Regulation of CDK Activity and the Transitions Between the Stages of the Cell Cycle

Question 1

How are CDKs mainly regulated?

Answer 1

By activating and inhibiting phosphorylation.

Question 2

Phosphorylation of a threonine residue near the active site of the CDks is required for enzyme activity. What is this phosphorylation mediated by?

Answer 2

CDK-activing kinase.

Question 3

CAK activity is _____ throughout the cell cycle and phosphorylates the CDK as soon as ___________.

Answer 3

1. Constant
2. A cyclin-CDK complex is formed.

Question 4

What is Cdc25? What does it do?

Answer 4

It is a phosphatase that removes inhibitory phosphorylation of Y (Tyrosine) and T (Threonine) residues in the ATP binding site of the CDKs.

Question 5

The dephosphorylation of Y and T residues in the ATP binding site does what?

Answer 5

Activates G1/S and Mitoic CDKs

Question 6

Which cell cycle transitions does the dephospho rylation of CDKs by Cdc25 regulate? There are 2**

Answer 6

1. G1 –> S
2. G2 –> M

Question 7

Describe the G1/S degradation of Cyclins and CDK inhibitors.

Answer 7

1. G1/S phase CDKs activate the expression of S phase cyclin CDKs components.
2. G1/S phase CDKs phosphorylate S phase inhibitor.
3. SCF-proteasome degrades phosphorylated S phase CDK inhibitors.

Question 8

Describe the degradation of cyclins and other proteins during mitosis, specifically the metaphase/anaphase transition.

Answer 8

APC/C^Cdc20 proteasome degrees securin.

Question 9

Describe the mitotic exit associated with degradation of cyclins and other proteins during mitosis.

Answer 9

Phosphatases activate Cdh1 and APC/C^Cdh1 proteasome degrades mitotic cyclins

Question 10

In the G1/S phase transition, the commitment to divide is taken late I the G1 phase: Restriction or start point. Cells that never divide such as highly differentiated cells arrest before this point where?

Answer 10

In the G0 phase.

Question 11

The control of G1/S transitions in mammals involves Rb, E2F, Cyclin E, Cyclin A, etc. Give a description of the process involved in control of the G1/S phase in mammals.

Answer 11

1. In the early G1 phase, the genes required for S-phase are suppressed by RB binding to the transcription factor E2F.
2. Growth factors and signal transduction stimulate cyclin D expression –> cyclin D/Cdk4 and CycD/CDK6
3. E2F stimulates transcription of cyclin E, cyclin A, CDK2
4. Cyclin E-CDK2 further phosphorylates RB resulting in the commitment to pass the restriction point and you will notice a rapid rise in E2F and cyclin E/CDK2

Question 12

Describe the loading of ORC and MCM-helicase based on the initiation of DNA replication.

Answer 12

• Degration of mitotic cyclins
• Origin recognition complex (ORC) binds to the origins
• The loaders Cdc6 and Cdt1 load INACTIVE MCM-helicase to ORC
• The pre-replication complex (pre-RC) is formed

Question 13

Describe initiation of DNA replication based on the CDKs and kinases.

Answer 13

1. S-phase cyclins are synthesized
2. Inhibitors of CDK are degraded
3. Two kinases (DDK and S phase CDK) phosphorylate
4. Cdc6 and Cdt1 –> degration (no loading of MCM after firing of the origin)
5. MCM–> Activation of helicase fires the origin DNA replication
6. CDC45, Sld2 –> Recruit GINS to MCM to facilitate elongation

Question 14

During the S phase of DNA replication, DNA ________ are recruited to the forks and elongation commences (semi-conservative DNA replication, leading and lagging strands, Okazaki fragments, etc).

Answer 14

Polymerases

Question 15

Linkage of replicated sister chromatids is facilitated by _______. This is structured as a dimer of SMC proteins (Smc1/Smc3).

Answer 15

Cohesin

Question 16

Describe the mechanism in which cohesin is used for linkage of sister chromatids.

Answer 16

• Cohesins associate with replicated chromatid in late G1
• During DNA synthesis Smc3 is acetylated, leading to encirclement of the replicated sister chromatids by a cohesin ring

Question 17

The Cohesin ring in the S phase of the cell cycle is ______ in mitosis.

Answer 17

Degraded

Question 18

In mitosis, which two kinases are activated by CDK?

Answer 18

Polo and Aurora

Question 19

Polo and Aurora phosphorylate cohesions leading to their ______________.

Answer 19

Dissociation and degradation by APC.

Question 20

What are the purposes of checkpoints in the cell cycle?

Answer 20

• Ensure the next stage of the cell cycle does not initiate prior to the completion of the proceeding one.
• Checkpoints are always comprised of event sensors, a signalling pathway, and an effector that halts cell cycle progression and activates repair on demand.
• Arrest the cell cycle in response to DNA damage or improper spindle assembly.

Question 21

Describe the composition of checkpoints in the cell cycle.

Answer 21

• Event sensors
• Signalling pathway
• Effector

Question 22

Describe the G1/S checkpoint of cells in general.

Answer 22

• G1/S transition requires nutrient growth and factor signalling.
• The cells make sure they have enough proteins and energy to go through the cell cycle.
• The G1/S point is inactivated by DNA damage.

Question 23

Describe the G1/S checkpoint of budding yeast cells.

Answer 23

• Nutrients stimulate the synthesis of Cln3
• TOR regulates the activity of ribosome and the synthesis of rRNA (makes sure that the cell can synthesize all proteins necessary for cell progression).

Question 24

Describe the G1/S checkpoint in mammalian cells.

Answer 24

• Growth factor signalling stimulates the synthesis of Cyclin D > pRb
• TOR regulates the activity of CDKs and the synthesis of various genes