Cell cycle Flashcards
Control of cell cycle
The cell cycle ensures that 3 critical events happen:
- Chromosome replication
- Chromosome segregation
- Cell division
The cell cycle is a clock
Almost all normal cells don’t proliferate unless promoted by an extrinsic factor
Other signalling proteins can overrule stimulatory factors and force halt to proliferation
Extracellular signals can induce post-miotic differentiated state = no proliferation
Mixture signals has to be integrated by cell to make decision proliferate, be quiescent of differentiate
Existence of master governor makes major decision regarding cell fate = cell cycle clock, operates in nucleus
Structure of cell cycle
Eukaryotic cell cycle usually consist of 4 phases
Interphase - main part of cell cycle:
- G1 phase
- Cell increases in size
- Ribosomes, RNA produced
- Preparation of DNA synthesis
- S phase
- DNA synthesised, chromosomes duplicated
- G2 phase
- Cell checks fidelity of DNA
- Preparation for nuclear division
Mitosis: Cell division
- Sub-phase:
- Prophase, prometaphase, metaphase, anaphase, telophase
- Cytokinesis
WE have mapped key events and duration in cell cycle
G1 - 10 hours
S - 7.5 hours
G2 - 3.5 hours
M - 1 hour
This is time in cultured cell
Checks:
- Allow for increase to scheduled length of phase
- Facilitate repair processes
- Are known as checkpoints
G1: growth vs quiescence decision
Discrete window to consult extracellular environment - from onset G1 to hour or 2 before G1toS transition
G1 decision making machinery apparent in responses of cultured cells to extracellular signals:
- Serum and growth factors removed before the cells have completed 80-90% of G1 fail to proceed further and revert to G0 state
- Serum and growth factors removed in final hour G1 proceed to S, G2 and M phase
Deregulation of R-point decision-making machinery accompanies formation most types cancer cells
Challenges associated with finding useful model system
Genetic approach - Required cells that have mutation in putative cell cycle transition gene
Biochemical approach - Requires supply large number cell undertaking same transition at same time
Advantages of using yeast as a genetic model for cell cycle
Rapid division rate at <1 hour
Cell cycle control genes highly conserved
Yeast can be grown as haploid or diploid
How can we study genes that are crucial for cell survival?
Genetic tricks allow identification potentially lethal mutations:
- Diploids used to maintain lethal mutations that are then studied as haploids
- Temperature sensitive mutations allow growth at permissive temperature
Cell-free mitosis
Can deplete cytoplasm of different proteins using antibodies
Can remove cytoplasm at different stages to study changes over time e.g. in protein phosphorylation
Biochemical approach - early embryonic cells allow us to purify cell cycle regulators
In frogs a oocyte can grow without dividing for months it’s arrested in G2
Once fully grown to an egg it is arrested in M-phase until fertilised by a sperm where it is released from M-phase arrest
It was discovered by Yoshio Masui that there is something in the eggs cytoplasm that can catalyse the transition from G2 to M phase
This factor called Maturation Promoting Factor (MPF)
G2/M transition
(Fission) Yeast genetics ID’d cdc2
cdc2 (CDK1) encoded p34cdc2
Regulated by Tyr phosphorylation + cyclin binding
Shown to be similar to the frog 32 kDa fragment
Bioinformatics:
- A cell cycle transition controlled by protein kinase-based machine
- Similar to proteins identified in yeast which perform similar functions
(Look at diagram)
We can visualise and quantify kinase activity
Selective extraction of kinase -> Incubation with a protein substrate and ATP -> Electrophoresis of substrate and imaging
Cell cycle transitions involve irreversible destruction of cyclins
Mostly during mitosis as the kinase is inactive during the other phases
What about other cell cycle transitions?
Would expect that distinct proteins would need to be switched on/off in each transition
SO different transitions catalysed by distinct regulator kinases
In yeast, only one cell cycle regulator kinase gene called Cdk1 but are multiple cyclin
In mammals, are multiple Cdks as well as multiple cyclins
Cyclin levels fluctuate during cell cycle
Cyclin E: low levels throughout most of G1, rapid increase after R point
Cyclin A: levels increase in concert with entrance in S phase
Cyclin B: levels increase in anticipation mitosis
Collapse cyclin levels as cell progresses through cell cycle degradation (ubiquitination-dependent)
The cell cycle can only progress in one direction
