Three states that cells exist in regarding cell division
1) Always in cell cycle
2) Post-mitotic; never divide again
3) Resting; can be induced to divide
Substance that will cause a resting cell (in G0) to re-enter cell cycle and divide
When in cell cycle do you synthesize more histones?
When does histone phosphorylation by His kinase occur?
Throughout cell cycle, with peak phosphorylation at the end of G2 and none by end of mitosis
Proteins involved in G1-S transition
E2F: transcription factor that activates genes necessary for G1-S transition
RB (restriction boundary protein): normally is unphosphorylated and represses E2F, but when phosphorylated, doesn't bind E2F (must be dephosphorylated again before next cell cycle)
What is a double thymidine block?
Techinque to synchronize cells by causing them to accumulate at the G1/S interface
Factor in G2 cells that prevents the re-replication of already replicated DNA prior to mitosis
What is checked for at G2/M checkpoint?
Make sure DNA is replicated and environment favorable for mitosis
What is checked for at the S checkpoint?
Make sure DNA is not damaged and see if it can be repaired
What is checked for at the M checkpoint?
Make sure all chromosomes attached to spindle
M-phase promoting factor
Controls transition from G2 to M; is a cyclin-dependent kinase (cdc2) that must bind cyclin B in order to act; is specifically a histone kinase
What is checked for at G1/S checkpoint ("Restriction Point")?
Is environment favorable for DNA synthesis? If not, can go into G0.
When does CDC2-Cyclin B act?
G2 to M
When does CDK4-Cyclin D1 act?
Restriction point; this is what phosphorylates RB (thus allowing E2F to transcribe genes needed for S phase)
What does CDC2-Cyclin B do?
Phosphorylates histones (it is a histone kinase), so must contribute to chromatin condensation in mitosis
After Cyclin B binds CDC2, what else must happen before the complex is active?
CDC2 must be dephosphorylated at 2 sites
What are three ways to negatively regulate CDKs?
1) Phosphorylation at 2 sites makes the CDK inactive
2) CKI (cyclin dependent kinase inhibitor) can bind and inactivate CDK
3) Degrade the cyclin
What is special about E3 enzyme in the ubituitination system?
E3 enzymes specifically target a certain protein for degradation
(ie E3 will degrade a specific cyclin at a specific point in the cell cycle)