BISC 130 - Chapter 11: Sexual Reproduction

• 4/11/25 NOTES (Continued):

   

  • Virtually all multicellular Eukaryotes are capable of sexual reproduction

    • Results in more diverse offspring than asexual reproduction

      • High diversity among offspring increases likelihood of survival

   

  Sexual Reproduction

   

  • Sperm + egg = Zygote

  • 1n + 1n = Zn

    • Haploid sperm  + Haploid egg = Diploid Zygote

   

  • Need a way to produce haploid egg/sperm cells from diploid cells

    • This process is called Meiosis

      • This is done by special "germ cells"

   

  Eukaryotic Germ Cell Cycle

   

  1. Interphase

   

  1. Meiosis 1

     1. PPMAT 1

   

  1. Prophase 1

     1. Nucleus, golgi, and ER break down

     2. Chromosomes condense

     3. Synapsis. Formation of tetrads.

        1. "Sister chromatids" are exactly the same

        2. "Homologous chromosomes" have the same genes but their exact sequences are different

        • Allows for crossover

          • Increases genetic variation

   

  • Prometaphase 1

   

  • Microtubules attach to tetrads

   

  • Metaphase 1

   

  • Tetrads align at the center of the cell.

    • Their orientation is random - different daughter cells will receive different homologs.

   

  • Anaphase 1

    • Tetrads pulled apart

      • Sister chromatids stay together, they go to the same side.

   

  • Telophase 1

    • Chromosomes de-condense

    • Nucleus, golgi, ER

   

  • Cytokinesis

    • Results in two "haploid with replicated DNA" cells.

    • *No interphase between Meiosis1 and Meiosis 2

   

  • Meiosis 2

    • PPMAT 2

    • Every step in Meiosis 2 is just like mitosis.

   

  • Cytokinesis

    • Reuslt: 4 haploid cells

   

  ---------------------------------------END OF LECTURE 3-2--------------------------------------------------------

   

  Attendance quiz code: 34300

LECTURE TRANSCRIPT (CHAPTER 11):

start up on chapter 11, which is actually still about cell division. It's just a slightly different type of cell division. So this has to do with sexual reproduction.

So here's a slide from the very first day of class. Reproducing was something that all living things did. This was an example of asexual reproduction, but now we're going to focus on sexual reproduction. So virtually all, there are some exceptions, but virtually all multicellular eukaryotes, plants and animals, are capable of sexual reproduction. This is a big deal because sexual reproduction, as we will see, mixes things up. Asexual reproduction makes a clone of yourself. You know, binary fission, the stuff we saw in the last chapter, those made identical daughter cells.

The big picture point of what we're going to see in this chapter is not to make identical cells, it's to have diverse offspring. It's to randomize things. It's to mix things up. Diversity is very valuable in organisms because you don't know what the next year is going to bring, what the next season is going to bring. If you have lots of offspring that have lots of different combinations of attributes, you're sort of hedging your bets and increasing the likelihood that one of those is going to have what it takes to survive whatever the world throws at them down the line.

So again, this is sort of the big picture sales pitch on why all of this is needed, why we can't just do asexual reproduction for everything. Sexual reproduction mixes things up, creates diversity in offspring, and high diversity in offspring increases likelihood of survival.

So, what is sexual reproduction? Well, let's break this down into the most exciting way possible, mathematically. So, at the end of the day, sexual reproduction is very simply:

• Sperm plus egg equals zygote.

This is how it happens in animals. This is how it happens in plants. One cell plus other cell equals this cell. The zygote is this cell here. Now, a term that I introduced in the last chapter was the term diploid. I told you that virtually all of our cells are diploid. The abbreviation for this is 2N. That meant two copies of each chromosome. And we saw that in the slide on human chromosomes, two copies of each one of those.

So, if our sperm are diploid, 2N, and the egg is diploid, 2N, 2 plus 2 equals 4, that would make a zygote that is 4N? Four copies of each chromosome? Do we double the chromosome number in our offspring? No, we don't actually do it. Immediately crossing this out, we don't do this. This is the reason why I said virtually all of our cells are diploid. Yes, our skin cells are diploid, our muscle cells arediploid, our neurons are diploid. The exception is the eggs and sperm. These things are not diploid. They are 1N or haploid. That's because we need to generate a zygote that's 2N. We need to make offspring that is diploid. And the only way to add two numbers together and have it equal 2 is each of those numbers has to be 1. So sperm is haploid. These cells have one copy of each chromosome. Eggs are haploid. They have one copy of each chromosome so that when they come together, the zygote is diploid, two copies of each chromosome.

