Module 5 - Mechanics of cell division

Question 1

What drives the cell cycle?

Answer 1

Phosphorylation of key proteins by cyclin-dependent Kinases (Cdks)

Occurs as the OH side chain of either serine, tyrosine, or threonine is phosphorylated

Question 2

What happens before mitosis?

Answer 2

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

Question 3

Centrosomes

Answer 3

Like DNA, they duplicate after phosphorylation by Cdks

Each cell gets a centrosome so they remain on polar ends of a dividing cell

Question 4

Microtubule formation: the three types and what they do

Answer 4

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

Question 5

Setting up the mitotic spindle

Answer 5

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

Question 6

What does M-Cdk do?

Answer 6

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

Question 7

Prophase

Answer 7

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

Question 8

What pushes the centrosomes apart?

Answer 8

The kinesin Eg5 cross-links MTs and pushes them apart

Question 9

Prometaphase

Answer 9

Uses Eg5 which stabilises microtubules by antiparallel interactions

The nuclear envelope disassembles in most species and in animal cells the nuclear lamina disassembles

Question 10

Monoastrol: what does it do and how is it used?

Answer 10

Inhibits Eg5

Used to block spindle formation

Question 11

The golgi in cell divsion

Answer 11

Fragments during prometaphase, givimg each daughtyer cell part of the golgi

Question 12

Prophase to metaphase transition

Answer 12

The nuclear envelope breaks down and the Golgi apparatus shuts down

Nuclear envelope dissembles
Nuclear lamina dissembles
Golgi apparatus dissembles
Secretion and endocytosis stop

Question 13

Kinetochore

Answer 13

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

Question 14

Kinetochore MT movement

Answer 14

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

Question 15

Kinetochore properties

Answer 15

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)

Question 16

What conditions prevent the entry into anaphase?

Answer 16

• 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

Question 17

The role of Mad2

Answer 17

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

Question 18

What happens if kinetochores are unattached/attached?

Answer 18

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

Question 19

APC

Answer 19

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

Question 20

Inhibiting Mad2

Answer 20

Results in cytokinesis without microtubules

Question 21

Stage duration

Answer 21

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

Question 22

Cohesins

Answer 22

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

Question 23

Aneuploid daughters

Answer 23

Occurs when anaphase is entered prematurely; cells are left with different numbers of chromosomes

Question 24

Anaphase

Answer 24

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)

Question 25

Cell cortex pulling force

Answer 25

MT growth and interpolar forces along with Dynein assistance (attaches to spindles and then is attracted to the negative end of the cell?)

Question 26

Telophase

Answer 26

The genome is already separated so the nuclear envelope and lamins reassemble, the Golgi apparatus reassembles, and secretion and endocytosis continue

Question 27

Cleavage furrow

Answer 27

Location signalled by actin and myosin ring and the central spindle recruits and activates proteins that signal contractile ring assembly

Question 28

End of cytokinesis

Answer 28

Actin and myosin are dynamic in this situation and the contractile ring reduces in size over time and eventually pinches off the cells

Question 29

Plant cell division

Answer 29

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)