules that attached to both of the opposite sides of each pair of sister chromatids.

38 minutes 34 seconds

And so we can see that some of them are starting to grab onto the size of the sisters and some of them are still actually touching each other.

38 minutes 41 seconds

These non kinetochore microtubules are still really important.

38 minutes 45 seconds

And you can also see that there looks like there's some microtubules in this structure called the aster that looks like a star that seem like they're lost, like they're going in the wrong direction.

38 minutes 54 seconds

Like why are microtubules going that way when the chromosomes are over here?

38 minutes 58 seconds

But that's also intentional.

39 minutes 1 second

Those microtubules are pushing at the borders of the cell to try to help elongate the cell so you can eventually go right down the middle.

39 minutes 9 seconds

So it's very intentional.

39 minutes 10 seconds

So these guys at the top and the bottom are pushing the cell to elongate it.

39 minutes 14 seconds

The ones that are bouncing off of each other here are actually trying to push off of each other to help elongate the cell.

39 minutes 20 seconds

Also, 'cause they're growing, they're getting longer from these these from these centrioles.

39 minutes 26 seconds

That's their job is to actually build them.

39 minutes 28 seconds

So they're getting longer and longer.

39 minutes 30 seconds

But those motor proteins, or they're just kind of pushing the ends off of each other to help make the cell longer.

39 minutes 34 seconds

And then the third guys are the ones that are actually attaching to the sister chromatids.

39 minutes 39 seconds

And so the mitotic spindle is very, very important.

39 minutes 42 seconds

If you didn't have the cytoskeletal element and you couldn't sort of breakdown all the microtubules you already had and reassemble them into these ones, you would not be able to separate the chromosomes or make the cell elongate so that you could do cytokinesis down the middle.

39 minutes 56 seconds

And so the centrosomes are what are called microtubule organizing centers.

40 minutes 1 second

Their job is to take those little subunits of tubulin and shove them together and stick them together so they form actual long tubes.

40 minutes 8 seconds

Tubulin, as you learned before, forms a tube, and these things can be used in this case as fishing rods.

40 minutes 13 seconds

They're actually going out and catching chromosomes.

40 minutes 16 seconds

They're pushing off the ones on the other side, and then they're help pushing toward the back so that the cell will elongate.

40 minutes 21 seconds

It's helping to control.

40 minutes 22 seconds

Not only the movement of the microtubule, the the chromosomes, but also the elongation of the cell.

40 minutes 27 seconds

So these are some examples.

40 minutes 28 seconds

This is really neat is looking at these mitotic spindles that are inside cells of sand dollar.

40 minutes 36 seconds

This is just an easy Organism for viewing these, but it's a really elegant structure.

40 minutes 40 seconds

And then if you zoom down a little bit more and you look specifically at where the microtubules are going to grab a hold of the sister chromatids, there are these protein complexes that come in temporarily and sit on the outside of each centromere on the sisters.

40 minutes 55 seconds

And they form an adapter complex that can grab a hold of the microtubules that are reaching out to try to attach onto the chromosomes.

41 minutes 5 seconds

And so the kinetochore is a complex of proteins that sits on the centromere.

41 minutes 10 seconds

It's temporary, it's not always there, but it forms these like little baskets that can actually attach to spindle microtubules.

41 minutes 18 seconds

And if spindle microtubules attach in a incorrectly, they can de attach and try to reattach again until everything is attached in the way it's supposed to.

41 minutes 28 seconds

And so this is a really cool electron scanning electron or a transmission electron micrograph looking at these complexes.

41 minutes 34 seconds

So you can see these sort of arrays of proteins that are sticking off of this kinetochore or of the kinetchore that's stuck to it.

41 minutes 41 seconds

Those are the microtubules that are actually stuck to either side of this kinetochore complex.

41 minutes 46 seconds

And what this effectively does is by the kinetochore attaching on the microtubules on either side, these microtubules keep getting shorter and longer and shorter and longer, and they're playing this sort of game of tug of war to get all of the sister chromatins to line up in the middle of the metaphase.

41 minutes 59 seconds

Now, when you look at a cell, you can kind of tell that they're lined up along the middle, but in reality, the chromosome arms are free.

42 minutes 5 seconds

So they're all just kind of like sticking out in all locations.

42 minutes 10 seconds

But they're really, they're lined up in the middle by their centromeres.

42 minutes 13 seconds

OK.

42 minutes 14 seconds

So they're stuck to their centromeres in the middle.