Now, the tricky part is we need a way to make these haploid cells. Everything we talked about in the last chapter, mitosis as part of the overall eukaryotic cell cycle, that made identical copies of cells. Here, we need to make haploid cells. No other cells in the body are haploid. We need a way to make haploid eggs and haploid sperm from diploid cells. The way that we are going to do this is through a process called meiosis. Some people say meiosis. That's another ligand-ligand thing. I say meiosis, but meiosis is fine. Meiosis is the process that's going to create these haploid cells. And this is done by special germ cells. So not everything is doing meiosis. Only germ cells within the testes or the ovaries are able to do this very special type of cell division.

So yeah, here's an overview of the human sexual life cycle:

1. You get a zygote, this single cell.

2. It's going to divide.

3. It's going to grow into an embryo, a fetus, a baby, an adult.

4. All these cells are diploid.

5. Then within the germline, we have meiosis to produce sperm or egg as the case may be.

6. These are haploid.

7. The sperm is haploid.

8. The egg is haploid.

9. Here's the 1 plus 1 equals 2.

10. Fertilization, we're back to diploid.

11. And it's back to the rest of this cycle.

So meiosis is the process here, and germ cells are the cells that do meiosis. Again, not everything does this. So if we want to talk about the process of meiosis, I talked about how much I liked this particular figure in the last chapter. So I'm actually just going to modify this. So this was mitosis as part of an overall eukaryotic cell cycle. Really, we just cross these ones out and replace them with this. So this is what happens specifically in eukaryotic germ cells. We have G1, S, G2, and then we have meiosis 1, cytokinesis, meiosis 2, then another cytokinesis. And so obviously, this is going to be a two-step process. It's not just one division. It's two sets of divisions, two cytokinesis. This is going to result in four cells by the time we're done with this. It's two rounds of cell division. So we'll get some visuals on this soon enough. But yeah, I like modifying this familiar figure. This is how it happens when a cell is doing meiosis, when a germ cell is doing meiosis.

So all right, eukaryotic germ cell cycle. We got interphase. Not going to say anything more about that. We talked about G1, S, and G2 in the last chapter. I'm not going to rehash any of that again. It's the same interphase. I talked about that. Copy and paste all that in.

All right, next we have meiosis 1. So meiosis 1 has the same PPMAT that mitosis had. So in some ways, this is convenient that it has so much in common with mitosis. But in other ways, it can sometimes be confusing because it has so much in common with mitosis. I am going to do my best to highlight the similarities and the differences between the PPMAT of meiosis 1 and the PPMAT of mitosis from the last chapter. The way that we distinguish these in text is with the Roman numeral 1. So if we're talking about prophase 1, that means we're talking about the prophase of meiosis 1. If you see pro... and so on my multiple choice test questions, you have to read these questions carefully. I'm not trying to trick anyone ever. I just want you to read them carefully. If the question says metaphase 1, it means I'm talking about the metaphaseof meiosis 1. And, you know, spoilers for later on. If I say metaphase 2, that means I'm going to be talking about the metaphase of meiosis 2. if I just say metaphase flat with no additional Roman numerals, that means I'm talking about the metaphase from the last chapter, the metaphase of mitosis. So again, not trying to get confusing, but the Roman numeral is how we distinguish these from mitosis.

So, all right, let's take a look at this process. We got prophase. Pro-metaphase is not pictured here, I think just to save space on this figure. Metaphase, anaphase. Well, I'm sorry, prophase 1, metaphase 1, anaphase 1, telophase 1, and then finally cytokinesis.

So let's start in on prophase 1. So prophase 1, we are going to have the chromosomes condense, the nucleus, Golgi, and ER break down. Okay, this should sound familiar, but I guess that's good. Prophase 1 is very similar to prophase from mitosis, nucleus, Golgi, ER break down, chromosomes condense. So far so good, has a lot in common with mitosis.

