Lecture 28

Question 1

describe separase and the role it plays in the cell cycle

Answer 1

separase is a protease enzyme

separase cleaves cohesin, allowing sister chromatids to separate and become daughter chromosomes

Question 2

True or False: during the course of mitosis, there is only 1 set of PRE-RC’s synthesized, to ensure there are no duplicate strands of DNA created. explain your answer

Answer 2

False

NO NEW PRE-RC’s are made during mitosis

PRE-RC’s are made only once in the G1 phase

Question 3

describe cohesin and the role it plays in the cell cycle

Answer 3

cohesins are members of the SMC (structural maintenance of chromosomes) proteins

large cohesin protein complexes are what glue sister chromatids together along their length

(cohesin forms rings around sister chromatids “rungs of a ladder”)

Question 4

describe the role of securin

Answer 4

Securin protects cohesin by inhibiting separase

Question 5

explain the levels S-Cdk and M-Cdk during the S and M phase. What effect does this have on the amount of PRE-RC’s that are formed?

Answer 5

The levels of S-Cdk and M-Cdk are high during both the S and M phase of the cell cycle

This has no effect on the amount of new PRE-RCs formed during these phases because PRE-RCs are only formed during G1 phase

Question 6

describe the order of events involving APC/C, Cdc20, securin, separase and cohesin in sister chromatid separation.

Answer 6

APC/C, once activated by Cdc20, ubiquitinates securin

securin was bound to and inhibiting separase, however it is degraded, which activates separase

separase then cleaves cohesin, which allows sister chromatids to separate during anaphase

Question 7

List and describe the 4 regulating methods of cyclin-Cdk complexes (be sure to mention when the levels drop for the first 3 complexes)

Answer 7

G1/S cyclin-Cdk complexes: start the cell cycle in late G1 phase by commiting to the cycle
(levels drop in S phase)

S cyclin-Cdk complex: stimulates chromosome duplication
(these levels remain high until mitosis)

M-Cyclin-Cdk complex: stimulate the entry into mitosis at G2/M checkpoint
(these levels drop at mid mitosis)

4-G1 Cyclin-Cdk complex: governs the activity of G1/S cyclins

Question 8

explain the levels S-Cdk and M-Cdk during the S and M phase. What effect does this have on the amount of PRE-RC’s that are formed?

Answer 8

The levels of S-Cdk and M-Cdk are high during both the S and M phase of the cell cycle

This has no effect on the amount of new PRE-RCs formed during these phases because PRE-RCs are only formed during G1 phase

Question 9

when do APC/C levels rise?

Answer 9

mid mitosis

preparing to trigger anaphase

Question 10

What is the role of PRE-RC when it comes to ensuring that chromosomes only duplicate one time?

Answer 10

PRE-RC: (prereplicative complex) is a CRITICAL structure for controlling mitosis

at G1 phase in the cell cycle, PRE-RC’s assemble are origins of replication and ensure that there is only 1 copy of DNA synthesized

Question 11

State the stages and names of the cell cycle in their chronological order

Answer 11

S Phase: DNA synthesis occurs here

G2 phase: growth phase that occurs between S and M phase

M phase: chromosomes are separated from one another and pulled to opposite ends of the cell as cytokinesis occurs, yielding 2 identical daughter cells

G1 phase: growth phase that occurs between M and S phase

(GAP Phases include G1 and G2 (the 2 other phases)

Question 12

state and describe the checkpoints of the cell cycle

Answer 12

Checkpoint 1: known as the “start checkpoint” or “restriction point” that occurs at the end of the G1 phase

Basically evaluates the environment (nutrient levels) and decides if the cell will enter into the cell cycle (beginning with S phase) or not

Checkpoint 2: occurs between G2 and M phase

evaluates the quality of DNA replication (wants 1 perfect round of DNA replication)

Checkpoint 3: occurs at the metaphase-anaphase transition and triggers the separation of sister chromatids

evaluates if all chromosomes are properly attached to the spindle before anaphase pulls them to opposite ends of the cell

Question 13

Describe the role of CAK’s in terms of it’s control mechanism of the cell cycle.