42 minutes 16 seconds

And the images in our textbooks, they always look nice and flat because that's the easiest way for us to learn it.

42 minutes 21 seconds

In reality, like I said, their arms are just kind of like, yeah, I'm in the middle, but my arms are floating out in space.

42 minutes 26 seconds

And then when you start to separate them, you start to see the drag, right?

42 minutes 29 seconds

You see the arms that are like dragged.

42 minutes 32 seconds

So it's kind of a neat structure to look at, but these microtubules are going to end up helping to pull these actual sister chromatids apart.

42 minutes 40 seconds

But something has to happen before that's actually allowed.

42 minutes 44 seconds

So when they're all lined up in the middle, we refer to this as metaphase, right?

42 minutes 48 seconds

They're sort of lining up in their pairs, right?

42 minutes 50 seconds

Doesn't matter which way they face because they're all identical to each other.

42 minutes 54 seconds

And the goal here is to make sure that when we pull a set this way and pull a set this way, we've got a full set going in each direction.

43 minutes 23 seconds

We can kind of imagine a line even through the the natural cell.

43 minutes 27 seconds

We can sort of see a line through the plane even though the arms are sticking out like 3 dimensional space.

43 minutes 32 seconds

Looking kind of creepy actually.

43 minutes 34 seconds

It looks like a spider with too many legs.

43 minutes 37 seconds

But the spindle microtubule has very much intentionally formed and looks really great.

43 minutes 42 seconds

We've got the asters that we can see growing off on either direction that are stained in green and then it mixed in there.

43 minutes 48 seconds

You can't really tell.

43 minutes 48 seconds

Are these non kinetochore spindle microtubules that are sort of pushing off of each other?

43 minutes 53 seconds

They're going to help elongate the cell while the chromosomes separate.

43 minutes 57 seconds

Now before we can go into the next phase, which is anaphase.

44 minutes 1 second

And this is one everybody recognizes, right?

44 minutes 3 seconds

There's something has said, OK, cut the loops around the centromeres and let these spindle microtubules start pulling.

44 minutes 9 seconds

And it's sort of a race to pull them to other ends.

44 minutes 12 seconds

These spindle microtubules are going to get shorter near the Centro centrioles and also shorter where they're attached.

44 minutes 18 seconds

You can chop them off, but what has to happen first before you can actually do this is we have a cell cycle checkpoint that is very important and must happen.

44 minutes 26 seconds

So this is the M checkpoint.

44 minutes 29 seconds

So this starts to get regulated and controlled as you're doing pro metaphase.

44 minutes 33 seconds

And then you you finally arrive at metaphase and everybody's lined up and the cell is going to pause and go, all right, is everybody connected correctly?

44 minutes 42 seconds

If not, they need to redo the attachments with the spindle microtubules because the consequences are really severe if all of the sister chromatids are not attached appropriately.

44 minutes 53 seconds

All right, So this pause in metaphase, this second pause right here.

45 minutes

So you've got to stop here in this M checkpoint.

45 minutes 2 seconds

And when everything is cool, we're allowed to go past metaphase into anaphase and actually separate.

45 minutes 8 seconds

Because what it's guaranteeing is that a full set is going to go to 1 cell and a full set is going to go to the other cell because we were all lined up appropriately with fishing rods pulling on either side of each.

45 minutes 20 seconds

OK?

45 minutes 20 seconds

So when all of the chromosomes are attached to the spindle microtubules, they get the go ahead and then they're going to separate.

45 minutes 26 seconds

And then there's the simultaneous signal that's activated too, that is getting the cell ready to go.

45 minutes 31 seconds

Hey, we're going to finish mitosis here.

45 minutes 33 seconds

We're going to go back into G1 phase.

45 minutes 35 seconds

Each new daughter cell is going to get like ready to go back into G1 at this point to gear up for for regular life.

45 minutes 41 seconds

So why does there have to be a pause?

45 minutes 42 seconds

Well, this is kind of a consequence.

45 minutes 44 seconds

This is a really cool image of a cell where you can see all the microtubules in this gold yellow color and you can see that in fact, the asters are actually there's rice, there's tons of them going in the opposite directions and we can see that the spindle microtubules have started to pull these sets of blue chromosomes apart.

46 minutes 1 second

The opposite of and as a cell.

46 minutes 3 seconds

Now, I grew up in the 80s and the 90s and I'm absolutely 100% a Star Wars baby and every time I see an image like this I can't help but think that it looks exactly like this ugly dude who is from Return of the Well, it's from what's from.

