Cell Cycle

Question 1

4 stages of the cell cycle

Answer 1

G1: growth prior to DNA synthesis
S: DNA replication (leaves each chromosome with identical sister chromatids linked at centromere)
G2: second growth period preceding cell division
M: mitosis

Question 2

G0

Answer 2

Cells that have differentiated and are no longer dividing have exited the cell cycle and are said to be at this stage (way to remember: G0 = G “not” growing!)

Question 3

Stages of interphase

Answer 3

G1, S, and G2 (time between each mitosis)

Question 4

Cell cycle checkpoints

Answer 4

G1/S: start/restriction point where cell determines if conditions are favorable
G2/M: all DNA has been replicated and still good to divide
Metaphase/anaphase transition point: all chromosomes attached to mitotic spindle

Question 5

DNA damage checkpoint

Answer 5

Cells with damaged DNA arrest until damage can be repaired or enters apoptosis if too severe

Question 6

Cyclin-dependent kinases (CDK)

Answer 6

Serine/threonine kinases that function in association with cyclins (regulatory subunits); different CDK/cyclin complexes are active at different stages of cell cycle due to presence or absence

Question 7

CDK/cyclin complexes that are most important at M-phase

Answer 7

Cyclin A/CDK1 and cyclin B/CDK1

Question 8

CDK/cyclin complexes that are most important at mid-G1 phase

Answer 8

Cyclin D/CDK4 and cyclin D/CDK6

Question 9

CDK/cyclin complexes that are most important at late G1 phase

Answer 9

Cyclin E/CDK2

Question 10

CDK/cyclin complexes that are most important at S phase

Answer 10

Cyclin A/CDK2

Question 11

How are cyclins degraded?

Answer 11

By ubiquitin-dependent proteolysis after function has been discharged

Question 12

Mitogenic signals

Answer 12

Tissue-specific growth factors that are typically required to stimulate cell growth and division

Question 13

Contact inhibition

Answer 13

Inhibitory effect of interactions between cell and extracellular matrix of neighboring cells that promotes cell division; loss of inhibition leads to cancer

Question 14

How do growth factors stimulate cell division?

Answer 14

Growth factors are added to cells to stimulate transcription of early-response genes, such as transcription factor c-Fos (also c-Jun and c-Myc); altogether these factors stimulate transcription of delayed-response genes

Question 15

Role of tyrosine kinase receptor in cell division stimulation

Answer 15

Receptor for growth factors; ligand binds to trigger dimerization and autophosphorylation of the tyrosine residues in the receptor that form docking sites where signaling molecules can bind

Question 16

Growth factor signaling/cell division process

Answer 16

1. GRB2 binds to phosphotyrosine residues in activated receptor
2. GRB2 also binds Sos protein to bring it to cell membrane
3. Sos meets Ras (small G-protein) and activates Ras
4. Ras activates Raf
5. Raf activates MAP kinase kinase
6. MAP kinase kinase activates MAP kinase which enters nucleus and regulates transcription
7. Early-response genes are transcribed (including c-Fos, c-Jun and c-Myc)
8. c-Fos + other transcription factors stimulate transcription of delayed-response genes which encode additional transcription factors and certain cyclins

Question 17

Early-response genes

Answer 17

Transcription factors c-Fos, c-Jun, and c-Myc

Question 18

Delayed-response genes

Answer 18

Additional transcription factors, mid-G1 cyclins and CDKs (D-type cyclins, CDK4, and CDK6) and late-G1 cyclins (cyclin E/CDK2)

Question 19

Main point of growth factor signaling

Answer 19

Activation of MAP kinase stimulates c-Fos transcription, leading to progression through restriction point

Question 20

Retinoblastoma protein (Rb)

Answer 20

Key target for cyclin D-CDK4/6 complex; binds to histone deacetylase and methylase to promote chromosome condensation and transcription inhibition; also binds protein products of delayed-response genes (E2F proteins) causing:

• -Repression of transcription of genes for DNA replication
• -Repression of transcription of late G1 cyclin (cyclin E), S-phase cyclin (cyclin A), S-phase CDK (CDK2), and E2F’s own genes

Question 21

Why is phosphorylation of Rb so important?

Answer 21

1. Phosphorylation by cyclin D-CDK4/6 causes dissociation of Rb from E2F proteins to activate transcription of genes to enter into S phase of cell cycle
2. Cyclin E-CDK2 is produced as result of E2F action that phosphorylates Rb (so when cyclin E-CDK2 is present, Rb phosphorylation becomes independent of cyclin D-CDK4/6 and does not need growth factors to progress through cell cycle)

Question 22

What kinases phosphorylate Rb?

