Cell Biology Chapter 18 (Exam 2)

What is the cell cycle?

The mechanisms in which all living life on earth is created

Do all human body cells divide?

No, not all go through cell cycle

What happens during G1?

A cell grows larger, and synthesizes the RNA and proteins needed for sustaining itself and for DNA replication. Transcription and translation occur largely here

What happens during S?

DNA replication. The cell makes a copy of the cell’s complete DNA. The amount of genetic material doubles, the amount of chromosomes stay the same

What happens during G2?

Quality control mainly. Checks for any errors that might have occured during S phase. The amount of genetic material and chromosome count does not change. Stays the same as it was from S phase

What happens during mitosis?

The process where a single cell divides its duplicated genetic material to create two new daughter cells

Explain the chromosomes during G1

Long and linear. 46 total chromosomes, 46 chromatids

Explain the chromosomes during S

The entire genome is being actively replicated. The amount of A, C, T, and G doubles. 46 chromosomes, 92 chromatids. X shape

Explain the chromosomes during G2

Still has 46 chromosomes and 92 chromatids.

Explain the chromosomes during mitosis

The number of genetic material stays the same (92) while the chromosomes double. They split up and each now has its own centromere. 92 chromatids, 92 chromosomes (divided between each new daughter cell)

Difference between the chromosomes in Interphase and the chromosomes in Mitosis

During interphase, chromosomes are loose. During mitosis, the chromosomes are very tightly packed in order to move around.

When do the chromosomes become X shaped?

Forms at the end of Interphase (S/G2). It is temporary.

Different cell types divide at…

Different rates

The rate of cell division depends on its…

specific function or environment

Frog embryo divison rate

Every 30 minutes

Yeast divison rate

Every 1-1.5 hours

Human gut (intestinal) cells division rate

Every 12-24 hours

Liver cells division rate

Once a year

Neurons division rate

Once a lifetime

Why do gut cells divide so frequently?

The stomach and intestines have harsh acids and food so these cells are constantly damaged. Therefore, they must divide continuously to replenish the lining

Once your brain is developed, where do your neurons go?

They exit the cell cycle and enter G0, resting state.

What is flow cytometry?

A laboratory technology that measures the characteristics of cells as they flow in a liquid stream through a laser beam

How does flow cytometry work?

Dye the cells with a fluorescent dye. Dye binds depending on how much DNA is in the cell (darker for more, lighter for less). As each cell passes through the laser, the computer counts it. It can count 10,000+ cells per minute. A dim amount of dye means a less DNA count, a bright dye means a greater DNA count. Tells you which cells are actively going through DNA replication

How is each phase in your cells carefully controlled?

There are checkpoints in each phase before the next phase starts. For example, before S phase, the cell asks itself, is the environment favorable.

Can you go back once you start S phase?

NOOO

What two proteins come together to make cyclin-dependent kinases? (CDKs)

Cyclin-dependent Kinase and the Cylin

What is the structure of the CDKs an example of?

Quaternary regulation

What do kinases do?

They phosphorylate

What does it mean by cyclin-dependent kinase?

A kinase who needs a cyclin to become active

What is the structure called when the CDK and Cyclin come together?

CDK-Cyclin

What do the CDK and cyclin provide to each other?

Checks and balances - they control and regulate each other - they need each other to carry out their duties.

Why is it important for the cell cycle to be controlled? What happens if not controlled properly?

Cancer happens. The cells divide very quick and use all of the resources

How can something like cancer be controlled by the CDK cyclins?

The CDK and cyclin must come and work together to regulate the cell cycle and prevent diseases like cancer.

In order for system of checks and balances to work, Cyclins and CDK’s must be…

Independent - independently regulated and doing their own thing.

Cyclins and CDKs have different levels of…

Control - Only when diff levels of control are high, can the cell cycle proceed.

What do Cyclin-CDKs do?

Drive each phase of the cell cycle forward by phosphorylating target proteins needed for each phase.

Can a kinase or a cyclin decide individually when the next phase of the cell cycle moves forward?

NO! They must work together to drive the phases forward

How many cyclins and how many CDK’s do you need for each phase of the cycle?

1 Cyclin, 1 CDK per each phase of the cycle.

Each phase of the cell cycle has different… to move the cycle forward?

Different cyclins and CDKs - They are all independently regulated. Different cyclins and CDKs per each phase add another layer of control and checks and balances in the cell cycle. Each phase has its own different cyclin and different CDKs

Why is the system of checks and balances important for regulating the cell cycle and preventing cancer?

If too much division occurs, that causes cancer. Your cells need BOTH of these things in order to move the cell cycle forward and adds another layer of control so we do not end up dividing uncontrollably

What is the point of having different cyclins and CDKs for each phase in the cell cycle?

This adds a another layer of control and checks and balances so the cell ensures that it doesn’t divide very quickly and uncontrollably

How does for example, S phase begin with the cyclin and CDK?

The different Cyclin and CDK must come together and that will start S phase now

Cyclins and CDK’s are…

Independently regulated

How can too much of division be bad for the cell?

If cells divide too much - Cancer - Individual dies.

After cyclin CDKs drive the cell cycle forward, what happens to these proteins

Must be destroyed by adding the ubiquitin group to them so proteases can destroy them. This carefully controls them even after you use them.

