Cell cycle in Eukaryotes Flashcards
What is the cell cycle
- Repeated process by which the cell duplicates itself and divides into two daughter cells
- Continuous cell divisions since beginnings of life on earth
- The cell cycle is controlled
- embryos show co-ordinated waves of cell division
What can the loss of cell cycle control result in
- Loss of cell cycle control is a key step in cancer formation
- Identifying regulators of the cell cycle is important:
- tumour suppressors or proto-oncogenes
- predict people at risk
- potential drug targets
What are the basic events in the cell cycle
- Duplicate cytoplasmic components (cell growth)
- Chromosome duplication
- Chromosome segregation
- Cytoplasmic segregation
What happens in Prophase
- The chromosomes begin to condense.
- The centrosomes begin to assemble at the poles of the cell.
- Nuclei starts to breakdown- no longer nuclear envelope
What happens in Prometaphase
- Microtubules from centrosomes start to project and radiate out into cells
What happens in Metaphase
- The chromosomes align along the plane that bisects the cell and become attached to the microtubule fibres of the mitotic spindle.
What happens in Anaphase
- Segregation occurs; the sister chromatids are pulled apart by the mitotic spindle to the two opposite poles of the cell.
What happens in telophase
- The chromatids de-condense and a new nuclear membrane forms around each new set of chromosomes.
How is the cell cycle regulated
- Controlled by checkpoints
- If problem- there is a pause for it to be rectified or for cell to exit cell cycle and not divide
- Checkpoints there to detect problem in External and Internal environment
What happens in the first checkpoint
- Check if favourable internal environment
- Right number of organelles
- Correct amount of duplication etc
What is checked at the second checkpoint
- All DNA needs to have been duplicated
What is checked at the third checkpoint
- Make sure anaphase is triggered at correct time
What may happen without the first checkpoint
- Start checkpoint
- Inappropriate cell proliferation
- lack of nutrients
- → cell death
- → malformed tissues
- → cancer
What may happen without the G2/M checkpoint
- Division without complete DNA duplication
- daughter cells lack full complement of chromosomes (aneuploidy)
- → cell death
- → cancer
What may happen without the Metaphase/Anaphase transition checkpoint
- Division with incomplete chromosome attachment
- daughter cells with uneven amounts of DNA (aneuploidy)
- → cell death
- → cancer
How is progression through the checkpoints controlled?
- Cyclins and cyclin-dependent kinases
- Cyclin-dependent kinases (cdk) phosphorylate target proteins
- Cyclins are regulatory proteins which bind cdks and allow them to become active
The level of what changes throughout the cell cycle
- Cdk levels do not change throughout the cell cycle.
- It is the levels of cyclins that go up or down; this in turn controls the activation status of cdks.
- Transcription levels of many other genes are also controlled during the cell cycle.
Describe levels of G1/S cyclins and complexes through progression of cell cycle
- G1/S-cyclins levels rise during late G1 Phase and form G1/S-Cdk complexes.
- The G1/S-Cdk complexes will reach high levels that that allow progression through the Start checkpoint.
- G1/S-cyclins levels they start off low in G1, rise and remain high until the end of M phase where they drop to low levels again.
Describe levels of S-cyclin and complexes through progression of cell cycle
- S-cyclin levels start to rise following progression through the Start checkpoint.
- S-Cdk complexes reach high levels at the start of S phase triggering downstream events.
- S-cyclin levels begin to drop following the G2/M checkpoint and reach basal levels at the metaphase-anaphase transition checkpoint.
Describe levels of m-cyclin and complexes through progression of cell cycle
- M-cyclin levels begin to rise in G2.
2. M-Cdk complexes are formed but remain inactivated until the end of G2.
What is APC/C
- APC/C initiates cell progression through the metaphase-anaphase transition checkpoint.
- Anaphase promoting complex
How are Cdk levels regulated
- Phosphorylation
- Binding of inhibitory proteins
- Proteolysis
- Localisation
- Feedback loops
- Transcription
How is cdk regulated by binding of inhibitory proteins
- Cdk regulation- binding of inhibitory proteins
- Bind to entire complex- cyclin-cdk complex
- Not active if bound
How is cdk regulated by phosphorylation
- reaction in which a phosphate group is covalently coupled to another molecule
- Cdk needs a kinase to add a phosphate group to allow it to activate
- Other regions on cdk other phosphate groups can be added which can be inhibitory and put it in inactive state even with binding of cyclin and activating phosphate