We are going to see the chromosomes. Oh, okay, so this is actually a little different now that we look at it. Back in mitosis, we had sister chromatids appearing at this point. We had these two chromosomes partnered up, but if we look closely at this, that's not two, that's four total chromosomes. If we want to zoom in on this so we can get a better look at this, yeah, that's one, two, three, four, four total chromosomes paired up together. This structure of four is, you know, this is new to meiosis 1. This structure of four is called a tetrad. And tetra means four, so there are four total chromosomes here. This consists of two pairs of sister chromatids. So yeah, these two red ones here, these are the sister chromatids. The two blue ones back here, these are sister chromatids.

So what are the red versus blue? Why are these different colors? Well, they're labeled here. The red versus blue are homologous chromosomes. So, okay, let me write this down, then I'll explain what homologous chromosomes are, because this is a very important term. We've got prophase 1 still. We're going to form tetrads, which the key terms define as two duplicated homologous chromosomes (four chromatids), and tetra means four, bound together during prophase 1. So this set of four. Synapsis is the name of this process. So synapsis occurs during prophase 1, and it leads to the formation of these tetrads.

So, okay, what are homologous chromosomes? Well, homologous chromosomes are not identical. Sister chromatids are identical. They're exactly the same. Homologous chromosomes have the same genes, but their exact sequences are a little bit different. This needs an analogy. These are sister chairs. These two chairs are exactly the same. No differences at all. Just like these two sister chromatids, they just went through DNA replication. These are exactly the same. Sister chairs are exactly the same. Sister chromatids are exactly the same. Homologous chairs are the same piece of furniture, but they are not exactly the same. These are both chairs. They are the same thing. They're the same type of thing. But, you know, legs are connected in a slightly different way here. The slats run vertical versus horizontal. I think this one's maybe a little bit wider than this one. They both do the same job. They both fulfill the same function of you sit on this thing. But they are obviously not exactly the same. That's what homologous means. They have the same genes. They have the same basic structural foundations of four legs, a seat,and a back. Homologous chromosomes are the same type of thing, but their exact sequences are a little bit different. And these textbook figures, that's what the red and the blue, these different colors, are always trying to make clear. If it's the same color and the same size and they're together, they're exactly the same. Sister chromatids, sister chairs. The red and blue is meant to show that there are differences here. The red and the blue are homologous chromosomes, the same genes, but different exact sequences.

And again, this is a good summary of what this is. Homologous chromosomes have the same genes as one another, but the exact sequences are a little bit different. So this tetrad has two sets of homologous chromosomes and their sister chromatids. So two sets of sister chromatids, four of these things total.

Okay, I spent so much time on this because this is one of the most important things, one of the most important differences between this and mitosis. So why? What was the purpose of doing this? What was the purpose of forming this tetrad? We didn't do it in mitosis. Why are we doing this? We go through synapsis and we form these tetrads in order to be able to do something called crossover or crossover. There's another term for it I'm not going to use.

Crossover is exactly what it sounds like. It's when these homologous chromosomes swap a little bit of information with one another. If we were looking at chairs, I would rip off this front left leg, swap it with this front left leg. You're left with two still completely functional chairs, but they've swapped a couple of parts with one another. And the purpose of this is to make slightly different chromosomes. You think about parents and offspring. This chromosome is coming from one parent. This chromosome is coming from the other parent. You're trying to make offspring of your own. If you're doing crossover, here's a slightly different chromosome. It's not the same as any chromosome you have. It's not the same as any of your parents' chromosomes. You're mixing things up. You're increasing genetic variation.

And every time your body goes to make eggs or sperm, it's doing this crossover in a slightly different way. It's not always down here at this exact position. So this, again, increases genetic variation. The whole point of this is to not have the same offspring twice. We're not having millions of offspring, but there are other organisms out there, especially big trees, that can make millions of sperm cells, millions of egg cells. And you want to have as much variation as possible. So this synapsis allows for crossover.

The purpose of crossover, which I'm sorry, I think I forgot to read the key terms definition of crossover. The definition of crossover from the key terms is the exchange of genetic material between homologous chromatids, resulting in chromosomes that incorporate genes from both parents. Allows for crossover. And the point of this is to increase genetic variation.

