Cell Division

Question 1

What is contact inhibition of cell growth?

Answer 1

Cells grow normally by detecting neighbouring cells

Question 2

What happens during the three stages of interphase?

Answer 2

G1 – the cell makes sure that it has everything that is necessary for duplication
S – DNA replication, protein synthesis and replication of organelles
G2 – the cell checks that everything is ready to enter mitosis

Question 3

What are centrosomes and what do they consist of?

Answer 3

Centrosomes are organelles near the nucleus of a cell, which contain centrioles (mother and daughter), and from which spindle fibres develop in cell division

Question 4

What happens to the centrosomes during G1 and S?

Answer 4

The mother and daughter centrioles separate in G1
Then the mother produces another daughter and the daughter produces another mother, resulting in the formation of 2 centrosomes (the duplication takes place during S phase)

Question 5

What are the points around the centrosome from which microtubules arise?

Answer 5

Nucleating sites

NOTE: nucleation is the assembly of microtubules

Question 6

Describe the condensation of chromatin that takes place duringprophase.

Answer 6

The double helices wrap around histones to form beads-on-a-string
This is then further compacted to form 30 nm fibres
It is then compacted to form a chromosome scaffold and then further wrapped until you get a chromosome (1400 nm)

Question 7

What is a kinetochore?

Answer 7

Protein complexes that are attached to each sister chromatid – they are important in detecting the attachment of microtubules

Question 8

Describe the arrangement of centrosomes at the end of prophase.

Answer 8

They are on opposite sides of the nucleus

Question 9

What is formed when microtubule arrays from the two centrosomes meet in the middle?

Answer 9

Polar microtubules

Question 10

What happens to the sister chromatids as soon as they are captured by microtubule arrays from both centrosomes?

Answer 10

They slide towards the middle of the cell

Question 11

What are the three main types of half-spindle?

Answer 11

Kinetochore microtubule – attached to kinetochores
Polar microtubule – attached to a microtubule array from the other centrosome
Astral microtubule – microtubule originating from a centrosome that does not connect to a kinetochore

Question 12

What keeps the sister chromatids stuck together?

Answer 12

Cohesin (protein complex)

Question 13

What happens in anaphase A?

Answer 13

Cohesin is broken down and the microtubules get shorter so the chromatids start moving towards their respective poles

Question 14

What happens in anaphase B?

Answer 14

Daughter chromatids continue to migrate towards the poles

The centrosomes migrate apart

Question 15

Describe what happens in telophase.

Answer 15

Daughter chromatids arrive at the pole and the nuclear envelope reassembles
Assembly of a contractile ring of actin and myosin filaments where the cells are going to split
The contractile ring squeezes the cell so it divides into two daughter cells
NOTE: cleavage furrow = where the cell is going to be cleaved

Question 16

What is the midbody?

Answer 16

The point where the actin-myosin contractile ring is formed

Question 17

Describe how the spindle assembly checkpoint works.

Answer 17

The kinetochore has proteins that emit a signal when the kinetochore is
NOT attached to microtubules
When a microtubule attaches to the kinetochore, it stops emitting the signal
At the end of metaphase, you want all the kinetochores to stop sending signals before you can proceed to anaphase

Question 18

Name two proteins that allow the kinetochores to detect spindle attachment.

Answer 18

CENP-E
BUB-protein kinase (this dissociates from the kinetochores when the chromatids are properly attached to the spindle – then they go on to signal progression to anaphase)

Question 19

What can cause aneuploidy?

Answer 19

Mitotic checkpoint defect  
Mis-attachment of the spindles (so chromatids end up at different poles to the ones that they should be at) 
Aberrant mitosis (production of an abnormal number of centrosomes leads to abnormal division of the chromatids, and abnormal cytokinesis)

Question 20

Name four different types of spindle attachment.

Answer 20

Amphitelic – normal spindle attachment
Syntelic – both kinetochores of the sister chromatids are attached to spindles from one centrosome
Merotelic – one kinetochore of one of the sister chromatids is attached to spindles from both centrosomes
Monotelic – one kinetochore of one of the sister chromatids is attached to a spindle, the other is unattached

Question 21

Broadly speaking, explain how cell cycle checkpoints can be a target for anti-cancer therapy.

Answer 21

By targeting the cell cycle checkpoints, the cancer cells can be arrested in mitosis
Cells are very vulnerable when they are in mitosis and are more easily killed

Question 22

Give an example of anti-cancer drugs that target cell cycle checkpoints.

Answer 22

Taxanes and Vinca alkaloids

These alter microtubule dynamics and produce unattached kinetochores, which leads to long-term microtubule arrest

Question 23

What can happen to cells that are held up at a checkpoint?

Answer 23

They can undergo DNA repair and then proceed through the cell cycle
If the damage is irreparable, they can undergo apoptosis

Question 24

Where are the main checkpoints within the cell cycle?

Answer 24

During G1
Just before mitosis to check for DNA damage
Metaphase-anaphase checkpoint (spindle assembly checkpoint)