Flashcards in Unit 7 - Regulation of the cell cycle by cyclin-dependent kinases Deck (18):
what are the three major checkpoints of the cell cycle?
1. G1/S "start" commitment phase
-trigger DNA replication machinery
2. G2/M "entry into mitosis"
-trigger mitosis machinery
3. metaphase/anaphase "exiting mitosis"
-trigger completion of mitosis and proceed to cytokinesis
what is cyclin-dependent kinase? how does its activity change in interphase and mitosis?
a serine/threonine specific protein kinase w/o activity unless combined with matching cyclin regulatory subunit
-CDK activity increases suddenly in mitosis (due to slowly increasing concentrations of cyclin from interphase through mitosis)
--after mitosis is done, levels of both CDK and cyclin decrease
mitotic CDK activation
mitotic CDK + M-cyclin = inactive M-CDK
-inhibitory kinase Wee1 and activating kinase phosphorylate it to make it inactive M-CDK-P-P
-activating phosphatase Cdc25 removes the inhibitory Wee1 phosphate, leaving active-M-CDK-P
-active M-CDK-P is positive feedback to activate more Cdc25 phosphatases to activate more M-CDKs
mitotic CDK inactivation
1. cyclin subunit is ubiquitylated by E3 ubiquitin ligase
-targets cyclin subunit for proteolytic destruction by proteosome
2. activating phosphate is removed from kinase subunit, leaving inactive CDK
what is APC?
anaphase promoting complex (E3 ubiquitin ligase that turns off mitotic CDK at 3rd checkpoint from metaphase to anaphase)
-helps triggers start of anaphase by turning off CDK activity, and by promoting Xm separation by activating protease separase
securin and separase interaction and what do they do?
securin (inhibitory protein) is bound to inactive separase (proteolytic enzyme)
-active APC ubiquilates and degrades securin, activating separase
-active separase helps trigger transition from metaphase to anaphase
what is the original trigger for metaphase to anaphase transition? how is this a checkpoint?
tension on all kinetochores, which inhibits kinase activity
-if one Xm fails to attach to both poles, the kinase remains active, and blocks progression to anaphase
-when all kinetochores are under tension, APC becomes active, and triggers separation of chromatids through proteolysis of cohesion complex
how many CDKs are needed to regulate cell cycle progression?
mulitple; each CDK has a unique substrate specificity and a unique time during cell cycle when it's active
-different cyclin/CDK combos are needed at different points in the cell cycle
what CDK/cyclin combos are needed in G1-->S, progression through S, and entry into mitosis?
G1-->S: cyclin D + CDK4/6
through S: cyclin A/E + CDK 2
into mitosis: cyclin B + CDK 1
concentrations of S VS M cyclin throughout cell cycle
S cyclin builds up in middle of G1 phase, and peak/plateau at start of S phase until slowly drops at start of M phase
M cyclin: starts increasing at beginning of G2, then peak/plateau at beginning of M phase, and steeply drops in middle of M phase
how can CDKs be regulated?
-damaged DNA (G1/S/G2)
-unfavorable extracellular environment (G1)
-incompletely replicated DNA (S/G2)
-Xm improperly attached to mitotic spindle (M)
how can DNA damage prevent mitosis?
DNA damage activates protein kinases that phosphorylate p53 to stable and active form
-active p53 binds to regulatory region of p21 gene
--if binds too long, causes apoptosis
--if can manage to transcript/translate p21 mRNA, makes p21 (CDK inhibitor protein)
---p21 binds the active G1/S-CDK and S-CDKs, thus inactivating them, and blocking entry into S phase until DNA is repaired
breast cancer and DNA damage repair
BRCA 1/2 proteins are important in dsDNA break repair
-if either one is completely absent, cell is unable to repair dsDNA breaks, so mutation rates increase
-most cells with DNA damage (especially if very extensive) don't progress through cell cycle, since prolonged checkpoint activation leads to apoptosis
what is PARP? what does inhibiting it do?
-pathway is important for ssDNA break repair
-if inhibited in BRCA-mutant cancer cells, the rate of apoptosis can be increased
what is Cisplatin and what does it do?
cancer drug that induces dsDNA damage that cannot be repaired, and is so extensive that the cell undergoes apoptosis
external controls of cell cycle?
work through modulation of cell growth
-needed to pass Start checkpoint in G1
-directly activate mitogens
-growth factors increase rate of PRO synthesis and reduce rate of PRO degredation, so cells can reach critical growth status for entry into cell cycle
what do mitogens do?
directly stimulate progression from G0 into cell cycle
-include ligands of receptor tyrosine kinases that activate the MAPK pathway