Mitosis

Question 1

What is a chromosome?

Answer 1

linear DNA molecule

Question 2

Centromere

Answer 2

region where spindle attaches

Question 3

Homologous chromosomes

Answer 3

‘same’ genes arranged in same order, 1 from father 1 from mother

Question 4

Chromatids

Answer 4

newly copied DNA strands still joined by a centromere

Question 5

Describe prophase

Answer 5

condensation of sister chromatids (identical copies)

Question 6

Describe metaphase

Answer 6

attachment of the mitotic spindle to the kinetochore by microtubules

Question 7

Describe anaphase

Answer 7

separation of sister chromatids to opposite poles

Question 8

Describe telophase

Answer 8

nuclear envelope reassembly, start of cytokinesis

Question 9

What does M-Cdk (or MPF) trigger?

Answer 9

Entry into mitosis:
Assembly of mitotic spindle (allow chromosomes to align + separate)
Each sister chromatid is attached to opposite pole
Chromosome condensation
Breakdown of the nuclear envelope
Rearrangement of the actin cytoskeleton + golgi

Question 10

Describe how M-cdk triggers into mitosis

Answer 10

1. M-cdk levels increase through G2 and M phase (due to increase in cyclin B expression, the cylcin that binds with M-Cdk)
2. Cak adds an activating phosphate to M-Cdk
3. Wee1 adds an inhibitory phosphate
4. M-cdk is inactive
5. Cdc25 removes the inhibitory phosphate from M-Cdk, activating it
6. This creates a positive feedback loop where Cdc25 is activated more and wee1 is inhibited

Question 11

What is the metaphase/anaphase transition driven by?

Answer 11

protein destruction

Question 12

Why is securin destroyed in the metaphase/anaphase transition?

Answer 12

Its destruction activates a protease that separates the sister chromatids = can now be pulled apart

Question 13

Describe the 2-hit hypothesis.

Answer 13

most genes need mutations on BOTH alleles to cause phenotypic change

Question 14

Describe a loss of heterozygosity (LOH)

Answer 14

change in chromosomes that is working towards homozygosity (2 mutant alleles)

Question 15

What is hemizygosity?

Answer 15

the loss of the allele = 1 mutant copy

Question 16

What is chromosome non-disjunction?

Answer 16

Chromosome ends up in wrong daughter cell e.g lagging chromosomes during anaphase

Question 17

Describe LOH by mitotic recombination

Answer 17

(G2, M phases)
→ associated with meiosis but happens here also
(pic)

Question 18

What are the 3 main structural components of the mitotic spindle?

Answer 18

Interpolar microtubules
Astral microtubules
Kinetochore microtubules

Question 19

Describe the function of interpolar microtubules in mitotic spindles

Answer 19

slide past each other to separate the chromatids

Question 20

Describe the function of astral microtubules in mitotic spindles

Answer 20

Contact cell cortex to position the spindle (anchor)

Question 21

Describe the function of kinetochore microtubules in mitotic spindles

Answer 21

attach to the chromosome at kinetochore (centromeres)

Question 22

describe how inappropriate attachment of the spindles are sensed

Answer 22

→ Tension
Correct attachment:
Kinetochores pulled in opposite directions
But, sister chromatids resist = tension

Incorrect attachment:
Tension is lower → inhibitory signal → loosens the microtubule attachment site

Question 23

Why is the destruction of securin neccessary to the cell cycle?

Answer 23

Securin inhibits separase (which cleaves the cohesin rings holding chromosomes together)

Securin must be depleted before anaphase to ensure chromosome segregation occurs with anaphase

Question 24

Describe what happens to the kinetochore microtubules in Anaphase A

Answer 24

Kinetochore microtubules shorten & start to pull chromatids apart

Question 25

Describe what happens to the astral tubules in Anaphase B

Answer 25

astral tubules slide over each other and finally separate the chromatids

Question 26

What is APC?

Answer 26

Anaphase-promoting complex used in the M/A transition

Question 27

What does APC destroy and why?

Answer 27

1. M cyclins = pushes cell out of anaphase
2. Securin = activates separase: separates the sister chromatids allowing them to move to opposite poles

Question 28

Describe why Cdks are needed in the cell cycle

Answer 28

Cdks are enyzymes, alone = inactive.
Once bound to a cyclin = active and can phosphorylate target proteins making them more/less active

Question 29

What are the key events of M phase and what are they driven by?

Answer 29

• Spindle forms
• Chromosomes condense
• Nuclear membrane breaks down
  = MPF complex drives this

Question 30

How does APC/C do its job?

Answer 30

It tags proteins with a polyubiquitin chain, marking it for degradation in the proteasome