SM 15 Flashcards
(6 cards)
Differentiate cohesins from condensins.
Cohesins keep sister chromatids together; starts in S phase/G1 with cohesion loading, after S phase they make ring of cohesion proteins along entire chromosome, then only in the missile then during anaphase the cohesins dissolve. Condensin compact the chromosomes so they are well organized in this mitotic structure.
How do we activate cyclin-b-cdk1?
After Cdk1 binds cyclin B1, it is rapidly phosphorylated and activated by a protein kinase called CAK (Cdk-activating kinase) and another inhibitory kinase, Wee1. Full activation of CDK1 occurs when a phosphatase Cdc25 removes the inhibitory phosphorylation. once active the cdk-cyclin will inhibit wee1 and activate cdc25.
What happens if the cell detects damage at the G2 checkpoint?
if the sensors detect damage, they activate one of several protein kinases of the inositol 1,4,5-triphosphate kinase (IP-3) family. The major kinase in this process is ATM (ataxia telangiectasia mutated) and another is ATR (ataxia telangiectasis and Rad3-related. The sensor kinases then phosphorylate at least two substrate effectors: tumor suppressor protein p53 and Chk1 (checkpoint kinase 1).
The effector Chk1 phosphorylates Cdc25 on a specific residue inactivating the phosphatase. This phosphorylation also produces a binding site on Cdc25 for an adapter protein of the 14-3-3 group of proteins. This adaptor protein anchors Cdc25 to cytoplasmic microtubules preventing its entry into the nucleus. p53 is not required for the G2-M arrest, but it does function to stabilize the arrest. p53 is a transcription factor that induces the transcription of a Cdk inhibitor called p21. p21 binds Cdk-cyclin complexes and inhibits their activities.
When the DNA damage is repaired, other kinases reverse the arrest caused by Chk1 and p21 and allow mitosis to initiate.
How do we trigger anaphase?
MCC is a wait signal that inhibits APC. The triggering event in anaphase is the sudden loss of Cdk1 activity. This begins with the gradual destruction of cyclin A from prometaphase through metaphase and finishes with the activation of the APC. The APC is a ubiquitin-ligase complex that targets cyclins and the cohesion complex.
Separation of sister chromatids requires the degradation of cohesion complex that hold chromatids together. Cohesin is degraded by a protease called separase. Separase is inhibited by a protein called securin. Securin is ubiquitinated by the APC targeting it for proteasome degradation, thus activating the separase.
So APC degrades securin and the m-phase cyclins. It has a cofactor cdc20 that helps specify targets including cyclinb/cdk1 and securing.
What part of the cytoskeleton is involved in cytokinesis?
The contractile ring is composed of an actin-myosin complex that can contract. The contraction of the ring is initiated by a local release of calcium that activates myosin light chain kinase, which in turn activates myosin II in the ring.
What are the only chromosomes that tolerate trisomy?
13, 18, 21, X and Y
