Lecture 2- Mitosis Flashcards
What are the 2 phases of anaphase?
- Chromosome condensation
2. Sister-chromatid resolution
Define chromosome, centromere, homologous, chromatids.
Chromosome- linear DNA molecule
Centromere- region where the spindle attaches
Homologous chromosomes- have the same genes arranged in the same order (1 inherited from each parent)
Chromatids: are the newly copied DNA strands still joined to each other by a centromere
What are the stages of M phase (mitosis and cytokinesis)?
- Prophase- condensation of sister chromatids (identical copies)
- Metaphase- attachment of the mitotic spindle to the kinetochore by microtubules
- Anaphase- separation of sister chromatids to opposite poles
- Telophase- nuclear envelope reassembly, start of cytokinesis
Why are yeast a good genetic model for cell cycle studies?
Rapid division rate, cell cycle control genes are highly conserved and yeast can be grown as haploids or diploids.
How can we study genes crucial for cell survival?
- Diploid can be used to maintain lethal mutations that are then studied as haploids
- Temperature sensitive mutations allow growth at permissive temperatures.
Both these genetic tricks allow lethal mutations to be identified.
Why are xenopus a good biochemical model for cell cycle studies?
- Easy to collect eggs
- Rapid division rate
- Large size makes protein purification easier
- Can be manipulated by injection of RNAs or chemicals into the oocyte
What is cell-free mitosis?
The ability to deplete the cytoplasm of different proteins using antibodies at different stages to study changes over time.
How does M-Cdk drive entry into mitosis?
- Assembly of the mitotic spindle
- Each sister chromatid is attached to an opposite pole
- Chromosome condensation
- Breakdown of the nuclear envelope
- Rearrangement of the actin cytoskeleton and golgi
How does M-cyclin/Cdk trigger entry into mitosis?
- M-cyclin levels increase through G2 and M (by increase in cyclin B expression) to create a pool of inactive M-Cdk complexes
- In late G2, the Cdc25 phosphate is triggered to activate a positive feedback loop which rapidly activates mitosis
What causes Cdc25 to activate during mitosis?
Possible S-Cdk complexes. Once started, positive feedback inhibits Wee1 and activates more Cdc25
What drives progression through metaphase and anaphase transition?
Driven by protein destruction
What is APC and what are its 2 targets?
APC is a ubiquitin ligase.
- S + M cyclins:
If these are destroyed most CDKs are inactivated leading to the dephosphorylation of CDK targets by phosphates.
APC/C kept on in early G1 gets turned off as G1/S-CDK is activated to allow cyclin accumulation
- Securin:
Protects the protein linkages that hold sister chromatids together.
Destruction activates a protease that separates the sister chromatids (anaphase)
What is the 2-hit hypothesis?
Most genes need mutations on BOTH alleles to cause a phenotypic change.
In the case of tumour-suppressor gene it would be any mutations that lead to inactivation of the proteins functions . This is called loss of heterozygosity.
What is loss of hemizygosity?
Loss of the allele so you have one copy. If this is mutated, problems can occur.
What is chromosome non-disjunction?
Most common error during mitosis where chromosomes end up in the wrong daughter cell e.g lagging chromosomes during anaphase.
