Cell Cycle Flashcards
What are DNA damage checkpoints? What are their functions during cell cycle progression?
The progression of the cell cycle is regulated by extracellular signals (growth factors) and internal signals. The 1st checkpoint is at G1 to make sure there are growth factors (in the absence of GFs, cells enter G0 and stop proliferating). The 2nd checkpoint is in S to make sure DNA is replicated properly. The 3rd checkpoint is at G2 to make sure DNA is not damaged. The 4th checkpoint is at spindle assembly, and stops mitosis if chromosomes are not properly aligned.
What are the steps of maturation promoting factor (MPF) regulation? (Be familiar with Thr 161, Tyr 15, and Thr 14, and kinases, phosphatases, and ubiquitinases)
MPF is a dimer consisting of cyclin B and the Cdk1 protein kinase. Cdk1 forms a complex with cyclin B during G2, and this complex is the inactive MPF. Wee1 kinases phosphorylate Tyr-15 and Thr-14 onto the inactive MPF. Cdc25 phosphatase is activated and removes inhibitory phosphates from Tyr-15 and Thr-14. This dephosphorylation activates MPF at the G2 to M transition. MPF activity is terminated toward the end of mitosis proteolytic degradation of cyclin B, which is followed by dephosphorylation of Cdk1 (Thr-161 is removed).
How does Cdk4,6/cyclin D activity drive G1 progression?
The Cdk4,6/cyclin D complex is synthesized in response to growth factor (ras, Rf, MEK, ERK) pathways. It phosphorylates Rb ( a tumor suppressor), which leads to Rb released E2F transcription factors, which activate the transcription of genes necessary for DNA synthesis and S phase entry.
What are the main targets of Cdk4,6/cyclin D and what are their roles in the regulation of transcription?
The target is Rb. When unphosphorylated, Rb binds to E2F, which represses transcription (no tumor). But when Cdk4,6/cyclin D phosphorylates Rb, E2F is released, which activates transcription (tumor).
Compare the two different responses to DNA damage.
1) Cell cycle arrest and repair has two parts. ATR and ATM (protein kinases) are activated by DNA damage, which arrests the cell cycle by inhibiting cdc25 (which is needed to activate cdk1 and cdk2). ATM also phosphorylates the p53 protein, which activates transcription of the gene encoding the CDK inhibitor, which leads to inhibition of the CDK2/cyclin E complex. This causes cell arrest until DNA is repaired.
2) Apoptosis: p53 triggers pro-apoptotic genes, the mitochondria releases cytochrome c; this activates caspases and initiates the apoptosis cascade, and the cell is phagocytized.
What’s MDM2’s role in the regulation of p53, both in the absence of DNA damage and when there are high levels of DNA damage? How is p53 modified by MDM2?
MDM2 binds to p53 and tags it for ubiquitination and degradation, not allowing cell arrest (because cells are healthy). In the presence of DNA damage, active p53 can escape MDM2 binding and arrest cell growth. MDM2 attaches ubiquitin molecules to lysine residues on p53, marking p53 for destruction by the 26S proteasome.
What role does the spindle assembly checkpoint have in the metaphase to anaphase transition?
The spindle assembly checkpoint monitors the alignment of chromosomes on the metaphase spindle. Once this happens, the cell initiates anaphase to complete mitosis. (MCC is the mitotic checkpoint complex.) Progression to anaphase is mediated by the activation of APD/C ubiquitin ligase, where degradation of MPF leads to degradation of cohesion between sister chromatids (once chromatids begin to be pulled apart, anaphase begins).
What would you expect to see if cells had mis-aligned chromosomes?
Cells would be stuck in metaphase and have long mitotic cell arrest
