4. Cell Division

Question 1

How does the rate of cell division vary from cell to cell?

Answer 1

• Embryonic vs adult cells (early embryo cells divide every 30 minutes)
• Complexity of system (e.g. yeast cell divides every 1.5-3 hours)
• Necessity for renewal (intestinal epithelium - every 20 hours, hepatocytes - every 1 year)
• State of differentiation (some cells never divide i.e. neurons and cardiac myocytes)
• Tumour cells have an inability to regulate the cell cycle

Question 2

What is the cell cycle?

Answer 2

Orderly sequence of events in which a cell duplicates its contents and divides into two.

Question 3

Briefly describe the eukaryotic cell cycle.

Answer 3

• Cells are normally resting in G0
• The mitosis itself happens very fast (roughly 5 mins relative to the 24 hour clock model) because it is a dangerous time for the cell
• When a cell decides to enter the cell cycle if they want to divide, they go into Gap phase 1 (G1)
• Following G1 is the S phase (synthesis) where duplication takes place
• Once all the duplication has taken place, it enters G2 (decision point) where the cell checks that everything is OK and ready to go into mitosis

Question 4

Describe the S phase.

Answer 4

Question 5

Describe the function and structure of the centrosome.

Answer 5

• Centrosome - an organelle near the nucleus of a cell which contains the centrioles, and from which the spindle fibres develop in cell division
• They are at 90 degrees to each other
• They are referred to as mother and daughter centrioles
• They regulate the microtubule network to orchestrate cell division
• The centrioles themselves are made of microtubules

Question 6

Describe the duplication of the centrosome and DNA.

Answer 6

• When the cell initiates duplication and enters the cell cycle, it needs to duplicate the centrosome
• In G1 phase there is separation of the motor and daughter centrioles (they are normally stuck together)
• When they separate they start to duplicate - the mother centriole will produce a daughter and the daughter centriole will produce a mother
• The duplication takes place in the S phase
• There is a cloud of protein complexes around them and there are points where they make nucleating sites for the microtubules
• When you put microtubules together it is called nucleation
• As the cell encounters a need for mitosis, the microtubules start to grow from these points and form an array of microtubules (looks like a sea urchin)

Question 7

Name the 6 phases of cell cycle.

Answer 7

1. Prophase
2. Prometaphase
3. Metaphase
4. Anaphase
5. Telophase
6. Cytokinesis

Question 8

Explain what happens at prophase.

Answer 8

• During the S phase the DNA has been duplicated and now it needs to be condensed in prophase
• They need to be condensed so that you can minimise DNA damage during mitosis
• The chromosome is held together by a centromere (acts like a belt) - it is a constriction around the chromosomes
• At the centromere there are a load of protein complexes that forms the kinetochore
• The kinetochore is a complex of proteins and that is a key regulator of the processes around segregation of chromosomes in the cell cycle
• So, the condensed chromosomes are present as a pair of sister chromatids
• The centrosome has been duplicated
• The microtubules are radiating away from the centrosome
• During late prophase, the nuclear envelope breaks down and by doing so, the chromosomes come out into the cytoplasm
• As the nuclear envelope breaks down, the centrosomes migrate to opposite sides
• They then begin to organise the spindle
• The spindle is like a highway that guides the chromosomes to where they have to go

Question 9

Describe condensation of chromatin.

Answer 9

• The double helices are wrapped around histones to forms ‘beads-on-a-string’ form of chromatin
• This compact the chromatin from being 2 nm wide to 11 nm wide
• The string is then further wrapped around itself to form 30 nm fibres
• The 30 nm fibres are then extended as a scaffold forming a chromosome scaffold - compacting it to a 300 nm wide fibre
• It is then further wrapped until you end up with a chromosome

Question 10

Describe the formation of the spindle.

Answer 10

• Radial microtubule arrays (asters) form around each centrosome
• The radial arrays from the two centrosomes meet in the middle (and form stable interactions)and when they meet each other they are then called polar microtubules
• These form highways telling the chromosomes which way to go

Question 11

Describe what happens at prometaphase.

Answer 11

Prometaphase can split into early and late prometaphase

• Early Prometaphase
  
  • ​Spindle formation largely
  • Attachment of chromosomes to spindles via kinetochores
  • One microtubule array will attach to the kinetochore on one side, and another microtubule array will attach on the other side
• Late Prometaphase
  
  • ​The sister chromatids have been captured by the microtubules arrays
  • Once captured, the chromosomes slide rapidly towards the middle of the cell (even if they were captured somewhere other than the middle of the cell)
  • In the kinetochores there are specialised proteins, which sense the attachment of microtubules e.g. CENP-E - this senses whether the kinetochore is attached to microtubules or not
    *

Question 12

What are the types of half-spindles?

