Protein control of cell division Flashcards
What does the cytoskeleton do?
Gives mechanical support and shape to cells.
What does the cytoskeleton consist of?
Different protein structures including microtubules, which are found in all eukaryotic cells.
What are microtubules?
Hollow cylinders composed of the protein tubulin. They radiate from the MTOC or the centrosome.
What do microtubules do?
Control the movement of membrane-bound organelles and chromosomes.
Cell division requires remodelling of the cytoskeleton, how is this done?
Formation and breakdown of microtubules involves polymerisation (growth) and depolymerisation (shrinkage) of tubulin.
What do microtubules form and why?
Spindle fibres that are active during cell division.
The cell cycle consists of?
Interphase and Mitotic (M) phase.
Interphase?
Involves growth and DNA synthesis including G1 (growth phase); S phase (during which DNA is replicated) and G2 (further growth phase)
What does the mitotic (M) phase involve?
Mitosis and cytokinesis.
What happens in mitosis?
(Includes cytokinesis)
Chromosomal material is separated by spindle microtubules. This is followed by cytokinesis in which the cytoplasm is separated into 2 daughter cells.
What does mitosis consist of?
Prophase, Metaphase, Anaphase and Telophase.
What happens in prophase?
- DNA condenses into chromosomes each consisting of 2 sister chromatids.
- Nuclear membrane breaks down; spindle microtubules extend from the MTOC by polymerisation and attach to chromosomes via their kinetochores in the centromere region.
What happens in metaphase?
Chromosomes are aligned at the metaphase plate.
What happens in anaphase?
As spindle microtubules shorten by depolymerisation, sister chromatids are separated and the chromosomes are pulled to opposite poles.
What happens in telophase?
- The chromosomes decondense and nuclear membranes are formed around them.
- Cytokinesis also takes place which involves the separation of the cytoplasm into 2 daughter cells.
Progression through the cell cycle is controlled by?
Checkpoints
What are checkpoints?
Mechanisms within the cell that assess the condition of the cell during the cell cycle and halt progression to the next phase until certain requirements are met.
What is involved in regulating the cell cycle?
Cyclin proteins that accumulate during cell growth.
How does progression through the cell cycle get controlled?
Cyclins combine with and activate CDKs. Active cyclin-CDK complexes phosphorylate proteins that regulate progression through the cycle. If sufficient phosphorylation is reached, progression occurs.
G1 check point - what happens there to halt the cell cycle?
Rb acts as a tumour suppressor by inhibiting the transcription of genes that code for proteins needed for DNA replication.
G1 check point - what happens there to continue the cell cycle?
Phosphorylation by G1 cyclin-CDK inhibits the Rb. This allows transcription of genes that code for proteins needed for DNA replication.
What happens at the end of the G1 check point?
Cell cycle progresses from G1 to S phase.
What happens at the G2 checkpoint?
Success of DNA replication and any damage to DNA is assessed.
DNA damage triggers the activation of several proteins including p53 that can:
- Stimulate DNA repair
- Arrest the cell cycle
- Cause cell death (Apoptosis)
What does the metaphase checkpoint control?
Progression from metaphase to anaphase
What happens at the metaphase checkpoint?
Progression is halted until the chromosomes are aligned correctly on the metaphase plate and attached to the spindle microtubules.
What does an uncontrolled reduction in the rate of the cell cycle cause?
Degenerative disease.
What does an uncontrolled increase in the rate of the cell cycle cause?
Tumour formation.
What is a photo-oncogene?
A normal gene usually involved in the control of cell growth or division which can mutate to form tumour-promoting oncogene.
What is apoptosis triggered by?
Cell death signals that can be internal or external.
Why is apoptosis essential during development of an organism?
To remove cells no longer required as development progresses or during metamorphosis.
When might cells initiate apoptosis?
In the absence of growth factors.
Example of external death signal?
Production of death signal molecules from lymphocytes.
How do external death signal molecules work?
Bind to a surface receptor protein and trigger a protein cascade within the cytoplasm.
Example of an internal death signal?
DNA damage.
How does the internal death signal resulting from DNA damage work?
Causes activation of p53 tumour suppressor protein.
What do both types of death signal result in?
Activation of caspases that cause destruction of the cell.
