Cell Cycle Control

Question 1

How is the cell cycle accomplished?

Answer 1

By a network of regulatory proteins known as the cell cycle control system

Question 2

Functions of the cell cycle control system?

Answer 2

1) Ensures that each phase is completed before the next one is begun
2) Ensure correct progression of each phase

Question 3

What proteins are associated with the cell cycle control system?

Answer 3

cyclically activated kinase proteins

Question 4

Are the protein kinases always present?

Answer 4

Yes, however, they are only activated at appropriate times during the cell cycle, after which it is rapidly inactivated

Question 5

How is the action of protein kinases on proteins reversed?

Answer 5

By protein phosphotases

Question 6

Concentration & Activity of Kinases

Answer 6

Concentration is constant throughout cell cycle

Activity rises & falls with each phase

Question 7

How are cyclically activated protein kinases switched on & off?

Answer 7

By proteins called cyclins that need to bind to the kinases in order for them to be enzymatically switched

Question 8

What are cyclically activated protein kinases known as?

Answer 8

Cyclin-dependent protein kinases (Cdks)

Question 9

Concentration & Activity of Cyclins

Answer 9

Concentration of cyclins vary in a cyclic fashion during cell cycle

Question 10

When does cyclin B synthesis begin?

Answer 10

S phase

Question 11

What happens to cyclin B during S & G2?

Answer 11

Cyclin B accumulates and forms complexes with M phase Cdk

Question 12

What state is the complex in when cyclin B just associates with M Cdk in G2?

Answer 12

Enzymatically inactive

Question 13

As the Cyclin B-M Cdk complex forms, the complex is phosphorylated by:

Answer 13

1) An activating kinase, at the site that it is required for activity (addition of activating phosphate)
2) An inhibitory kinase, that overrides the activating kinase & inhibits activity (addition of inhibitory phosphate)

Question 14

What happens to the cyclin B-M Cdk complex at the end of interphase (in G2)?

Answer 14

It is activated by Cdc25 (a protein phosphatase) that removes the inhibitory phosphate group, leaving the activating phosphates

Question 15

Once the cyclin B-M Cdk complex is activated …

Answer 15

It phosphorylates & activates multiple target proteins, including Cdc25

Question 16

What is the result of cyclin B-M Cdk complex activating Cdc25?

Answer 16

Phosphorylating Cdc25 further activates the phosphatase activity, thus producing a + feedback loop, where active cyclin B-M complex further activates itself

Question 17

What happens toward the end of mitosis?

Answer 17

The M phase Cdk is inactivated by proteolytic degradation of cyclin B leading the cell to exit mitosis, undergo cytokinesis and return to G1 phase

Question 18

What happens toward the end of G1 phase?

Answer 18

M phase Cdk is dephosphorylated by a phosphatase -> it is now inactive and ready to form a complex with cyclin B

Question 19

Targets of cyclin B-M phase Cdk complex:

Answer 19

1) Chromatin condensation (condensins)
2) Mitotic spindle formation (MT associated proteins)
3) Nuclear envelope breakdown (lamins)

Question 20

Condensin

Answer 20

Activated during prophase, and participates in chromatin condensation leading to the visible M-phase chromosomes

Question 21

MT associated proteins

Answer 21

Activated during prophase and promotes their ability to cross-link the MTs growing from opposite spindle poles, leading to mitotic spindle formation

Question 22

Nuclear lamins

Answer 22

Activated during pro metaphase, and triggers the disassembly of the nuclear lamina, which causes nuclear envelope to fragment into vesicles

Question 23

List the 4 major cyclins in vertebrates

Answer 23

cyclin D
cyclin E
cyclin A
cyclin B

Question 24

List the 4 major Cyclin Cdk partners & their complex

Answer 24

cyclin D -> Cdk4, Cdk6 -> G1-Cdk
cyclin E -> Cdk2 -> G1/S-Cdk
cyclin A -> Cdk2 -> S-Cdk
cyclin B -> Cdk1 -> M-Cdk

Question 25

What is the function of protein kinases?

Answer 25

Phosphorylate & activate proteins involved in DNA replication, mitosis & cytokinesis