9.12.16 Lecture

Question 1

What are the phases of the cell cycle?

Answer 1

1. G0 (quescent)
2. G1 (gap between M and S)
3. S (DNA replication)
4. G2 (gap between S and M)
5. M (mitosis - nuclear division and cytokinesis - cytoplasmic division)

Question 2

Approximately how long is interphase? M-phase?

Answer 2

22-24 hours; 1 hour

Question 3

What happens during G1?

Answer 3

Nutrients and cytoplasmic organelles are produced to increase the cell volume.

Question 4

Before G1 ends, what question must be answered?

Answer 4

Is the environment favorable?

Question 5

What happens during S?

Answer 5

DNA is replicated and chromosomes are duplicated

Question 6

What happens during G2?

Answer 6

Cell volume increases

Question 7

Before G2 ends, what questions must be answered?

Answer 7

Is DNA properly replicated?

Is the environment favorable?

Question 8

What happens during M?

Answer 8

Metaphase, Anaphase, Telophase, Cytokinesis

Question 9

Before metaphase ends, what question must be answered?

Answer 9

Are chromosomes attached to spindles?

Question 10

The cell cycle ___ triggers essential processes of the cycle.

Answer 10

Control system

Question 11

What are the two key components of the cell-cycle control system and how do they interact?

Answer 11

Cyclin (regulatory protein) and cyclin-dependent kinase (CDK); cyclin binds CDK to activate it

Question 12

Levels of ___ regulate the cell cycle; ___ levels stay constant.

Answer 12

Cyclin; CDK

Question 13

There are specific cyclins and CDKs for each phase of the cell cycle - what are the phases?

Answer 13

G1, G1/S, S, M

Question 14

Describe the level of G1/S-cyclin across the course of the cell cycle, beginning with G1.

Answer 14

Begins to increase in G1, peaks during G1, begins to decreases early in S, remains low until the cycle repeats

Question 15

Describe the level of S-cyclin across the course of the cell cycle, beginning with G1.

Answer 15

Begins to increase in the transition from G1 to S, peaks during S, remains high until it decreases shortly before the metaphase/anaphase transition, at which point it remains low until the cycle repeats.

Question 16

Describe the level of M-cyclin across the course of the cell cycle, beginning with G1.

Answer 16

Begins to increase in G2, peaks during M, begins to decrease at the metaphase/anaphase transition, remains low until the cycle repeats.

Question 17

Describe the process of activating/inactivating CDKs.

Answer 17

CDK and cyclin bind to form an inactive dimer. CAK (CDK-activating kinase) phosphorylates CDK to activate it. Wee 1 kinase phosphorylates it again to inactivate it. Cdc25 phosphatase dephosphorylates it to activate it.

Question 18

___ bind to both cyclin and CDK in a complex, distorting the active site. It also inserts into the ___-pocket, further inhibiting it.

Answer 18

CKIs (p27, p21, p16); ATP

Question 19

What is the result of CKIs binding to CDK and cyclin?

Answer 19

Cell cycle arrest in the G1/S-phase (typically leads to long-term inhibition, which leads to cell differentiation)

Question 20

Describe control of proteolysis by APC/C.

Answer 20

Inactive APC/C dimerizes with Cdc20 (activating subunit), forming activate APC/C. This ubiquitinates M-cyclin (using ubiquitin, E1, and E2). This leads to degradation of M-cyclin in the proteosome. This inactivates M-CDK. Cytokinesis can proceed.

Question 21

What are APC/C and SCF?

Answer 21

E3 ligases

Question 22

Describe control of proteolysis by SCF.

Answer 22

SCF binds F-box protein to form an activate SCF complex. CKI is phosphorylated. This is recognized by the SCF complex, which polyubiquitinates CKI (using ubiquitin, E1, and E2). CKI is degraded in the proteasome. This increases CDK/cycling activity. The cell cycle proceeds.

Question 23

What are the protein kinases and phosphatases that modify CDKs?

Answer 23

CAK (on), Wee1 (off), Cdc25 (on)

Question 24

What are the CKIs?

Answer 24

p27, p21, p16 (all off)

Question 25

What are the ubiquitin ligases and their activators?

Answer 25

APC/C, Cdc20, Cdh1, SCF (all on)

Question 26

Describe control of chromosome duplication.

Answer 26

1. In G1, prereplicative complexes (DNA helicases) assemble at replication origins. S-CDK is activated and phosphorylates proteins of complexes to initiate the replication fork. It also prevents multiple origin activations. Initiation begins.
2. The replication forks form. Elongation occurs. The entire chromosome is duplciated.
3. M-Cdk is activated, chromosomes segregate.
4. APC/C is activated, CDK is inactivated.
5. New Pre-RCs assemble at origins.

Question 27

Describe the activation of M-CDK, including its positive feedback.

Answer 27

CDK and M-cyclin bind to form inactivate M-CDK. CAK and Wee1 both phosphorylate to keep M-CDK inactive. Cdc25 dephosphorylates M-CDK to form active M-CDK. Active M-CDK helps activate Cdc25 and inactivate Wee1.

Question 28

A decrease in M-CDK in late mitosis leads to…

Answer 28

…an increase in Cdh1-APC/C. This ensures continued suppression of CDK activity after mitosis.

Question 29

Describe the process of mitogen stimulation of cell cycle entry.

Answer 29

Mitogen binds to a receptor (like an RTK). This activates Ras, which activates MAP kinase (Mitogen Activated Protein Kinase). This leads to activation of transcription regulatory proteins. This leads to immediate early gene expression, which includes creation of Myc. This leads to expression of other genes, leading to active G1-Gdk. This phosphorylates Rb. Rb releases a now-active E2F. This leads to S-phase gene transcription. This leads to G1/S-cyclin and S-cyclin, which leads to active S-CDK, which leads to DNA synthesis.

Question 30

What are mitogens?

Answer 30

Growth factors such as EGF, insulin, and HRG.

Question 31

Activation of p21 by ___ leads to inactivation of ___ complex.

Answer 31

p53; G1/S-CDK

Question 32

Describe the activation of p21 by p53.

Answer 32

DNA damage activates ATM/ATR kinase. This activates Chk1/Chk2 kinase. This phosphorylates p53, which dissociates from Mdm2. p53 accumulates, which stimulates transcription of the p21 gene. p21 is produced, which binds and inactivates G1/S-Cdk and S-Cdk. This leads to arrest of the cell in G1. This can either lead to apoptosis or repair of DNA and a decrease of p21 by the SCF-F-box complex.

Question 33

Describe the mechanism by which p53 arrests cells in G1/S.

Answer 33

DNA damage activates p53 which activates the waf1 gene. This encodes for p21 (CKI). This inhibits G1/S CDKs and G1/S cyclins from forming their complex. This blocks phosphorylation of Rb. Rb binds and inhibits EsF from activating gene transcription of essential S-phase genes. This blocks the G1/S transition.

Question 34

Describe the process of cell cycle arrest or apoptosis induced by excessive mitogenic stimulation.

Answer 34

Excessive production of Myc activates Arf. This binds to and inhibits Mdm2; note that Mdm2 is normally bound to p53, leading to p53 degradiation. This leads to an increase in p53 levels, which either leads to cell cycle arrest or apoptosis.