Answer 12

• Kinetochore microtubules - bound to the kinetochore
• Polar microtubules - a microtubule that has met and connected with a microtubules from the other centrosome
• Astral microtubules - a microtubule that is originating from the centrosome that does not connect to a kinetochore

Question 13

Describe what happens during anaphase.

Answer 13

• Paired chromatids separate to form 2 daughter chromatids
• Cohesin is a protein complex that holds the sister chromatids tightly bound togther - needs to be dissolved
• Chromosomes (sister chromatids) are starting to split into 2 separate chromosomes
• Anaphase can be split into Anaphase A and B

Question 14

Describe what happens at telphase.

Answer 14

• Daughter chromosomes arrive at the pole
• Nuclear envelope reassembles at each pole
• The centrosomes are moved apart and the cells try to revert to their normal size
• There is a condensation of material where the cells are going to split and you get assembly of a contractile ring of actin and myosin filaments
• The contractile ring then squeezes the cell so that it divides into 2 daughter cells
• The cleavage furrow is where the cells are going to be cleaved

Question 15

Describe Cytokinesis

Answer 15

• This is the last phase of mitosis
• You get insertion of the new membrane at the cleavage furrow
• Midbody = where the actin-myosin ring is formed

Question 16

Describe the spindle assembly checkpoint.

Answer 16

• One of the checkpoints is when the cell wants to exit metaphase and enter anaphase - this is the spindle assembly checkpoint (aka anaphase checkpoint)
• This checkpoint sense the completion of chromosome alignment and also checks for spindle assembly
• The kinetochore has proteins that emit a signal when the kinetochore is NOT attached to microtubules
• Once the kinetochore attaches to microtubules, it stops emitting the signal
• At metaphase, you’re hoping that all of the all the kinetochores will stop sending signals so that they can proceed to anaphase
• There are many proteins involved in this signalling process but two important ones are:
  
  • CENP-E
  • BUB Protein Kinase
    
    • BUBs dissociate from the kinetochore when chromatids are properly attached to the spindle
    • They then go on to signal progression to anaphase

Question 17

What is the consequence of a mitotic checkpoint defect

Answer 17

This happens if anaphase initiates before the spindles attach properly

It results in abnormal division of the chromosomes between the daughter cells

Question 18

Describe how the microtubules may misattach to the kinetochore.

Answer 18

• Normally, you have a chromosome made up of 2 chromatids
• There is normal attachment, where a microtubule array one centosome is attached to the kinetochore of one sister chromatid, and the microtubules array from another centrosome is attached to the kinetochore of the other chromatid
• This type of normal attachment will allow the sister chromatids to be split apart and go to opposite poles
• Syntelic Attachment - both the kinetochores are hooked by 2 microtubules arrays from the SAME centrosome
• Merotelic Attachment - there is more than one microtubule array attached to the same kinetochore- this means that one of the chromatids is being pulled in 2 different directions
• Monotelic Attachment - only one of the kinetochores of one chromatid is attached to a microtubule array, the other kinetochore is unattached.

Question 19

Explain what aberrant centrosomes do?

Answer 19

If the centrosomes are not duplicated properly you could end up with 4 centrosomes in one cell

This can lead to very abnormal attachment of the microtubule arrays to the kinetochores leading to abnormal cytokinesis

Question 20

Explain how anti-cancer therapy induces gross chromosome mis-segregations

Answer 20

• You can slow down cancer by inhibiting the proliferation of tumour cells
• One mechanism of cancer therapy is exploiting checkpoint control
• The kinetochore signalling tells the cell when metaphase is complete so if you have an inhibitor for this checkpoint, then you make the nucleus think that it is correctly hooked onto microtubules
• This allows the cells to proceed into anaphase
• By altering the microtubule dynamics you can cause long-term mitotic arrest
• If you keep the cells in this disorganised and vulnerable position for a prolonged perioud, they are more easily killed.

Question 21

Describe the cell cycle checkpoints and tumour progression.

Answer 21

• The first checkpoint is during G1 - checkpoint for whether the cell should enter the cell cycle.
• Then the next checkpoint is just before mitosis, to check for DNA damage before entering mitosis
• There there is also a metaphase-anaphase checkpoint
• Tumours develop by means in which they can bypass these checkpoints

Question 22

What happens do the G0 phase during tumorigenesis?

Answer 22

• Normally, when cells come out of G1 they enter G0 (during this phase they are still being very active)
• Tumours block the ability of the cells to leave the cell cycle and enter G0 - once they finish mitosis, they enter another cell cycle
  *

Question 23

Explain how a cell begins to divide?

Answer 23