Okay. All of this was happening during prophase 1. Next, we've got prometaphase 1. This one's easy. Microtubules attach to the kinetochore of these tetrads. Pretty similar to the last chapter, microtubules attach. But again, it's tetrads they're attaching to, not sister chromatids. Got this set of four here during prometaphase 1.

Okay. After prometaphase 1 comes metaphase 1. And yep, very similar to the last chapter. Everything's lining up in the middle. But there's an important detail about this lining up in the middle. So mitosis, metaphase, it didn't matter whether this chromatid went to the left or whether this chromatid went to the left because these two chromatids are exactly the same. The two sides are going to get exactly the same stuff. But here, it does matter how they are facing. We could picture, sorry, this one's horizontal instead of vertical, this figure. But we could imagine what's going to happen next. They're going to get pulled apart from one another. And it looks like the daughter cell up here is going to get red for this chromosome.And the daughter cell down here is going to get blue for these chromosomes. So it does matter which way they line up, which way they are facing. So how do we make sure that each daughter cell gets about the same amount of red and blue? How do we ensure an equal distribution of these different homologues? Well, the answer is we don't. It's completely random which way they are facing.

So let's compare these chromosomes once again to pieces of furniture. So yeah, you have:

• Two different chairs.

• Two different tables.

• Two different nightstands.

• Two different chested drawers.

This is where I got bored of Google image searching different furnitures. Suffice it to say, in a human with 23 different chromosomes, this would be a pretty crowded room indeed. But yeah, these line up in such a way that you're going to make an egg or sperm cell with this chair, this table, this nightstand, this chest of drawers, and a different egg or sperm cell with this chair, this table, this nightstand, this chest of drawers. Or you could do this with this combination of furniture and another cell with this combination of furniture. Or it could be this combination and this combination. Any of these are possible.

Here is a simple organism with only three chromosomes showing all the possible combinations:

• Blue, blue, blue

• Red, red, red

• Red, red, blue

• Blue, blue, red

• Blue, blue, red

You see what I mean. Eight possible combinations if there are three chromosomes. I did the math for you. In an organism like ourselves with 23 different chromosomes, there are over 8 million possible combinations for how these homologs can be sorted. That means you could go through the process of making egg or sperm, as the case may be, 8 million times and not make the same cell twice. The whole purpose of this is to increase genetic variation.

So in metaphase 1, these tetrads align at the center of the cell. Which way they're facing, their orientation, is random. So different daughter cells will receive different homologs. They're not going to be the same every time. The purpose of this is to increase genetic variation.

Okay, that was PPMAT. After metaphase 1, we come to anaphase 1. And yep, the tetrads are pulled apart from one another. Importantly, the sister chromatids stay together. They'll get pulled apart in the next one. But yeah, the tetrad is broken apart and the sister chromatids stay together, pulled to opposite sides during anaphase 1. Yep, tetrads pulled apart, sister chromatids go to the same side.

Okay, and yep, here if we want to zoom in on this, this was pro metaphase where the microtubules attached. And there is anaphase 1 where the sister chromatids sticking together, but the tetrad is broken apart. And finally, telophase 1, followed afterwards by cytokinesis. Very similar to last chapter. Telophase 1, chromosomes decondense. Nucleus, Golgi, ER are going to reform, X2. And then cytokinesis is going to pinch that cell into two.

So at this point, we have finished meiosis 1 and the first of our two cytokinesis. The question is, what kind of cells are these, these daughter cells? Are these daughter cells haploid or diploid? Don't answer yet, because this is a little tricky. At first count, you may look at this and say, oh, this daughter cell has one, two chromosome, one, two, one, two. This looks like a diploid cell. Technically, diploid means you have two different things. You have two different chairs. You have two different nightstands. You have two different tables. This is our default cells. We have two different versions of everything. These cells here at the end of meiosis 1 and the first cytokinesis are not actually diploid. These two chromosomes arethe same. A little bit of difference because of crossover, but these are the same chromosome. These are the same chromosome. But these are the same chromosome. These are the same chromosome. These daughter cells are, and this is an awkward phrase, but it is what it is, these cells are haploid with replicated furniture, or haploid with replicated DNA. So yeah, this is not a diploid room. This is a haploid room with replicated furniture:

• Two of the same chair.

