Mitosis

Question 1

Define chromosome and homologous chromosomes

Answer 1

-chromosome = linear DNA molecule
-homologous chromosome = have the ‘same’ genes arranged in same order

Question 2

Define centromere and chromatids

Answer 2

-centromere = region where spindle attaches
-chromatids = newly copied DNA strands still joined to each other by a centromere

Question 3

What drives entry into mitosis?

Answer 3

M-Cdk

Question 4

What occurs in the transition from G2 to M phase?

Answer 4

-triggers assembly of mitotic spindle
-each sister chromatid is attached to an opposite pole
-chromosome condensation
-breakdown of nuclear envelope
-rearrangement of actin cytoskeleton + Golgi

Question 5

How do the changes that happen in the transition from G2 to M phase occur?

Answer 5

-increase in cyclin B causes M-cyclin levels to increase through G2 + M phase
-creates a pool of inactive M-Cdk complex
-Cdc25 phosphatase is triggered in late G2 phase
-activates a +ive feedback loop, rapidly activating mitosis

Question 6

What is APC and what does it do?

Answer 6

-anaphase promoting complex
-its an ubiquitin ligase
-its destruction causes the progression from metaphase to anaphase

Question 7

What are the 2 targets of APC?

Answer 7

-S + M cyclins
-securin

Question 8

What happens if S and M cyclins are destroyed by APC?

Answer 8

-most CDKs are inactivated
-CDK targets are dephosphorylated

Question 9

What does securin do and what happens if it’s destroyed by APC?

Answer 9

-protects protein linkage that holds sister chromatids together
-destruction activates a protease that separates sister chromatids

Question 10

What can go wrong in mitosis?

Answer 10

-loss of heterozygosity - mutations on both alleles cause a phenotypic change
-loss of allele - only 1 copy, so if mutant, can cause a problem
-chromosome non-disjunction - chromosomes end up in wrong daughter cell

Question 11

Describe the basic structure of the mitotic spindle

Answer 11

-interpolar microtubules that bridge over each other in the middle of the cell
-kinetochore microtubules that attach to chromosomes at the kinetochores (centromere)
-astral microtubules that contact cell cortex to position of spindle
-centrosome - centriole surrounded by pericentriolar matrix

Question 12

How is inappropriate attachments of the spindle pole to a kinetochore sensed?

Answer 12

Sensed by incorrect tension

Question 13

What happens when there’s correct or incorrect attachment of spindle poles to the kinetochore?

Answer 13

-correct = kinetochores pulled in opposite directions but sister chromatids resist, creating tension
-incorrect = lower tension causes inhibitory signal which loosens microtubule attachment site

Question 14

What happens during anaphase A and anaphase B?

Answer 14

-A = kinetochore microtubules shorten, so chromatids dragged apart
-B = astral microtubules pull apart, causing poles to become further apart

Question 15

How can heterozygosity be lost in mitosis?

Answer 15

-non disjunction
-mitotic recombination
-gene conversion

Question 16

How can heterozygosity be lost by nondisjunction?

Answer 16

-cells with incorrect chromosomes should be eliminated by apoptosis
-can result in a loss of extra chromosome - 1 cell can have 2 mutant alleles
-by becoming hemizygous (1 chromosome) where you only have the mutant allele, no protein can be made

Question 17

How can heterozygosity be lost by gene conversion?

Answer 17

-DNA polymerase begins replication on template strand of chromosome B
-DNA polymerase jumps to template strand of chromosome A
-DNA polymerase copies some of A then jumps back to template strand of chromosome B
-mostly fine as long as copied part of A isn’t mutated