Lecture 3 - Eukaryotic Cell Division Flashcards
What does Cyclin A-Cdk2 cause?
It triggers replication origin firing to initiate DNA replication during S phase
What happens at the G2/M phase checkpoint?
- Ensure that all of the chromosomes have been accurately replicated without mistakes or damage
- Cell size and energy/protein reserves are assessed
- Can sister chromatids separate correctly?
- Are all chromosomes attached to the mitotic spindle?
Between Interphase and Prophase
What happens at the Metaphase-Anaphase checkpoint? Also known as Spindle Assembly Checkpoint (SAC)
- Checks that conditions are OK to enter M phase
- Has DNA replication occurred correctly?
- Are preparations for M phase complete?
- Cell size
If yes, then the cell enters the late stages of M phase
Between Metaphase and Anaphase
How was the dominant M Phase Promoting Factor (MPF) found?
When M phase cells and cells in any other phase were mixed it was observed that the interphase cell would always start to segregate its chromosomes early (even if replication was incomplete). Both nuclei would enter mitosis synchronously
How does MPF work?
MPF promotes the entrance into mitosis (M phase) from the G2 phase by phosphorylating multiple proteins needed during mitosis
How is MPF controlled?
It is controlled by the periodic accumulation and degradation of Cyclin B during cell cycle progression
How does Cyclin B work?
Their levels gradually rise during G2 and then associates with the M phase Cdk, Cdk1
What does the mitotic spindle do?
It moves segregated sister chromatids to opposite ends of the dividing cell in Anaphase
What is the mitotic spindle composed of?
Microtubules - dynamic polymers of tubulin
Where do the microtubules form the mitotic spindle interact with the sister chromatids?
At protein complexes called Kinetochores
Where to the microtubules from the mitotic spindle interact with the spindle poles?
At the centrosomes
How are microtubules arranged?
They are composed of linear chains of tubulin dimers that are organized into a hollow cylinder
How are microtubules regulated?
- A rapidly growing microtubule has GTP-bound subunits at its plus end forming a ‘GTP-cap’ structure.
- New subunits can be added to this structure to promote MT growth from the + end
- If this ‘GTP cap’ is lost (due to GTP hydrolysis to GDP) then the subunits begin to disassemble and the MT shrinks
When do kinetochores form?
G2 phase
Where do kinetochores form?
At the centromeres of replicated chromosomes
Explain + and - ends of microtubules
+ ends project away from the spindle pole
- ends are anchored at the spindle poles by centrosomes
What do Interpolar microtubules do?
Interpolar microtubules from the two poles interdigitate (interlock like fingers) at the spindle equator
What do astral microtubules do?
They radiate out from the poles into the cytoplasm
What ensures correct bio-orientation of sister chromatids?
A trial and error mechanism
What does the SAC do until correct bio-orientation is achieved?
The SAC functions to inhibit progression into the later stages of mitosis
Why do replicated sister chromatids not separate from each other before Anaphase?
Adhesion of sister chromatids to each other prevents chromosome separation before Anaphase
Function of cohesin
Holds sister chromatids together
Two distinct mechanisms by which Cohesin is removed at the SAC
- Cohesin rings at the tips of the chromosomes are easily lost/removed without any enzymatic activity - they are loosely held together
- Cohesin rings at centromeric regions are very tightly bound and need to be removed by a proteolytic enzyme known as Separase
What is the function of Separase?
It removes cohesin rings at centromeric regions
How does the APC/C initiate sister crhomatid separation?
- Active M-phase cyclin B/cdk1 complexes initiate the assembly of the mitotic spindle and these MTs attach to sister chromatids via their kinetochores
- Active M-phase cyclin B/cdk1 complexes phosphorylate and activate the APC/C complex
- APC/C binds to Cdc20 protein
- APC/C induces ubiquitinylation and destruction of a protein known as Securin
- This allows Separase to cleave the remaining Cohesion complexes that are holding the sister chromatids together
What does Securin do?
Normally holds the proteolytic enzyme Separase in an inactive state during metaphase
What else does the APC/C result in during Anaphase?
Destruction of remaining cyclins (S phase or M phase) leaving the cell with little or no Cdk activity during Anaphase
