Mitosis

Question 1

Where does mitosis happen in the cell cycle?

Answer 1

Remember this is in M phase

Question 2

What attach to centromeres?

The centre of the chromosome

Answer 2

Spindle fibres attach here to pull apart sister chromatids

Question 3

What are homologous chrososomes?

Answer 3

They have the same genes (note this doesnt say alleles) arranged in the same order

These chromosomes are structurally similar

On chromosome come from the mother and one from the father

Question 4

So what do centromeres join together?

Answer 4

Chromatids

Question 5

What happens in the different stages of mitosis?

Answer 5

Prophase

• there is condensation of sister chromatids (this just means that they become more condensed (fatter)
• identical copies of the chromatids are made

Metaphase

• this is when there is attachment of mitotic spindles to the centromere’s kinetochore
• this is done by microtubules

Anaphase
- this is when there is the separation of sister chromatids to opposite poles

Telophase
- this is when the nuclear envelope reassembles, at the start of cytokinesis

Question 6

What does the decatenation checkpoint ensure? And what happens if this checkpoint is bypassed?

This is before the M phase in the cell cycle

Answer 6

This checkpoint is before M phase of the cell cycle. It makes sure chromosomes aren’t tangled.

The checkpoint DELAYS entry into mitosis if the chromosomes arent disentangled.

A bypass of the decatenation checkpoint causes chromosome breakage and nondisjunction during mitosis. This results in aneuploidy (abnormal numbers of chromosomes) and chromosome rearrangements in daughter cells.

Question 7

Another use of yeast…

What is the advantage of using yeast as model for the cell cycle?

Answer 7

Yeast can grow as haploid (having a single set of chromosomes)

Or as diploid (having a double set of chromosomes)

Yeast also have a rapid division rate of 1 hour

Cell cycle control genes are highly conserved thus the genes that control CDKs in humans are the same as in yeast.

Question 8

What ways can we use yeast to study mutations in the cell cycle?

Answer 8

Can make yeast temperature sensitive via mutations. And then investigate the effects of said mutations on CDK formation for example

Diploid yeasts can also be used to maintain lethal mutations that are then studied as haploids.

Question 9

What is the advantage of using xenopus to study the cell cycle?

Answer 9

Their eggs are easy to collect and they are big

They have a rapid division rate

Their large size means they can purify proteins more easily

They can be manipulated by injections of RNAs or chemicals in oocytes.

Question 10

How can we study the affect of proteins in the cell cycle?

Answer 10

You can deplete the cell cytoplasm of different proteins using antibodies

Or you can remove cytoplasm at different stages of the cell cycle

Question 11

What is triggered in late G2 of the cell cycle to activate mitosis?

What triggers this? And what does a positive feedback inhibit?

Answer 11

Cdc25 phosphatase in a positive feedback loop mechanism.

This is triggered by S phase cdk complexes

Once this is started a positive feedback loop inhibits Wee1 genes (which stop mitosis) and activate more Cdc25

Question 12

What is the anaphase promoting complex? And how does it become activate?

Note this says a complex

Answer 12

Thus this is a complex which sees the destruction of proteins

This allows for entry into anaphase

It is a ubiquitin ligase

It becomes active when a Cdc20 sub unit binds to it

Question 13

What two proteins are targeted by the anaphase promoting complex?
Note one of these protein targets is useful and one isnt.

Also how is the protein complex regulated?

Answer 13

Securin

• which is a protein linkage that holds sister chromatids together
• destruction of the proteins activate a protease that separates sister chromatids leading to anaphase

However the negative protein targets is S phase and M phase cyclins

• if these cyclins are destroyed it means cyclins in S phase and M phase
• this means that the CDK’s targets arent phosphorylated

The complex is regulated by:

• being on in G1 phase
• but being turned off in S phase to allow cyclin accumulation.
• so when its turned back on in M phase there are enough cyclins for it to work.

