cell cycle control quick flashcards

how is the cell cycle regulated

cell

what happens if cells at different phases of the cell cycle are fused together

signals in the cytoplasm of the fused cell will cause both nuclei to enter the same phase of the cell cycle

what are the major checkpoints of the cell cycle

G1 checkpoint (before DNA replication)

G2 checkpoint (before mitosis)

M checkpoint (before cytokinesis)

what is the G1 checkpoint

known as restriction point (mammals) / start (yeasts)

most important checkpoint

if cell recieves go ahead signal, usually completes the whole cycle and divides

what is the G0 phase

if no go ahead or stop signal at G1, enter G0 phase

non dividing state, no active cell growth/division

only maintenance

nerve cells, cardiac muscle cells, liver cells are in G0 phase

explain G0 to G1 transition in the liver

most liver cells exist in a reversible G0 phase

liver has capacity to regenerate

prolonged alcohol misuse can reduce this ability

what are cyclins

proteins that regulate progression through checkpoints

levels cyclically change

CDKs activate them

CDK-cyclin active complex phosphorylates downstream targets

what are kinases

enzymes that add a phosphate group to a target protein

aka phosphorylation

what are G1/S cyclins

trigger progression through G1 checkpoint

what are M-cyclins

stimulate entry into mitosis at G2 checkpoint

what are CDKs

levels are constant

activity controlled by cyclic changes in cyclin levels

what regulates CDK regulators

cyclin binding

phosphorylation and dephosphorylation (G2)

binding of inhibitory proteins (G1)

destruction of cyclins - happens at M checkpoint (APC/C)

what happens if DNA damage is present at G1 checkpoint

activates p53, stimulating the transcription of several genes

p21 binds to G1/S-Cdk and S-Cdk and inhibits their activity

blocks entry to S phase

what happens if DNA replication is incomplete at G2 checkpoint

signals inactivate the phosphate Cdc25

blocks dephosphorylation and activation of M-Cdk, blocking entry into mitosis

what happens in the S phase

S-CdK trigger start of DNA replications at each origin

each origin fired once per cell cycle

duplicated sister chromatids are linked together by cohesin

what happens if chromosomes aren’t correctly attached at M checkpoint

“wait” signal → blocks APC/C → blocks sister chromatic separation

cells don’t enter anaphase until all chromosomes are correctly bi-orientated

what triggers the transition from metaphase to anaphase

by protein destruction

what is the APC/C

catalyses ubiquitylation and destruction of:

securin (activates protease that separates sister chromatids)

S and M cyclins

what is P53

trigger apoptosis in cells with damaged DNA = halt damaged cell growth and division

mutations in p53 are found in 70% of lung tumours

what is peto’s paradox

larger animals with more cells don’t have increased cancer risk

suggests they have more effective mechanisms to prevent cancer

elephants have 20 extra copies of the p53 tumour suppressor gene