Mitosis Flashcards
Overall problem
To divide properly can’t just replicate DNA
Cell has to be able separate long pieces DNA without breakage or tangling
Cell has to rearrange DNA into short, condensed chromosomes
These then pulled apart in anaphase
2 phases
- Chromosome condensation
- Sister-chromatid resolution
Definitions
Chromosome - linear DNA molecules
Centromere - region where the spindle attaches
Homologous chromosomes - have the “same” genes arranged in the same order
Chromatids - are the newly copied DNA strands still joined to each other by a centromere
M-Cdk drives entry into mitosis
M-Cdk trigger:
- 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 + Golgi
M-cyclin levels increase through G2 and M by increase in Cyclin B expression to create pool of inactive M-Cdk complexes
In late G2, Cdc25 phosphatase triggered to activate positive feedback loop rapidly activating mitosis (look at diagram)
Cdc25 activated by:
- Possibly S-Cdk complexes
- Once started +ve feedback will inhibit Wee 1 and activate more Cdc25
APC (Anaphase-Promoting Complex)
Metaphase - attachment of mitotic spindle to kinetochore by microtubules
Anaphase - separation sister chromatids to opposite poles
Progression through metaphase/anaphase transition -> driven by protein destruction
Contrast this to phosphorylation which used active/inhibit proteins such as M-cdk
APC is ubiquitin ligase
2 targets
1. S+M cyclins: if destroyed most CDKS are inactivated -> CDK targets are dephosphorylated by phosphatases
- APC/C kept on in early G1 -> turned off as G1/S-CDK activated to allow cyclin accumulation
2. Securin: protects protein linkages that hold sister chromatids together
- Destruction -> activates protease that separates sister chromatids -> anaphase
APC also ubiquitinates S-cyclin and Securin
What can go wrong in mitosis?
Don’t forget:
- 2-hit hypothesis, most genes need mutations on both alleles cause phenotypic change
- In case tumour-suppressor gene would be any mutations that lead to inactivation of proteins function e.g. Rb
- Call this change loss-of-heterozygosity
- Can be by several mechanisms - doesn’t matter
- Can also get loss allele i.e. hemizygosity, so in effect have one cop, if that’s mutated then you have problem
Most obvious error is chromosomes ending up in wrong daughter cell e.g. lagging chromosomes during anaphase
Called chromosome non-disjunction
Basic structure of mitotic spindle
Spindle central to separation of chromosomes
Interpolar microtubules - Overlap
Kinetochore microtubules - Attach to chromosomes at kinetochores (centromeres)
Astral microtubules - Contact cell cortex to position the spindle
Centrosome - Centriole surrounded by pericentriolar matrix, act to nucleate microtubules
Look at diagram to see which is which
Successful mitosis -> sister chromatids going opposite poles
Trial and error used to get 1 kinetochore to spindle pole
Inappropriate attachments sensed by tension
Correct attachment:
- Kinetochores pulled in opposite direction
- But sister chromatids resist -> tension
Incorrect attachment:
- Tension is lower -> inhibitory signal -> loosens microtubule attachment
Activation of APC/C by Cdc20 leads to ubiquitylation and destruction securin
Cdks can phosphorylate separase -> inhibition
But in anaphase -> cyclins destroyed -> Cdk inactivation -> less separase phosphorylation -> activation
What can go wrong in mitosis - LOH by nondisjuction
Loss of homology
Elimination by apoptosis when nondisjunction occurs (destroy the hemizygosity)
Then loss the extra chromosome which leads to one egg with LOH
Most genes is loss active copies that’s important when considering detrimental mutations
So by becoming hemizygous when left with mutated allele, effectively have inactivated protein as no active proteins can be made
2nd hit - 2nd copy inactivity - LOH by mitotic recombination
Look at diagram
2nd hit - 2nd copy inactivation- LOH by gene converstion
DNA polymerase begins replication on template strand of chromosome B
DNA polymerase jumps to template strand of chromosome A (homologous) (where the mutation is)
DNA polymerase copies some of A then jumps back to template strand of chromosome B
Summary
M-cyclin/Cdk triggers entry into mitosis and is controlled by Cdc25 and wee1
APC (Anaphase-Promoting Complex) controls the metaphase to anaphase transition
Mitosis can go wrong by inducing LOH:
- Nondisjunction
- Mitotic recombination
- Gene conversion