• Two of the same table.

• Two of the same nightstand.

And that's what these cells are at this point. So after this first cytokinesis, we have two cells. They are haploid with replicated DNA cells. They're not diploid cells. They don't have two different things. They have two of the same thing. Haploid with replicated DNA.

Okay. Back to this. We did meiosis I. We did cytokinesis. Now it's time for meiosis II. Importantly, there is no interface between meiosis I and meiosis II. You don't have another cell growth. You don't have another DNA replication. You don't have another G2. After cytokinesis, you go to meiosis II.

So what happens in meiosis II? Well, got another figure to show this. So we got:

• Prophase II,

• Pro-metaphase II, not pictured here,

• Metaphase II,

• Anaphase II,

• Telophase II,

• Cytokinesis.

All right. So what's going on here? Well, prophase II. It looks like the ER, Golgi, nucleus are breaking down. The chromosomes are condensing, becoming visibly distinct. This is exactly the same as it was in mitosis. Pro-metaphase II. Again, it's not pictured here, but the microtubules attach to the sister chromatids. No tetrads here. No crossover is happening here. No synapsis here. This prophase II, again, I'm sorry it's not pictured here, is exactly the same as the pro-metaphase from the last chapter.

Metaphase II. Sister chromatids line up in the center. Okay, still no tetrads, no crossover. These sister chromatids lining up exactly the same as it was in mitosis. Anaphase II. The sister chromatids are pulled apart exactly like it was in mitosis. And then telophase II and cytokinesis. Chromosomes decondense, nucleus, ER, Golgi, all the same stuff from mitosis.

So although meiosis I had several important differences comparing it to mitosis, meiosis II is actually the same. Everything that I would write down about prophase II is exactly what I wrote down for prophase in the previous chapter. Everything for pro-metaphase II is the same. If you're making your own notes and you want to do this, you can copy and paste this in if you want to. I'm going to save us all some time and just say each one of these phases is the same as the phase from mitosis.

There are no notable differences between prophase II, pro-metaphase II, metaphase II, anaphase II, and telophase II, and prophase, pro-metaphase, metaphase, anaphase, telophase. Exact same descriptions I gave in the last chapter.

So by the time we finish with this second cytokinesis happening right after telophase II, we are finally left with four haploid, truly haploid, not haploid with replicated DNA. We are truly left with four haploid cells.

So let's go back to furniture once again. We started with a diploid room. Two copies of each chromosome. We had interphase, which means we had DNA replication as part of interphase. We created a diploid room with replicated furniture. It's starting to look a little messy, but this is what happens during interphase. We replicated everything. After meiosis I and that first cytokinesis, that left us with a haploid with replicated furniture room. We only had one of each type of furniture, but wehad two copies of it. Haploid with replicated furniture. And by the time we have finished meiosis II and the cytokinesis. and the cytokinesis that comes after that, we have a truly haploid room. One single copy of each piece of furniture. Sorry, if this furniture analogy is getting a little tedious, but it's a good way I think to describe what's going on here. By the time we finish the second cytokinesis, we now have four haploid cells.

If you don't appreciate furniture, if you don't, that's fine. Here's an overview of meiosis compared side by side with mitosis. I find this very useful as a visual. Hey, if you can draw this yourself, if you could, and you know, I'm not a great artist. Even if you're not a great artist, you could draw a cell. It's just a circle. You could draw a chromosome. It's just a sausage. You could get two different colored markers or colored pencils or pens, whatever. If you could draw this out yourself, you'll really get a better appreciation for what's happening along the way.

Mitosis was:

1. Interphase

2. Prophase

3. Prometaphase

4. Metaphase

5. Anaphase

6. Telophase

7. Cytokinesis

Done. Meiosis was:

1. Interphase

2. Prophase 1

3. Prometaphase 1

4. Metaphase 1

5. Anaphase 1

6. Telophase 1

7. Cytokinesis

Each of these cells goes through:

1. Prophase 2

2. Prometaphase 2

3. Metaphase 2

4. Anaphase 2

5. Telophase 2