46 minutes 19 seconds

It's from Return of the Jedi, right?

46 minutes 21 seconds

Where Luke Skywalker shows up and kills the rancor.

46 minutes 25 seconds

And so at Java's palace, like, it looks like that to me.

46 minutes 28 seconds

I think this is just the ugliest thing ever.

46 minutes 29 seconds

But the chromosomes kind of look like teeth.

46 minutes 32 seconds

And you might notice here that this is kind of bizarre looking, that there's these blue, this blue chromosome that's kind of floating out in the middle that looks like it's been left behind.

46 minutes 41 seconds

It looks like it hasn't been attached properly to microtubules, and so there's this risk that a cell has that it could either lose a chromosome altogether, or what could happen is that you could pull a whole pair to one daughter cell and none to the other daughter cell, and that probably would lead to cell death.

47 minutes

Losing an entire chromosome is not something that our cells can withstand.

47 minutes 4 seconds

In fact, gaining an extra whole chromosome is also not something our cells can usually withstand.

47 minutes 12 seconds

Extra gene expression from any chromosome is usually problematic.

47 minutes 16 seconds

We have very few limitations on that being OK.

47 minutes 20 seconds

This is referred to as trisomy.

47 minutes 22 seconds

If you pick up a third version in your cells or an extra chromosome, particularly when you're when reproduction happens, it's problematic.

47 minutes 33 seconds

Generally only X chromosomes are OK.

47 minutes 35 seconds

You can get, you know, some of the smaller ones like chromosome 131521, Those can exist OK, in our cells.

47 minutes 42 seconds

In fact, this is only one cell, right?

47 minutes 43 seconds

This is only one cell during cell division getting an extra chromosome, but it's still not OK.

47 minutes 47 seconds

We've evolved to have this much gene expression from every chromosome, not here and not here.

47 minutes 52 seconds

It can be problematic, especially when you're talking about thousands of genes on one chromosome either being lost or gained an extra.

48 minutes

And so this is a danger thing, which is why we have this checkpoint to make sure that all the pairs line up correctly, all are attached correctly.

48 minutes 7 seconds

That way we can split them all correctly.

48 minutes 9 seconds

And so once it's determined that everything is lined up and attached correctly, there are proteins that come then break the lassos, they break the loops that are around the centromeres and they allow the spindle microtubules to pull in opposite directions.

48 minutes 23 seconds

And so as I mentioned, they're not only going to get shorter where they were created, these spindle microtubules are going to start losing tubulin where they're actually attached the to the to the kinetochores.

48 minutes 34 seconds

Now you think about that, you're like, well, how am I cutting off the tubulin?

48 minutes 37 seconds

If then make them still stuck.

48 minutes 38 seconds

And the reason is it's not just that the microtubules have been attached.

48 minutes 44 seconds

There are motor proteins that are actually walking on those microtubules, pulling the chromosomes.

48 minutes 49 seconds

And so some of these tubulin molecules are going to be chopped off as these motor proteins are really pulling and walking along the microtubules to bring them to opposite poles.

48 minutes 59 seconds

It's a pretty fascinating cell biological process, But you can kind of see that there's now a distance between the chromosomes, right?

49 minutes 6 seconds

Looks like a bad set of blue teeth, like a muppet or something.

49 minutes 9 seconds

You can start to see the chromosomes actually separating from each other in anaphase, and then by the time you get through telophase, it is very clear that the chromosomes are now in their own regions.

49 minutes 19 seconds

The cell hasn't fully separated, right?

49 minutes 21 seconds

Cytokinesis is going to start to overlap with telophase, but in telophase you've got your chromosomes now at opposite poles.

49 minutes 27 seconds

You can see nothing but microtubules between them at this point, right?

49 minutes 31 seconds

They're still separated.

49 minutes 33 seconds

And then we're going to see the nuclear envelope and the nucleolus start to come back.

49 minutes 36 seconds

This generation of the nucleolus means that all the genes involved in producing things for doing translation are going to be, you know, kicked on quickly because we got a baby cell now and we got to start making stuff that we need for the cell to get bigger and to kick out its kick on its life.

49 minutes 54 seconds

And then we're also going to see the nuclear envelope reform.

49 minutes 56 seconds

And so that double membrane bound barrier around your chromosomes is keeping your most sensitive information safe until the next time that you decide to do cell division or that you're told to do cell division.

50 minutes 8 seconds

So notice that this is starting to pinch together.

50 minutes 11 seconds

So cytokinesis is the actual separation of the cytoplasm, equitable division of the cytoplasm and the plasma membrane as best possible.

