AH Biology - Unit 1 - KA5(c) - Control of the Cell Cycle

What is the progression of a cell through the cell cycle controlled by?

The progression of a cell through the cell cycle is controlled by checkpoints.

What are checkpoints?

Checkpoints are mechanisms in the cell that assess the condition of the cell during the cell cycle, and can halt progression to the next phase until certain requirements are met.

What are the three key checkpoints during the cell cycle?

The three key checkpoints during the cell cycle are:

• G1 checkpoint

• G2 checkpoint

• Metaphase checkpoint

What does the G1 checkpoint control, and what does it require?

The G1 checkpoint controls progression fron G1 to S. It requires sufficient phosphorylation of retinoblastoma (Rb) to progress.

As the cell grows during G1, what happens?

As the cell grows during G1, it begins to accumulate cyclin proteins. Cyclin proteins are involved in regulating the cell cycle. Cyclins combine with and activate cyclin-dependent kinases (CDKs). These active cyclin-CDK complexes phosphorylate proteins that regulate progression through the cell cycle. If sufficient phosphorylation occurs, progression takes place from G1 to S phase.

What is retinoblastoma?

Retinoblastoma (Rb) is a protein that acts as a tumour supressor protein by inhibiting the transcription of genes that code for proteins needed for DNA replication.

What happens when retinoblastoma is phosphorylated by the active G1 cyclin-CDK complex, and what does this allow for?

When retinoblastoma is phosphorylated by the active G1 cyclin-CDK complex, it inhibits the retinoblastoma protein and prevents it from binding to the DNA and inhibiting transcription. This allows for transcription of genes that code for proteins required for DNA replication, and cells then progress from G1 to S phase.

What is the process of G1?

The process of G1 phase is:

1. Cells grow

2. Cyclins accumulate

3. Cyclins bind with CDK to form cyclin-CDK complexes.

4. Active cyclin-CDK complexes phosporylate retinoblastoma.

5. This prevents retinoblastoma from inhibiting transcription of genes that code for DNA replication proteins.

6. DNA replication can take place.

7. The cell moves from G1 to S phase.

What is stage 1 of G1?

Stage 1 of G1 is “Cells grow”

What is stage 2 of G1?

Stage 2 of G1 is “Cyclins accumulate”

What is stage 3 of G1?

Stage 3 of G1 is “Cyclins bind with CDK to form cyclin-CDK complexes.”

What is stage 4 of G1?

Stage 4 of G1 is “Active cyclin-CDK complexes phosporylate retinoblastoma.”

What is stage 5 of G1?

Stage 5 of G1 is “This prevents retinoblastoma from inhibiting transcription of genes that code for DNA replication proteins.”

What is stage 6 of G1?

Stage 6 of G1 is “DNA replication can take place.”

What is stage 7 of G1?

Stage 7 of G1 is “The cell moves from G1 to S phase.”

What does the G2 checkpoint do, and what does it require?

The G2 checkpoint controls progression from G2 to M. It requires successful DNA replication. The G2 checkpoint assesses if DNA replication has been successful and if there is any DNA damage. DNA damage triggers the activation of many proteins, including the P53 protein. The P53 protein can:

• Stimulate DNA repair

• Arrest the cell cycle

• Cause cell death

What does the M checkpoint do, and what does it require?

The M checkpoint controls progression from metaphase to anaphase. It requires alignment of chromosomes on metaphase plate and attachment of spindle microtubules to centromeres.

What can an uncontrolled reduction in the rate of the cell cycle result in?

An uncontrolled reduction in the rate of the cell cycle may result in a degenerative disease.

What can an uncontrolled increase in the rate of the cell cycle result in, and what is this caused by?

An uncontrolled increase in the rate of the cell cycle may result in tumour formation. This can be caused by a mutation in proto-oncogens (a normal gene usually involved in the control of cell growth/division to form on a tumour-promoting oncogene.