Answer 13

the phosphorylation of Cdk is caused by CAK (Cdk Activating Kinases)

Before a cyclin and Cdk come together (in order to activate the Cdk) the T loop of the Cdk is blocking the active site of the Cdk

the binding of cyclin to the Cdk moves the T loop out of the active site, and into a “cave site”

it is in this cave site that CAK phosphorylates, specifically at the T loop, the Cdk in the “cave site” and fully activates the Cdk

Question 14

Describe the role of SCF in terms of it’s control mechanism of the cell cycle. Include what is necessary for SCF activity to occur.

Answer 14

SCF- ubiquitin ligase is used to add ubiquitin to phosphorylated CKI’s, to target them for destruction

SCF protein MUST have the F-box subunit (helps SCF recognize target proteins) or else it will be inactive

SCF adds ubiquitin to CKI proteins in G1, which inadvertently restores the activity of S-Cdk (starts DNA replication)

Question 15

Describe the role of CKI’s in terms of it’s control mechanism of the cell cycle. State a general example of a CKI

Answer 15

CKI’s (Cdk Inhibitory proteins) cause cyclin-Cdk complexes to be inhibited by binding to both of them

P27 is a CKI that binds to and inactivates cyclin and Cdk

Question 16

When ubiquitination is used to destroy proteins as a control mechanism for the cell-cycle, what is another term for this process?

Answer 16

proteolysis (just the act of tagging and destroying proteins)

Question 17

Describe the role of ubiquitination in terms of it’s control mechanism of the cell cycle.

Answer 17

Proteolysis of CKI’s can turn on S-Cdk’s

SCF-ubiquitin ligase adds ubiquitin to CKI’s so they are destroyed.

once the CKI’s are degraded, S-Cdk’s stimulate DNA replication (entry into the cell cycle)

(any other protein that needs degraded can be ubiquitinated but doesn’t necessarily regulate the cell cycle)

Question 18

Describe the role of Wee1 Kinase in terms of it’s control mechanism of the cell cycle.

Answer 18

Wee1 kinase inhibits M-Cdk activity by phosphorylating the “roof site” of it, while it is bound to cyclin in a Cdk-cyclin complex

With the M-Cdk-cyclin complex inhibited, the cell cycle cannot progress

(part of the Wee1 kinase and Cdc25 phosphatase activation and inactivation cycle)

Question 19

True or False: during the course of mitosis, there is only 1 set of PRE-RC’s synthesized, to ensure there are no duplicate strands of DNA created. explain your answer

Answer 19

False

NO NEW PRE-RC’s are made during mitosis

PRE-RC’s are made only once in the G1 phase

Question 20

Describe the role of cdc25 phosphatase in terms of it’s control mechanism of the cell cycle.

Answer 20

Cdc25 phoshatase removes inhibitory phosphates from M-Cdk

M-Cdk is a kinase that directly triggers/controls mitosis

(part of the Wee1 kinase and Cdc25 phosphatase activation and inactivation cycle)

Question 21

Describe APC/C and the role it plays in the cell cycle

Answer 21

APC/C is a member of the ubiquitin ligase family of enzymes

APC/c catalyzes the addition of ubiquitin to proteins so that they will be degraded

In the cell cycle, APC/C catalyzes the ubiquitination of securin, which binding to and inactivating separase

after securing is ubiquitinated, it is destroyed and separase is activated (destroys cohesin)

Question 22

Describe Cdk’s and cyclins. Be sure to mention how they are related and the levels at which they occur in the cell.

Answer 22

Cdk’s (Cycling dependent kinases) main function is to phosphorylate proteins downstream to activate them and regulate cell cycle events.

The quantity of Cdk’s does not change, however the ACTIVITIES of Cdk’s rises and falls throughout the cell cycle (the cycling of their activity is dependent upon the presents/absence of cyclins)

Cyclins: proteins that regulate Cdk’s and serve to activate Cdk’s (without cyclins present, Cdk’s are inactivated)

The quantity levels of cyclins changes though out the cell cycle (this is where they get their name)

Question 23

State the 2 steps in DNA replication that ensure that DNA is only copied once per cell cycle. include what phase of the cell cycle both of these steps occur in.

Answer 23

prereplicative complexes assemble at the origins of replication during G1 phase

Replication forks are created at the sites of DNA replication during S phase

Question 24

What is INK4A and what phase of the cell cycle is it associated with? if it mutated to the point that is loses its normal function, what condition occurs?

Answer 24

INK4A is a CKI associated with the G1 phase of the cell cycle

Hereditary melanoma occurs if INK4A has a loss of function mutation