Do all cells take place in the cell cycle? Where do they go?

No. These cells go into the phase called G0.

What does it mean when a cell is in the G0 phase?

It has no plans to divide ever

Which types of cells can you see more cyclins and CDKs in them?

Cells that divide more frequently, like gut cells (every 12-24 hours). Have cyclin-CDKs almost all the time in them

Which types of cells can you see less cyclins and CDKs in them?

Cells that divide less frequently. Liver cells (once a year) or neurons (once a lifetime) - Have almost little to no

Why can damage to the brain or spinal cord be permanent?

Because neurons are very slow to divide. They do not fix those regions of the brain or spinal cord very quickly

How do cyclins and CDKs work? Give example of S phase

How do the proteins needed for S phase know when to start? The cell goes from G1 to S. While in G1, the cell gets ready for S phase. Many proteins needed for S will line up (ex: helicase). But these proteins are not on yet. Only when you get to S phase are they turned on. In order to turn these proteins on, needs to be phosphorylated. Phosphorylated by the Cyclin CDKs that have come together. Now the proteins can carry out S phase.

In order to go from G1 to S or S to G2, what needs to happen?

The proteins needed in the next phase need to be phosphorylated by cyclin-CDKs. This launches the next phase.

Are proteins needed, for example, in S phase, on anc active in G1 or G2?

NO. They are only turned on during S phase. This adds another layer of control for the cycle and ensures replication is not uncontrollable and rapid.

What and what process is needed for helicase to unwind DNA?

ATP through ATP hydrolysis.

Is ATP hydrolysis and Cyclin CDK the same thing?

No. Although they both work on the same proteins needed for each phase of the cell cycle, they both are needed. ATP provides a more long-lasting effect

What is the function of cyclin-CDKs?

Drive forward the cell cycle by helping the cell launch the next phase of the cell cycle

What does the cell need to have in order to move from G1 to S?

Need to have all of the proteins in the replisome turn on so the DNA can be replicated (ex. helicase, DNA polymerase, SSB, Primase). Must be phosphorylated to be turned on by the cyclin-CDK. And by turning the proteins on, this starts the next phase of the cell cycle because the cell has begun the actions of the next phase.

What is p53 and p21?

Proteins. p53 is the guardian of the genome

How do the cells respond to DNA damage using p53?

Damaged DNA acts as a ligand to bind to a surface receptor of a cell. This starts a signaling cascade. This cascade phosphorylates p53 to turn it on. p53 becomes a transcription factor and will bind to the regulatory region of the DNA and affect how we express our genes. The gene it will affect is p21. p53 will activate the transcription and translation of p21 by turning it on. p21 DNA to p21 mRNA to p21 protein. p21 will bind to cyclin-CDKs and inhibit their activity. Now the next phase of the cell cycle cannot be launched and the cycle will slow down. This is good because DNA was damaged and the cell does not want to make more of the damaged DNA

What can p53 act as once it is phosphorylated?

A transcription factor - controls how we express our genes by binding to DNA

p21 is a…

Cyclin CDK inhibitor

Why does p53 and p21 stop the cell cycle?

The DNA was damaged. Do not want to make more of the damaged DNA

Which gene is broken when an individual has cancer?

The p53 gene. When broken, it allows the cell to copy the damaged DNA instead of fixing it. About half the time an individual has cancer, the p53 gene is broken

Why is p53 so important?

It senses damage to the genome and inhibits the copying of the damaged genes by stopping the cell cycle from moving forward

What are two ways in which p53 or p21 can get damaged and be unable to fix damaged DNA?

p53 or p21 can have a mutation or you can lose any part of the cascade in which p53 and p21 are activated

Breaking p53 and p21 genes cause…

Cancer

Give an example of how the shape of proteins is important for regulating the cell cycle

Cyclins and CDKs must bind to each other and this causes a change in shape that activates the cyclin CDK. That shape change triggers the activation of the cyclin CDK. That activation will trigger the next phase of the cell cycle

In addition to proteins coming together to regulate the cell cycle, what is also key for regulating the cell cycle?

Post-translational modifications - Phosphorylation, methylation, acetylation. Can change shape and affect cell cycle

When the cyclin binds to the CDK, does that automatically mean the next phase can begin or is there more that comes to it?

No, you still need to have the rest of the shape working perfectly.

When it comes to proteins turning on and off, what two things play a role in doing so?

Quaternary regulation and post-translational modifications

What is an example of post-translational modifications turning proteins on?

Once a cyclin and CDK join together, the protein can still be turned on and off at that level. It can also have a phosphate group added by an inhibitory kinase. However, this phosphate group turns the protein off. Even though the cyclin and CDK have come together, because of this phosphate group, it is still off. We need an activating phosphatase to remove this phosphate group to turn on anc activate the cyclin CDK.

What does removing the inhibitory phosphate on an inactive cyclin-CDK via an activating phosphatase create?

A positive feedback loop because the Cyclin CDK is activated. This increases the activity of the pathway

When a cyclin CDK is turned on, what does it to do itself?

It creates a positive feedback loop on itself. It will take many phosphatases turn on them so they can remove the inhibitory phosphate off of the inactive cyclin-CDK. This becomes a loop of positive feedback where the phosphatases keep turning each other on and, as a result, we get a lot of active cyclin-CDKs