mitosis

Question 1

how is the DNA packaged in mitosis

Answer 1

The cell has to rearrange the DNA into short, condensed chromosomes
These are then pulled apart in ANAPHASE

Question 2

Homologous Chromosomes definition

Answer 2

Homologous Chromosomes: Have the ‘same’ genes arranged in the same order. 1 Inherited from father, 1 from mother

Question 3

Chromatids definition

Answer 3

Chromatids – are the newly copied DNA strands still joined to each other by a centromere

Question 4

How can we study genes that are crucial for cell survival?

Answer 4

Genetic tricks allow identification of potentially lethal mutations:
Diploids can be used to maintain lethal mutations that are then studied as haploids
Temperature sensitive mutations allow growth at permissive temperature.

Question 5

Xenopus laevis as a biochemical model for the cell cycle

Advantages:

Answer 5

Easy to collect eggs
Rapid division rate (~30min)
Large size makes purification of proteins easier
Can be manipulated by injection of RNAs or chemicals into the oocyte

Question 6

M-Cdk role?

Answer 6

M-Cdk drives entry into mitosis

Question 7

M-Cdk must cause:

Answer 7

M-Cdk must cause:
¥ Assembly of the mitotic spindle
¥ Each sister chromatid is attached to an opposite pole
¥ Chromosome condensation
¥ Breakdown of the nuclear envelope
¥ Rearrangement of the actin cytoskeleton + Golgi

Question 8

M-Cdk role? what is it controlled by

Answer 8

M-Cdk drives entry into mitosis

M-cyclin/Cdk triggers entry into mitosis and is controlled by Cdc25 and wee1

Question 9

how is positive feedback used to activate mitosis?

Answer 9

In late G2 the Cdc25 phosphatase is triggered to activate a positive feedback loop rapidly activating mitosis.

Once started +ve feedback will inhibit Wee1 and activate more Cdc25

Question 10

What is APC? What is it driven by

Answer 10

APC (Anaphase-Promoting Complex)

Progression trough metaphase/anaphase transition driven by protein destruction

Question 11

What is Securin?

Answer 11

• Securin: protects the protein linkages that hold sister chromatids together

Question 12

What does the APC do ?

Answer 12

activates a protease that separates the sister chromatids APC also ubiquitinates S-cyclin and Securin

Question 13

what is Loss-of-heterozygosity

Answer 13

be any mutations that lead to inactivation of the proteins -function -cause a phenotypic change

Question 14

what is hemizygosity

Answer 14

the loss of the allele – ie hemizygosity – so in effect you have one copy, if this is mutant then you may have a problem

Question 15

What is CHROMOSOME NON-DISJUNCTION

Answer 15

The most obvious error is chromosomes ending up in the wrong daughter cell
eg lagging chromosomes during anaphase

Question 16

what are the 4 basic mitotic spindle structures ?

Answer 16

Interpolar Microtubules: Overlap.
Kinetocore Microtubules: attach to chromosomes at kinetochores (at centromeres)
Astral Microtubules:Contact cell cortex to position the spindle
Centrosome: centriole surrounded by pericentriolar matrix. This act to nucleate microtubules

Question 17

How are inappropriate spindle attachments sensed

Answer 17

Tension
Kinetocores pulled in opposite directions BUT sister chromatids resists tension

yet if inccorrect attachment

Tension is lower, inhibitory signal, loosens the microtubule attachment site