1.4 - Cell Cycle Control Flashcards
4 phases of cell cycle (4)
- M - mitosis
- G1 - Gap1
- S - synthesis
- G2 - Gap2
commitment/control points of mitosis (checkpoints) (3)
- is the cell big enough? - cell growth
- is the environment favourable?
- DNA integrity? - DNA damage/problems during replication
genetic approaches to understanding cell cycle (studies in 2 single cell eukaryotic organisms crucial for understanding cell cycle control) (2)
- budding yeast (saccharomyces cerevisiae)
- fission yeast (schizosaccharomyces pombe)
budding yeast (saccharomyces cerevisiae) division
divides by budding
fission yeast (schizosaccharomyces pombe) division
cells grow lengthwise until they reach critical size before going into mitosis and divide by medial fission
what will a ts mutation in a random essential gene lead to?
the cell arresting across the cell cycle
what will a ts mutation in a cell cycle regulator gene lead to?
arrest at a discrete cell cycle stage
Cdc mutation at 36°C
cannot divide to form 2 cells
wee mutant at 36°C
divides too early to form 2 cells
master cell cycle regulator
Cdc2
mutations identified by Cdc2 screen (3)
- mutants that block in S
- mutants that block in G1
- mutants that block in G2
what is required for G1/S and G2/M?
Cdc25
can wee mutants delay cell cycle?
no, they divide to produce small G1 cells
Cdc2 mutant variations (2)
- Cdc
- Wee
role of wee1
negatively regulates movement from G2 -> M (inhibits mitosis)
role of Cdc25
positively regulates movement from G2 -> M (activates mitosis)
biochemical approach to understanding cell cycle control (2)
- biochemical analysis of how protein abundance changes fish/frog embryos
- gradual accumulation of protein activity until critical level -> rapid entry into mitosis (= Cdc2 activity)
cyclins
made and destroyed each cell cycle (cyclin proteins accumulated as cells progress through cell cycle)
mitosis promoting factor in mitotic nuclei
MPF - promotes mitosis in frog eggs
mitosis promoting factor (MPF)
cell cycle control element able to cause metaphase when injected into amphibian oocytes (female gametes) or when incubated with nuclei in a cell-free system
mitosis promoting factor (MPF) complex
complex between a 34kD serine/threonine protein kinase
mitosis promoting factor (MPF) identification
Identified as a Xenopus homolog of the cdc2 gene product, p34cdc2 and a 45kD substrate, identified as a Xenopus B-type cyclin’
Csc2 enzyme
cyclin-dependant protein kinase (CDK)
mitosis promoting factor (MPF) enzyme
active mitotic CDK cyclin complex
CDK
kinase that phosphorylates other proteins in a cell cycle dependant manner
what does Cdk regulation ensure?
ordered progression through the cell cycle (look at one note diagram)
what does M-CDK do when its activity has successfully brought about cell state required for cell division?
it helps trigger its own destruction by promoting the activation of APC which ubiquitylates cyclins leading to their destruction
ubiquitination
post-translational modification in which a small protein called ubiquitin is covalently attached to a target protein. This process regulates numerous cellular functions, including protein degradation, signalling, and trafficking.
what controls progression through mitosis?
MPF/Mitotic CDK
one of the most important events during G1/S
transition between preparing DNA for replication in G1 and performing DNA replication in S (making sure the cell does not try to re-replicate already replicated DNA)
licenses
cells are licenses to go through S phase once in a cell cycle
licensing of unreplicated DNA
occurs in G1 and inhibited in S phase (when the already licensed DNA is replicated)
2 main groups of cyclin/CDK complexes in humans? (2)
- G1/S - control entry into S phase
- G2/M - control entry into mitosis
cell cycle is a CDK cycle (7)
- low Cdk activity (loading of licensing factors at origin)
- S-cyclin increases
- S-Cdk activated replication
- S-Cdk activity high (no relicensing)
- M-cyclin increases
- M-Cdk activated, triggers mitosis
- after spindle checkpoint satisfied, APC activated and cyclins destroyed, resetting cycle
checkpoints
cells delay cell cycle progression in response to external signals (e.g. DNA damage)
major transition points/checkpoints in interphase (2)
- START - cells licensed in G1 to enter S phase
- G2/M transition
how is the Cdk/cyclin complex kept inactive/activated (2)
- kept inactive - Wee1
- activated - Cdc25
spindle checkpoint role (2)
- monitors whether cells assemble their chromosomes and spindles correctly (occupancy of kinetochores/tension)
- ensures all chromosomes lined up on metaphase plate and attached to microtubules to both poles before anaphase is triggered
role of MAD proteins
inhibits separation at unattached kinetochores (prevent activation of APC)
APC
anaphase promoting complex
