L26 Mitosis Flashcards
What is significant in cell division?
In order to divide properly its no good just replicating your DNA
The cell has to be able to separate long pieces of DNA without breakage, or tangling (which could lead to too many chromosomes ending up in one daughter cell.
The cell has to rearrange the DNA into short, condensed chromosomes
These are then pulled apart in ANAPHASE
2 phases
Chromosome condensation
Sister-chromatid resolution
Define chromosome?
linear DNA molecule
Define centromere?
Region where the spindle attaches
Define homologous chromosomes?
Have the ‘same’ genes arranged in the same order
1 Inherited from father, 1 from mother
What are chromatids?
Chromatids – are the newly copied DNA strands still joined to each other by a centromere
What are the processes in M - Phase(Mitosis + 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
What drives entry into mitosis?
M-Cdk drives entry into mitosis
Explain the trigger of M-Cdk.
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
What happens after M-cyclin/Cdk triggers 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 complex.
In late G2 the Cdc25 phosphatase is triggered to activate a positive feedback loop rapidly activating mitosis.
What causes Cdc25 to activated?
Possibly S-Cdk complexes
Once started +ve feedback will inhibit Wee1 and activate more Cdc25
What does APC (Anaphase-Promoting Complex) do?
Progression trough metaphase/anaphase transition driven by protein destruction
Contrast this to phosphorylation which used activate/inhibit proteins such as M-cdk
What is APC?
APC (anaphase-promoting complex) is a ubiquitin ligase with two targets.
What are the two targets of APC?
-
S+M cyclins: if these are 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. -
Securin: protects the protein linkages that hold sister chromatids together
Destruction - activates a protease that separates the sister chromatids anaphase
What can go wrong in mitosis?
- 2-hit hypothesis. Most genes need mutations on BOTH alleles to cause a phenotypic change.
- In the case of a tumour-supressor gene it would be any mutations that lead to inactivation of the proteins function. Eg Rb
- We call this change a Loss-of-heterozygosity
- It can be by several mechanisms – it doesn’t matter
- You can also get the loss of the allele – ie hemizygosity – so in effect you have one copy, if this is mutant then you may have a problem
What is chromosome non - disjunction?
The most obvious error is chromosomes ending up in the wrong daughter cell
eg lagging chromosomes during anaphase
This called CHROMOSOME NON-DISJUNCTION
Explain the basic structure of the mitotic spindle?
- Interpolar Microtubules: Overlap.
- Kinetocore Microtubules: attach to chromosomes at kinetochores (at centromeres).
- Astral Microtubules: Contact cell cortex to position the spindle.
- Centrosome: centriole surrounded by pericentriolar matrix. This act to nucleate microtubules.
What is a successful mitosis?
sister chromatids going to opposite poles, Trail and error is used to get 1 kinetochore to spindle pole.
How are inappropriate attachments sensed?
Answer = Tension
Correct attachment:
Kinetocores pulled in opposite directions
BUT sister chromatids resists -> tension.
Incorrect attachment
Tension is lower -> inhibitory signal -> loosens the microtubule attachment site
What does the activation of APC/C by Cdc20 leads to?
The activation of APC/C by Cdc20 leads to the ubiquitylation and destruction of securin.
Note:
Cdks can phosphorylate separase -> inhibition BUT
in anaphase -> cyclins destroyed
->Cdk inactivation -> less separase phosphorylation -> activation.
What is the difference between Anaphase A and anaphase B?
ANAPHASE A: kinetochore microtubules separate by shortening
In ANAPHASE B the ASTRAL microtubules pull the cells apart by motors and depolymerisation. The interpolar microtubules slide past each other.
It is in TELOPHASE that the nucleus (nuclear membrane etc) is reassembled and cytokinesis can begin
KEY POINT: This is complex and can go wrong
What can go wrong in mitosis? LOH by NONDISJUNCTION.
- Elimination by Apoptosis
- Loss of extra chromosome.
- Hemizygosity.
Explain hemizygosity?
Remember for most genes – it’s the loss of active copies that’s important when thinking about detrimental mutations
So by becoming HEMIZYGOUS, where you are left with the mutant allele, you effectively have inactivated a protein as no active protein can be made
Extra information
Explain LOH by Mitotic recombination?
- S-Phase chromosome replication.
- G2,M Mitotic recombination.
- End of Mitosis Chromatid segregation.
Suggestion - Look at slide 23 for better understanding.
Explain LOH by Gene conversion.
- DNA polymerase begins replication on template strand of chromosome B.
- DNA polymerase jumps to template strand of chromosome A (homologous.
- DNA polymerase copies some of A then jumps back to template strand of chromosome B.
