Cell Cycle

Question 1

Describe G1 phase.

Answer 1

Metabolically active

Largest segment of the cell cycle

Exception: early cell divisions of embryos proceed without growth phase

Question 2

Describe G2 phase.

Answer 2

Preparation for mitosis - makes required enzymes and proteins

4 hours

Question 3

What is minichromosome maintenance helicase? (MCM)

Answer 3

Enzyme that binds to replication origins which prepares the ORC for initiating replication

Question 4

How is the re-initiation of replication prevented?

Answer 4

Kinase activity during late G1/S

Question 5

What is the restriction point?

Answer 5

The point the cell is committed to dividing.

Located in late G1 phase

Question 6

What is Mitotsis or Maturation Promoting Factor?

Answer 6

CDK1 and Cyclin B

Question 7

What is CDK Activating Kinase (CAK)?

Answer 7

Phosphorylates CDK1 and activates it when its bound to Cyclin B

Question 8

What is Wee1 kinase?

Answer 8

Adds an inhibitory phosphate to CDK1 and allows CB/CDK1 complex to increase in the cell

Question 9

What Cyclin/Cdks are active during early G1 through R?

Answer 9

Cdk4,6

Cyclin D

Question 10

What Cyclin/Cdks are active during the G1 to S transition?

Answer 10

Cdk2

Cyclin E

Question 11

What Cyclin/Cdks are active during G2 to M transition

Answer 11

Cdk1

Cyclin B

Question 12

What are four mechanisms of Cdk regulation?

Answer 12

Association of Cdks with cyclins - cyclin synthesis and degredation

Phosphorylation of Cdk - CAK and Wee1

Dephosphorylation - Cdc25 phosphatase

Association with Cdk inhibitors

Question 13

What are the Ink 4 family of Cdk inhibitors?

Answer 13

Inhibitors that binds Cdk4,6 and inhibits progression through G1

Question 14

What are the Cip/Kip family of Cdk inhibitors?

Answer 14

p21, p27, p57

Binds Cdk2/Cyclin A or E and inhibits progress through G1 and S

Question 15

What external signal initiates the cell cycle?

Answer 15

Growth factor binding initiates a signaling cascade leading to synthesis of Cyclin D.

If growth factor signaling continues until the cell passes R, then the cell cycle will be completed

Question 16

What is Retinoblastoma protein? (Rb)

Answer 16

Key regulator of transcription of genes required for cell cycle progression and DNA synthesis

Question 17

What is the mechanism of Rb protein?

Answer 17

Not phosphorylated - binds E2F and represses transcription

Phosphorylation by Cdk4,6/CD - dissociates from E2F and transcription is active

Question 18

How does the cell pass the R point?

Answer 18

Prior to R, Cdk2/Cyclin E is inhibited by p27

As G1 progresses, more CE is made and p27 is releived

p27 synthesis decreases, p27 binds Cdk4,5/CD, p27 is degraded

Question 19

What are checkpoints?

Answer 19

Points in each phase that coordinate events to ensure that the next phase does not begin before that phase is completed

Question 20

What are the two types of checkpoints?

Answer 20

DNA Damage checkpoints - protect cell from replication errors

Spindle Assembly Checkpoint - ensures chromosomes are aligned properly

Question 21

Where are the DNA damage checkpoints?

Answer 21

G1, S, G2

Question 22

Where is the Spindle Assembly checkpoint?

Answer 22

M phase - monitors alignment of chromosomes on the metaphase plate

Question 23

What are ATM and ATR?

Answer 23

Kinases that are part of damage recognition complexes

Phosphorylate downstream kinases Chk1 and Chk2 when activated by DNA damage

Question 24

What is the role of Chk1 and Chk2?

Answer 24

phosphorylate Cdc25, preventing activation of Cdk1 and Cdk2

Question 25

What is p53?

Answer 25

A transcription factor that regulates the transcription of p21

Phosphorylated by ATM and Chk2

Increases levels of p21 which inhibits Cdk2/CE

Frequently mutated in cancers