Answer 22

Cyclin D-CDK4/6 and cyclin E-CDK2

Question 23

Describe how protein phosphorylation and p27KIP1 act to inhibit cyclin A-CDK2 and prevent entry into S phase.

Answer 23

Cyclin A-CDK2 is initially found in inhibited state due to complex it forms wih p27KIP1

Question 24

How is inhibition of cyclin A-CDK2 relieved?

Answer 24

1. Phosphorylation of p27KIP1 late in G1 by cyclin E-CDK2 complex triggers polyubiquitination and proteolysis of inhibitor
2. Additional regulation is done by a specific phosphatase that removes inhibitory phosphate from CDK2
3. DNA replication can begin after inhibitors are degraded

Question 25

What is the most important driver for events leading to mitosis?

Answer 25

CDK1 with cyclins A and B

Question 26

How is entry into mitosis coupled to completion of DNA replication?

Answer 26

Dephosphorylation of inhibitory phosphorylation of CDK1 subunits results in activation of cyclin/CDK1 complexes that phosphorylate many protein substrates such as histone H1, chromatin-associated proteins, nuclear lamins, microtubule-associated proteins, and proteins of inner nuclear membrane that drive mitosis (especially chromosome condensation, formation of mitotic spindle, and disassembly of nuclear membrane)

Question 27

Anaphase promoting complex

Answer 27

Participates in chain of events leading to degradation of cohesin complexes holding sister chromatids together at centromere to move them towards opposite ends of cell; polyubiquitinates cyclin A and B so they are destroyed by proteasome

Question 28

Would a normal cell divide if chromosomes were not correctly attached to mitotic spindle?

Answer 28

No, because certain protein factors recognize kinetochores that are not associated with spindle and initiate inhibition of anaphase promoting complex

Question 29

How is G2/M checkpoint regulated?

Answer 29

ATM detects replication forks and is active as long as replication forks are present; ATM activates “checkpoint” kinase that prevents dephosphorylation and activation of cyclin/CDK1 complexes needed to enter mitosis

Question 30

How is ATM activated?

Answer 30

In response to double-stranded DNA breaks (such as those caused by irradiation)

Question 31

How is ATR activated?

Answer 31

By DNA damage caused by UV light and certain drugs

Question 32

2 ways ATM/ATR kinases can prevent cell cycle in face of DNA damage

Answer 32

1. Activation of both of these kinases inhibits phosphatases that normally dephosphorylate and activate cyclin/CDK2 and cyclin/CDK1 complexes so that they are blocked
2. Together they phosphorylate p53, which stabilizes it to affect transcription of many different genes, targeting p21CIP1 which inhibits CDKs (BOTH cyclin/CDK1 and cyclin/CDK2) and stops transition of cell cycle

Question 33

Ataxia telangiectasia

Answer 33

Rare inherited disorder caused by mutations in gene encoding ATM so that response to double-stranded breaks of DNA is impaired; causes progressive difficulty in coordination, small and widened blood vessels, increased susceptibility to infection (especially in lungs), and increased risk of leukemia and lymphoma

Question 34

Apoptosis

Answer 34

Programmed cell death that is important for:

• -Removing cells that are damaged or infected
• -Normal cell turnover, embryonic development, and immune system function
• -The way cytotoxic chemicals exert their effects

Question 35

Steps of apoptosis

Answer 35

1. Chromosome condensation occurs
2. Cell shrinks in size due to cytoplasmic condensation
3. Nuclear envelope breaks down and nucleus fragments
4. Cell fragmentation occurs, forming apoptotic bodies that are phagocytosed by surrounding cells

Question 36

Intrinsic pathway of apoptosis

Answer 36

Can be initiated by internal signals such as p53

Question 37

Extrinsic pathway of apoptosis

Answer 37

Receptors known as death receptors bind external signals to trigger apoptotic process (ex. death signals such as tumor necrosis factor, TNF, and Fas ligand)

Question 38

What enzymatic activity do caspases possess?

Answer 38

Initiator caspases are activated by apoptotic signal and activate executioner caspases that hydrolyze cellular proteins (ex. components of nuclear lamina and cytoskeleton) and trigger DNA fragmentation

Question 39

Describe how activation of p53 leads to apoptosis.

Answer 39

1. Activation of p53 leads to induction of proapoptotic proteins PUMA, BID, and BAX (Bcl-2 family)
2. PUMA and BID stimulate activity of BAX, which allows release of cytochrome c from mitochondria into cytoplasm
3. When cytochrome c enters cytoplasm, binds Apaf-1, triggering its oligomerization and leading to formation of apoptosome
4. Apoptosome recruits and activates initiator caspase-9 activating executioner caspase-3 to cause apoptosis