Cell Cycle II Flashcards
which cyclins and cdks are essential for the cell cycle?
CDK1 - in absence of CDK2, 4 or 6 - can bind Cyclin D driving pRb phosphorylation but less efficient!
Cyclin A2 and Cyclin B1.
How did yeast studies help to identify START (G1 to S transition)?
Mating factors found to induce cell cycle arrest.
Exposure to factors early in cycle arrested in G1.
Exposure late in cycle arrested in NEXT G1.
How was START explored in mammalian cells?
Withdraw serum GFs <3h after mitosis - G1 arrest.
>4h after mitosis –> no arrest.
Progression through cell cycle changes from GF dependent (<3h) to GF independent (>4h).
What links are found implicating Cyclin D1 as an oncogene?
PRAD1 oncogene - gene involved in 11q13 chromosomal inversion found in subset of parathyroid Tu.
Shown to be Bcl-1 oncogene in t(11:14) bearing B-cell lymphoma.
Amplified in many mammary and squamous cell carcinomas.
Overexpressed in CRC and melanoma.
How are cyclin D1 levels regulated?
Transcription - D1 induced by AP1, D2 by Myc/Max, D3 by STAT3/STAT5
Translation - enhanced by GF stimulation involving PI3K dependent activation of eIF-4E.
Proteolysis - phosphorylated by GSK3B, then SCF dependent ubiquitination and proteasomal degradation. (PI3K signalling, PKB phosphorylates and inhibits GSK3B - preventing degradation)
What genes are transcribed in response to E2F?
Thymidine kinase, cdc6, DNA polymerase alpha (DNA synthetic products for S phase) / cyclin A, Cyclin E, Cdk1.
How is the restriction point passed?
Cyclin D phosphorylates pRb
Partial de-repression of E2F –> Cyclin E expression
Cyclin E also phosphorylates pRb –> more cyclin E.
+ve feeddback.
What happens to Cyclin E after G1 exit?
CyclinE/CDK2 phosphorylates Cyclin E –> ubiquitination and proteolysis.
What are replication origins and how are they formed?
Sequences in genome at which DNA replication initiated.
Origin licensing (recognition of sequences where replication occurs to form pre-RC).
Origin firing (activation of DNA synthesis - triggered by S-Cdk activity to form pre-IC)
How are cyclins involved in replication origins?
CyclinA-Cdk2 phosphorylates numerous preIC components inc. DNA polymerase, MCM2 helicase complex.
AND E2F phosphorylated –> degradation causing E2F dependent transcription to fall on S phase entry.
Briefly outline relicensing prevention.
CDK phosphorylates licensing components - initiating proteolysis.
Geminin binds CDT1 preventing it acting in licensing
CDT1, ORC, ORC6 ubiquitinated by SCF and degraded in S.
Geminin ubiquitinated by APC/C in M allowing CDT1 to act in licensing in next G1.
outline the cdc25 phosphatases.
3 in mammals
A functions at G1 necessary for activating CDK2 for S phase entry.
B and C function at G2/M
At G2/M checkpoint, if Chk1 activated - phosphorylates cdc25C, causing degradation and preventing it removing inhibitory phosphates on CDK1.