50 minutes 19 seconds

That's going to overlap with telophase so that you can produce 2 new daughter cells relatively quickly.

50 minutes 26 seconds

So cytokinesis forms what's called a cleavage Burrow.

50 minutes 30 seconds

So the process of cleavage in biology, it sounds like a funny name, right?

50 minutes 34 seconds

We often we think we use that term for anatomy sometimes to have cleavage.

50 minutes 40 seconds

But cleavage of biology usually means cutting something.

50 minutes 43 seconds

So enzymes cleave things by cutting them in half.

50 minutes 46 seconds

Cleavage happens in cytokinesis by cutting the cytoplasm in half.

50 minutes 49 seconds

And when you look at this image, I mean, I don't know what you think this looks like, but I, I think this looks like a coyote squash.

50 minutes 55 seconds

You probably thought it looked like something else.

50 minutes 57 seconds

Shame on you keep it G rated, right?

51 minutes

But it actually looks a bit like all right.

51 minutes 3 seconds

It looks kind of like a **** but no pun intended.

51 minutes 6 seconds

This has to happen in order for cytoplasm to be able to separate.

51 minutes 10 seconds

And so this happens with also you by also using cytoskeleton.

51 minutes 14 seconds

So the microtubules tend to be for doing big stuff inside the cell, but the actin filaments, the microfilaments, the smaller ones, they often live around the periphery of a cell.

51 minutes 24 seconds

And what they'll do is they'll start to form basically a waist or a belt around this, and they'll get shorter and shorter and shorter.

51 minutes 31 seconds

And they'll actually help pinch off the cytoplasm.

51 minutes 34 seconds

So it's not only the cytoplasm in the inside, but they're actually helping to pinch off the plasma membrane too because it's going to want to reform with itself once it gets close together.

51 minutes 42 seconds

So this cleavage furrow or this, you know, puckering is actually formed from those microfilaments.

51 minutes 50 seconds

It's going to form sort of like a purse string that you end up pulling and that's eventually going to cause the two daughter cells to separate from each other.

51 minutes 56 seconds

So it's absolutely necessary to have cytokinesis as well.

51 minutes 59 seconds

If you don't, you end up generating 1 cell with two whole nuclei in it and you know, two complete sets of all the chromosomes when really you don't want 46 chromosome or you don't want 92 chromosomes in a cell.

52 minutes 12 seconds

You not only 46, right, unless you have a cell type that is intentionally doing that.

52 minutes 16 seconds

And some organisms, we do see replication of the chromosomes without cytokinesis, but that tends to produce cell types and some organisms that have multiple sets of the genome in them.

52 minutes 32 seconds

For example, if you happen to notice that some plants have little hairs on them, like if you grow tomatoes at home or if you smoke marijuana and you see the leaf hairs that are on them.

52 minutes 45 seconds

I mean, I'm from Illinois, it's legal there.

52 minutes 46 seconds

It's pretty common thing.

52 minutes 48 seconds

Those hairs that are on the surface of a plant are called trichomes and they often have multiple, multiple, multiple sets, whole sets of chromosomes.

52 minutes 56 seconds

And so they will do mitosis without cytokinesis.

52 minutes 59 seconds

That is usually the consequence.

53 minutes

If you don't actually do cytokinesis, you have more sets of chromosomes than a normal cell would in plants.

53 minutes 6 seconds

Plants have to do this as well, but theirs is a little bit different.

53 minutes 8 seconds

So plant cells don't get to sort of move around like animal cells do, and they're bound by cell walls.

53 minutes 15 seconds

So not only do they have to do this in the context of having a cell wall around you, but they also have to build a new cell wall, and then they have to elongate and expand afterward.

53 minutes 24 seconds

And so in plant cells, there's not something called a cleavage furrow.

53 minutes 27 seconds

What they end up forming is something called a cell plate in the middle, and they line up a bunch of little vesicles that have components for building a new cell wall and may even also have gaps, right.

53 minutes 42 seconds

So plants have plasmodesmata, remember, where they can talk to each other.

53 minutes 45 seconds

And so there's usually endoplasmic reticulum that's like a placeholder where they end up building the new plant cell wall between the two daughters.

53 minutes 52 seconds

But they've got some, ER, placeholders so that we have communication between them.

53 minutes 56 seconds

But these vesicles you can actually see lined up here, they're little membrane bound vesicles, but they have cell wall building parts.

