Transcript Four Phases of Cell Division

Cell & Molecular Biology
Control of the Cell Cycle
M-checkpoint
G2 Checkpoint
1. Controls entry to mitosis
2. DNA replication assessed
3. Full genome must be present
to allow copy for each
daughter cell
G1 checkpoint
• Multicellular organisms, signalling molecules called ‘growth
factors’ from surrounding cells trigger the production of cyclins
which increase in concentration at the checkpoints.
• Cyclins bind to kinase enzymes known as cyclin dependent
kinases (CdK) forming cyclin-CdK complex
•Cyclin-Cdk complex activates key proteins by phosphorylation
Growth factors
Control of G2 checkpoint
• G2 cyclin levels build up and
bind to kinase enzymes (CdK)
•CdK plus M2 cyclin forms
active protein complex called
mitosis promoting factor or
MPF.
• Mitosis Promoting Factor is
essential for entry to mitosis.
Progression beyond G2
checkpoint involves activation of
cyclin dependent kinase (Cdk)
by G2 cyclins.
Complex is called Mitosis
Promoting Factor (MPF)
Increased concentration of MPF
causes
• chromosomes to condense
• Nuclear membrane breakdown
•Mitotic spindles form and
chromosomes move to the equator
The concentration of MPK does not change but its
activity rises and falls to to changes in the levels of
G2 cyclin. Click here for animation
M (metaphase) checkpoint
Controls entry to anaphase
Ensures chromosomes are
aligned correctly on equator
Ensures daughter cells
receive correct number of
chromosomes
• Triggers separation of
daughter chromosomes and
then cytokinesis
Abnormal Cell Division: Cancer
The cell cycle is under genetic control. There are two groups of
genes involves:
Proliferation & Anti-proliferation
Genes
Proliferation Genes
• Proliferation genes or proto-oncogenes – These switch on
the process of mitosis by coding for proteins that promote
cell division e.g. MPF
• These are dominant genes therefore a single mutation
would convert them into the mutant type ‘oncogenes’,
starting uncontrolled cell division
• Oncogenes are mutated genes where there is a ‘gain-offunction’
• Oncogenes promote cell division by the overproduction of
a stimulatory protein; such mutations can be at any level in
signalling and transduction.
Anti-proliferation Genes
•These are also known as tumour suppressor genes or antioncogenes and are involved in restricting cell division
Eg. p53 gene, act at checkpoints by generating proteins that
block progress through the cell cycle when conditions are not
met.
• When tumour suppressor genes mutate there is ‘loss-offunction’ allowing damaged or unrepaired cells to divide
• Two copies of the antiproliferation gene have to mutate before
a tumour starts to develop as they are recessive in nature
For nice game on cell cycle click here
Essay question due Friday
Write notes on cell cycle and control under the
following headings.
i) Interphase
ii) Mitosis
iii) Mutations
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