Exam #4 Cell Cycle

Question 1

Four Stages of the Cell Cycle

Answer 1

• M= Mitosis, when the cell dividies
• G1= Growth period the precedes DNA synthesis
• S= DNA synthesis/replication to form two sister chromatids attached at centromere
• G2= Second period of growth the precedes cell division

Question 2

Stages of the Cell Cycle that Comprise Interphase

Answer 2

Interphase is the time between cell divisions

• G1
• S
• G2

Question 3

Cyclin-CDK Complexes of M-Phase

Answer 3

• Cyclin A/CDK1

- Cyclin B/CDK1

Question 4

Cyclin-CDK Complexes of Mid G1 Phase

Answer 4

• Cyclin D/CDK 4

- Cyclin D/CDK6

Question 5

Cyclin-CDK Complexes of Late G1 Phase

Answer 5

Cyclin E/CDK2

Question 6

Cyclin-CDK Complexes of S Phase

Answer 6

Cyclin A/CDK2

Question 7

Contact Inhibition

Answer 7

• Cell - Cell contact usually inhibits cell division
• This “contact inhibition” is lost in cancer
• Cell - ECM contact does the opposite

Question 8

Growth Factors

Answer 8

• Molecules that stimulate cell growth/division

- PDGF, EGF, nerve growth factor, and insulin-like growth factor 1 are examples

Question 9

Growth Factor Receptors

Answer 9

• Intrinsic Tyrosine Kinase
• Ligand (growth factor) binds to receptor & triggers receptor dimerization & autophosphorylation or tyrosine residues within the receptor
• Phosphotyrosine residues are docking sites for signaling molecules

Question 10

GRB2, Sos, & Ras

Answer 10

• GRB2= adapter protein that binds phosphotyrosine residues
• Sos= Guanine Nucleotide Exchange Factor, activates Ras
• Ras= G-Protein that is activated by SOS

Question 11

Raf, MAP kinase kinase, MAP kinase

Answer 11

• Ras activates the protein kinase Raf
• Raf phosphorylates & activates MAP Kinase Kinase (MEK)
• MAP Kinase Kinase phosphorylates & activates MAP Kinase
• MAP Kinase translocated to the nucleus and stimulates c-Fos transcription
• End result= progression through the restriction point

Question 12

Early Response Genes

Answer 12

• Growth factor added to cell in G0 results in a burst of transcription
• Genes transcribed in this initial phase are “early response genes”
• Includes c-Fos, c-Jun, and c-Myc

Question 13

Delayed Response Genes

Answer 13

• Genes transcribed in response to c-Fos

- Encode more transcription factors & Mid/Late G1 Clycin-CDKs

Question 14

Why is phosphorylation of Rb important?

Answer 14

It stimulates progression through the restriction point and renders the cell cycle growth factor independent

Question 15

What kinases phosphorylate Rb?

Answer 15

• CDK 4 & CDK 6 (Cyclin D regulated) initially

- CDK 2 (Cyclin E regulated) secondarily

Question 16

How is cyclin A-CDK2 inhibited?

Answer 16

p27-KIP1

Question 17

What does cyclin A-CDK2 inhibition prevent?

Answer 17

DNA Replication

Question 18

How is cyclin A-CDK2 inhibition relieved?

Answer 18

Cyclin E/CDK 2 (late G1) phosphorylation of p27-KIP1

Question 19

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

Answer 19

• Cyclin A & B/CDK1 are required for entry into mitosis
• Both phosphorylated and inactive until replication is complete
• Once replication is completed, Phosphatase Cdc25C removes inhibitory phosphate

Question 20

Why would normal cells not divide if chromosomes were not correctly attached to the mitotic spindle?

Answer 20

• Anaphase is delayed until chromosomes attach to the spindle
• Attachment activates the anaphase promoting complex (APC)
• APC allows ubiquitination of cohesin that holds sister chromatids together at the centromere
• Sister chromatids then move apart

Question 21

ATM

Answer 21

• Senses replication forks i.e. ongoing replication
• ATM prevents dephosphorylation & activation of Cyclin A/B- CDK1
• ATM is active as long as replication forks are present; thus entry into M-phase is dependent on completion of replication

Question 22

Checkpoint Kinase

Answer 22

• ATM activated

- Blocks the action of Phosphatase Cdc25C i.e. blocks activation of Cyclin A/B-CDK1 when replication is ongoing

Question 23

p53

Answer 23

• Normally unstable transcription factor
• Stabilized by ATM/ATR of DNA damage checkpoint
• Upregulates p21-CIP1

Question 24

p21-CIP1

Answer 24

• Inhibitor of Cyclin/CDK 1 & 2

- Blocks entry into S & M phase

Question 25

Define Apoptosis

Answer 25

programmed cell death

Question 26

Outline Apoptosis

Answer 26

1) Chromosome & Cytoplasmic condensation 2) Nuclear Fragmentation 3) Cell Fragmentation & Blebbing 4) Apoptotic Bodies formed 5) Phagocytosis

Question 27

Caspases

Answer 27

- Caspase 9= Initiator Caspase | - Caspase 3= Executioner Caspase

Question 28

How does p53 activation lead to an efflux of cytochrome c from the mitochondria?

Answer 28

- One of the targets of p53 is the Bcl-2 family of proteins, including PUMA, BID, & BAX - BAX activation forms a channel in the mitochondira - Cytochrome C comes out

Question 29

G0

Answer 29

Cells that have differentiated and are no longer dividing exit the cell cycle and are in "G0"

Question 30

List the checkpoints in the cell cycle

Answer 30

- G1/S - G2/M - Metaphase/ Anaphase - DNA damage checkpoint

Question 31

G1/S Checkpoint

Answer 31

- Start/ Restriction point | - Cell determines if conditions are favorable for division

Question 32

G2/M

Answer 32

Ensures all the DNA has been replicated

Question 33

Metaphase/Anaphase

Answer 33

Chromosomes are evaluated to ensure they are attached to the mitotic spindle

Question 34

DNA Damage Checkpoint

Answer 34

- Occurs continuously throughout the cell cycle - Cells with damaged DNA arrest and repair - IF the cell cannot repair, undergoes apoptosis

Question 35

Cyclin-CDK

Answer 35

``` Cyclin= Regulatory subunit CDK= Serine/Threonine Kinase ```

Question 36

What mechanism degrades cyclins so they are only present at a specific point in the cell cycle?

Answer 36

Ubiquitin proteolysis

Question 37

How is CDK activity regulated?

Answer 37

Phosphorylation, Dephosphorylation & CDK Inhibitors

Question 38

c-Fos

Answer 38

Transcription factor that stimulates the transcription of the delayed response genes

Question 39

INK4

Answer 39

- Growth Inhibitory Factors - Compete with Cyclin D for binding to CDK 4/6 - Inhibit CDK 4/6

Question 40

DNA Damage Checkpoint

Answer 40

- Requires ATM & ATR Kinases - ATM senses ds-DNA breaks - ATR senses UV damage & certain drug damage - Block Phosphatase Cdc25 A & C, and phosphorylate p53

Question 41

Ataxia Telangiectasia

Answer 41

- Rare inherited disorder w/ effects on nervous & immune system - Ataxia= progressive balance problem - Telangiectasia= small widened blood vessels in skin - Mutation of ATM impairs cell response to ds-DNA break - Highly sensitive to radiation exposure