Question 14

Note we can get chromosome gene mutations in two ways which cause a PHENOTYPIC change, known as a two hit hypothesis.

What is a loss of heterozygocity mutation? And what is an example

And what is a hemizygosity mutations?

Answer 14

Heterozygosity mutation is when one allele is correct and one allele is incorrect causing a phenotypic change
- An example of this is with the tumour suppressor gene. A phenotypic change here is seen with inactivation of its proteins function EG RB

Hemizygosity mutations:
- is where you have one copy of an allele. If this is a mutant then you get a phenotypic change.

Question 15

How do heterozygosity mutations occur?

Answer 15

Chromosomes may end up in the wrong daughter cells

This is caused by the failure of chromosomes to split normally

Thus there is a lagging chromosome during anaphase

Thus chromosome is non disjunctional which means there is a failure for chromosomes to split normally

Question 16

What are the three types of mitotic spindle? And what is there structure?

Answer 16

Interpolar microtubules
- these overlap

Kinetochore microtubules
- these attach to chromosomes at kinetochores (the proteins found at the centromeres)

Astral microtubules
- contact cell cortex to position the spindle

Question 17

What is a centrosome?

Answer 17

This is a centriole - which is an organelle by the nucleus of the cell which helps to make microtubules

These are surrounded by the pericentriolar matrix

These nucleate microtubules.

Question 18

What is described as successful mitosis? Why is this considered a trail and error process?

Answer 18

When sister chromatids go to opposite poles

Is is a trail and error process to get 1 kinetochore to one spindle fibre

Question 19

What is correct attachment of centromere to spindle fibres and what does this cause?

Answer 19

Correct attachment of spindle fibres to the centromere causes tension

This tension is caused when kinetochore spindle fibres try to pull in opposite directions

But the sister chromatids resist which causes tension

Question 20

What causes incorrect attachment of spindle fibres to the centromere? And how is this overcome

Answer 20

This is when spindle fibres attach to the wrong position on centromeres

This decreases tension, especially when pulling the chromosomes apart

So then an inhibitory signal is sent

This loosens the microtubule attachment site.

Question 21

What enzyme does the anaphase promoting complex activate? And what negative effect of this complex poses a problem

Remember this is the complex which destroys the protein secruin which holds chromosomes together. This is activated when the Cdc20 protein binds to it.

Answer 21

The promoting complex can activate separase by phosphrylating it which break down the securin

Remember this APC also breaks down CDKs in metaphase

This causes CDK inactivation which means less separase phosphorylation occurs (remember from previous lectures you learnt Cdc20 is a CDK)

This means separase can’t work aswell

Question 22

What are the three ways is which hetrozygosity can occur?

Answer 22

By non disjunctional chromosomes (when chromsomes dont separate properly which can lead to cells with both allele copies being incorrect)

By mitotic recombination (this is when chromsomes accidentally swap out correct alleles_)

Cause by gene conversion (this is like mitotic recombination but DNA polymerase accidentally swaps out correct alleles for incorrect ones )

Question 23

How can hemizygosity occur?

Answer 23

This can happen due in instance of non disjunction

Remember this is when chromsomes dont separate properly

This means that 3 chromsome genes may end up in one cell

But in the other cell there may be only one chromosome gene

Which may be faulty

Question 24

How can loss of heterozygosity be caused by gene conversion?

Remember loss of heterozygosity means in the cell one chromosome has the corret gene allele and one has a mutant

Answer 24

DNA replication via polymerases may in rare instances jump between chromosomes during chromosome DNA replication

This means that the incorrect mutant alleles may be copied

This may cause cells to have a mutant allele and a correct allele

Question 25

How can loss of heterozygosity be caused by mitotic recombination?

Answer 25

This can happen between G2 and M phase

Chromatin can swap over in small pieces

In some instances genes may be swapped out to other chromsomes

So you may be left with a mutant chromosome and a correct one