Module 5 - Mechanics of cell division Flashcards
What drives the cell cycle?
Phosphorylation of key proteins by cyclin-dependent Kinases (Cdks)
Occurs as the OH side chain of either serine, tyrosine, or threonine is phosphorylated
What happens before mitosis?
The cell continues to grow in size and replicates its DNA
Cohesin rings are added when DNA is replicated and they hold sister chromatids together until anaphase
Centrosomes
Like DNA, they duplicate after phosphorylation by Cdks
Each cell gets a centrosome so they remain on polar ends of a dividing cell
Microtubule formation: the three types and what they do
Kinetochore MTs - have to find and attach to kinetochores (which will be attached to each chromatid) and must assemble and disassemble easily
Interpolar microtubules - growing microtubules must interact with other microtubules on the other end of the cell and form anti-parallel interactions (which will pull the cell apart)
Astral microtubules - involved in anaphase, dynamic
Setting up the mitotic spindle
As prophase begins and continues, more microtubules are nucleated by the centrosome
This is because mitotic kinase (M-Cdk) makes microtubules more dynamic which makes them also interact more
What does M-Cdk do?
Phosphorylates (activates) condensin and MT catastrophe proteins, causing chromosomes to condense and MTs to be more dynamic
Phosphorylates (inactivates) MAPs and nuclear lamins, making microtubules more dynamic and the nuclear envelope dissembles
Prophase
Chromosomes condense due to condensin and cohesin rings bind sister chromatids
The bipolar mitotic spindle begins formation and pole separation occurs due to anti-parallel interactions by each spindle
What pushes the centrosomes apart?
The kinesin Eg5 cross-links MTs and pushes them apart
Prometaphase
Uses Eg5 which stabilises microtubules by antiparallel interactions
The nuclear envelope disassembles in most species and in animal cells the nuclear lamina disassembles
Monoastrol: what does it do and how is it used?
Inhibits Eg5
Used to block spindle formation
The golgi in cell divsion
Fragments during prometaphase, givimg each daughtyer cell part of the golgi
Prophase to metaphase transition
The nuclear envelope breaks down and the Golgi apparatus shuts down
Nuclear envelope dissembles
Nuclear lamina dissembles
Golgi apparatus dissembles
Secretion and endocytosis stop
Kinetochore
Kinetochore: specialised protein structure that assembles to the centromere region of the chromosome in prophase
Dynein and kinesins at the kinetochore allow it to move along attached MTs
Kinetochore MT movement
The goal is for the two kinetochores on one chromosome to become attached to MTs coming from opposite poles
Co-ordinated behaviour of both kinetochores, but some competition generates tension
Microtubule assembly and disassembly is a major driver of chromosome movement
Kinetochore properties
Specialised chromosomal structure needed for spindle attachment
Binds multiple microtubules at once
Microtubule bundles attached to the kinetochore can switch between growing and shrinking in a regulated way
Kinetochores move in both directions along microtubules by harnessing MT assembly and disassembly, and using dynein and kinesins
Kinetochores properly attached to microtubules from both poles are under tension. Tension is needed before mitosis can proceed (next lecture)
What conditions prevent the entry into anaphase?
- Microtubules depolymerised (nocodazole)
- Microtubules stabilised (taxol)
- The spindle hasn’t assembled properly (e.g. inhibiting Eg5 with monastrol)
- If a single kinetochore is not attached to the spindle
The role of Mad2
A protein kinase that is needed for the metaphase checkpoint
It forms part of the spindle assembly checkpoint (SAC) complex that forms at the kinetochore
What happens if kinetochores are unattached/attached?
Unattached - Stop signal generated by the SAC complex, delaying anaphase by activating a complex that inhibits the anaphase-promoting complex (APC)
Attached - SAC proteins removed by cytoplasmic dyneins, anaphase commences
APC
Triggers proteolysis of specific proteins:
Covalently attaches Ubiquitin which tags protein to the proteasome for degradation - producing the cyclin subunit of M-Cdk and Securin
Inhibiting Mad2
Results in cytokinesis without microtubules
Stage duration
Most variable is prometaphase and the others are similar among all cell types
If the cell cannot rectify an issue preventing stage completion, apoptosis occurs
Cohesins
Stick the two sister chromatids together until anaphase
After anaphase cohesins are cleaved by separase after APC has broken down the inhibitory securin that inactivates separase
Aneuploid daughters
Occurs when anaphase is entered prematurely; cells are left with different numbers of chromosomes
Anaphase
Aa - Kinetochores shorten (from the plus end…), pulling sister chromatids to each pole of the cell
Ab - Kinetochores move further apart due to a pulling force at the cell cortex caused by interpolar MTs, this is further amplified by more MT growth (at the plus end)
Cell cortex pulling force
MT growth and interpolar forces along with Dynein assistance (attaches to spindles and then is attracted to the negative end of the cell?)
Telophase
The genome is already separated so the nuclear envelope and lamins reassemble, the Golgi apparatus reassembles, and secretion and endocytosis continue
Cleavage furrow
Location signalled by actin and myosin ring and the central spindle recruits and activates proteins that signal contractile ring assembly
End of cytokinesis
Actin and myosin are dynamic in this situation and the contractile ring reduces in size over time and eventually pinches off the cells
Plant cell division
No centrosomes or dyneins present - minus-end directed kinesins organise broad spindle fibres
Cytokinesis occurs by new membrane formation followed by cell wall formation (Golgi-derived vesicles control this, not actin and myosin)