Exam #4 Cell Cycle Flashcards
Four Stages of the Cell Cycle
- 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
Stages of the Cell Cycle that Comprise Interphase
Interphase is the time between cell divisions
- G1
- S
- G2
Cyclin-CDK Complexes of M-Phase
- Cyclin A/CDK1
- Cyclin B/CDK1
Cyclin-CDK Complexes of Mid G1 Phase
- Cyclin D/CDK 4
- Cyclin D/CDK6
Cyclin-CDK Complexes of Late G1 Phase
Cyclin E/CDK2
Cyclin-CDK Complexes of S Phase
Cyclin A/CDK2
Contact Inhibition
- Cell - Cell contact usually inhibits cell division
- This “contact inhibition” is lost in cancer
- Cell - ECM contact does the opposite
Growth Factors
- Molecules that stimulate cell growth/division
- PDGF, EGF, nerve growth factor, and insulin-like growth factor 1 are examples
Growth Factor Receptors
- 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
GRB2, Sos, & Ras
- GRB2= adapter protein that binds phosphotyrosine residues
- Sos= Guanine Nucleotide Exchange Factor, activates Ras
- Ras= G-Protein that is activated by SOS
Raf, MAP kinase kinase, MAP kinase
- 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
Early Response Genes
- 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
Delayed Response Genes
- Genes transcribed in response to c-Fos
- Encode more transcription factors & Mid/Late G1 Clycin-CDKs
Why is phosphorylation of Rb important?
It stimulates progression through the restriction point and renders the cell cycle growth factor independent
What kinases phosphorylate Rb?
- CDK 4 & CDK 6 (Cyclin D regulated) initially
- CDK 2 (Cyclin E regulated) secondarily
How is cyclin A-CDK2 inhibited?
p27-KIP1
What does cyclin A-CDK2 inhibition prevent?
DNA Replication
How is cyclin A-CDK2 inhibition relieved?
Cyclin E/CDK 2 (late G1) phosphorylation of p27-KIP1
How is entry into mitosis coupled to completion of DNA replication?
- 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
Why would normal cells not divide if chromosomes were not correctly attached to the mitotic spindle?
- 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
ATM
- 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
Checkpoint Kinase
- ATM activated
- Blocks the action of Phosphatase Cdc25C i.e. blocks activation of Cyclin A/B-CDK1 when replication is ongoing
p53
- Normally unstable transcription factor
- Stabilized by ATM/ATR of DNA damage checkpoint
- Upregulates p21-CIP1
p21-CIP1
- Inhibitor of Cyclin/CDK 1 & 2
- Blocks entry into S & M phase
Define Apoptosis
programmed cell death
Outline Apoptosis
1) Chromosome & Cytoplasmic condensation
2) Nuclear Fragmentation
3) Cell Fragmentation & Blebbing
4) Apoptotic Bodies formed
5) Phagocytosis
Caspases
- Caspase 9= Initiator Caspase
- Caspase 3= Executioner Caspase
How does p53 activation lead to an efflux of cytochrome c from the mitochondria?
- 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
G0
Cells that have differentiated and are no longer dividing exit the cell cycle and are in “G0”
List the checkpoints in the cell cycle
- G1/S
- G2/M
- Metaphase/ Anaphase
- DNA damage checkpoint
G1/S Checkpoint
- Start/ Restriction point
- Cell determines if conditions are favorable for division
G2/M
Ensures all the DNA has been replicated
Metaphase/Anaphase
Chromosomes are evaluated to ensure they are attached to the mitotic spindle
DNA Damage Checkpoint
- Occurs continuously throughout the cell cycle
- Cells with damaged DNA arrest and repair
- IF the cell cannot repair, undergoes apoptosis
Cyclin-CDK
Cyclin= Regulatory subunit CDK= Serine/Threonine Kinase
What mechanism degrades cyclins so they are only present at a specific point in the cell cycle?
Ubiquitin proteolysis
How is CDK activity regulated?
Phosphorylation, Dephosphorylation & CDK Inhibitors
c-Fos
Transcription factor that stimulates the transcription of the delayed response genes
INK4
- Growth Inhibitory Factors
- Compete with Cyclin D for binding to CDK 4/6
- Inhibit CDK 4/6
DNA Damage Checkpoint
- Requires ATM & ATR Kinases
- ATM senses ds-DNA breaks
- ATR senses UV damage & certain drug damage
- Block Phosphatase Cdc25 A & C, and phosphorylate p53
Ataxia Telangiectasia
- 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