54 minutes 2 seconds

And so once the vesicles fuse with each other, all the cell wall building parts are inside and they can build a cell wall between the two daughter cells.

54 minutes 10 seconds

And so you don't really get a cleavage furrow.

54 minutes 12 seconds

You get this cell plate that forms in between them that ends up generating a new cell wall.

54 minutes 17 seconds

And the image down here is a little misleading because there will be gaps.

54 minutes 20 seconds

They'll be little gaps in the wall that form these plasmodesmada that are controlled communication channels between the individual daughter cells that are formed in plants.

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And so plants are restricted in how they do this because the cell wall is there.

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And so after it happens and you form a new cell wall, if you look at dividing plant cells, you can see those little baby cells are a little smaller.

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What ends up happening is the cell starts to elongate.

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They'll often start to get longer, sometimes a little wider, but they stay in that same position and they'll get more mass by doing metabolism.

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And so mitosis is a really, really fascinating, absolutely necessary process, and you're doing it right now.

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As you're watching this.

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You're generating new cells in many of your tissues, as you should be.

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It's actually also part of the process of generating your gametes too.

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Even though your gametes are ultimately made by meiosis, mitosis is a part of it.

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You have a series of different cell types throughout your body that are referred to as stem cells that act as a repository for generating new cells in certain tissues.

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For example, you have hematopoietic stem cells in your bone marrow that divide to produce some cells that are going to stay stem cells and other cells that are going to develop all the different blood cells that you have in your body and in your immune system.

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And if you have a problem and those stem cells don't divide or don't work, you get something called leukemia, where you actually have a blood disorder.

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You don't produce all the immune cells you need and then you may need like a bone marrow transplant because those cells are not functioning.

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So mitosis is absolutely essential for stem cells that allow us to keep producing different types.

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If you look at the lining of your digestive tract, in every single one of your little fingers on your your digestive lining, you've got a pocket that's down below those called a crypt where you have stem cells.

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They're going to keep generating all the cells that are lining the gut of your intestines.

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And if you don't do that, you can't Slough off the old cells and produce new ones.

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And so mitosis is essential not only for development as you grow into a mature multicellular Organism, but also for maintaining the types of tissues that require regeneration.

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All right, because we use them so hard and so often.

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And so keep in mind, understand that the cells going to copy its DNA way earlier in the cell cell cycle up in South phase and that there are multiple checkpoints controlling being able to go through these different phases so that they are done correctly and the right timing and in the right place.

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And then we've got these five different phases of mitosis and the last of which overlaps with cytokinesis.

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And so those are different processes, right?

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This is the lining up of the chromosomes so that we get them all in the buddy system all along the middle and that everybody's attached appropriately.

56 minutes 59 seconds

And then we say peel bananas.

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We cut all the loops and we pull those sets into different daughters because the goal in this process is to make daughter cells that are genetically identical to each other.

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Everybody gets a full set, a full complement of 46 chromosomes that the original starting cell had.

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That is the point here is to produce two genetically identical daughter cells, not only with their chromosomes, but as closely as possible with the organelles.

57 minutes 27 seconds

Now, not all organelles are going to be duplicated.

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Some of them are just going to be sorted, but you have to sort and sometimes copy everything in the cytoplasm and give some to each daughter cell so that they both have an equal chance of surviving.

57 minutes 41 seconds

Now they'll do metabolism and they'll start to get bigger and grow and bulk up while they're doing G1S and G2.

57 minutes 47 seconds

But this is sort of the general overview of every almost every cell type that we have, how it is that they're able to produce new versions of themselves.

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I hope that a lot of that was a review for you.

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Some of the finer nitty gritty details of cell biology might not be something you learned the first time or you kind of glossed over it a little bit.

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The reason I bring that up now is because that will become important later when we think about other things that happen in myosis and cell division.

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When you think about the behavior of cells, if you're a bio major and you're going to take Physiology later, if you're going to take cell biology or if you're going to take plant Physiology, you'll still see overlap in these processes.

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We share these mechanisms with all other eukaryotes, all the way down from single celled yeast that makes our bread and our wine and our beer, to plants, to our single celled protist cousins that are out floating in ponds, all the way to all the animals that are on the planet.

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We use this exact same mechanism for for copying and sorting our chromosomes into new daughter cells.

58 minutes 45 seconds

And so that's something that is very, very ancient and works really well and we all share it in common.

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And I hope you find it fascinating and easy to understand.

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Thank you so much for your attention.

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Until next time, I hope you have a wonderful day and you will all be hearing from me again soon.