Mitosis

Question 1

Give a basic description of the cell cycle.

Answer 1

Chromosomes replicate to produce sister chromatids. The nucleus breaks apart and replicated chromosomes condense in preparation for mitosis. The sister chromatids separate during mitosis and two cells are formed during cytokinesis.

Question 2

What are cyclins?

Answer 2

Proteins associated with the cycle of cell division.

Question 3

What do cyclins do?

Answer 3

They bind and activate cyclin dependent kinases (CDKs).

Question 4

What do CDKs do?

Answer 4

They phosphorylate key players in the cell cycle - they initiate DNA replication and phosphorylate lamins which cause nuclear envelope breakdown.

Question 5

How are cyclins regulated?

Answer 5

Destruction - they are targeted by ubiquitination.

Question 6

What is ubiquitin?

Answer 6

A small molecule that attaches to proteins to inactivate them.

Question 7

What are the features of mitosis?

Answer 7

It ensures the accurate partitioning of the genome to daughter cells and is the shortest stage of the cell cycle (30 minutes to 2 hours). It is dynamic and highly ordered and has 6 phases.

Question 8

What is S-phase?

Answer 8

All the DNA is replicated and it occurs only one time.

Question 9

What is M-phase?

Answer 9

When the copies from one nucleus are divided into two.

Question 10

What is proliferating cell nuclear antigen? (PCNA)

Answer 10

It is part of the DNA replication complex and has a speckled texture during S-phase.

Question 11

What are homologous chromosomes?

Answer 11

A pair of chromosomes - one paternal and one maternal.

Question 12

What are sister chromatids?

Answer 12

Two copies of replicated chromosomes - two paternal and two maternal.

Question 13

What are cohesin rings?

Answer 13

A protein complex that keeps the replicated sister chromatids together.

Question 14

Why does replication of DNA occur once and only once within the cell cycle?

Answer 14

The cyclin-CDK complexes are destroyed by anaphase promoting complex/cyclosome (APC/C).

Question 15

What does polarisation of microtubules form?

Answer 15

A bipolar spindle.

Question 16

When does polarisation of microtubules being?

Answer 16

S?G2 with the duplication of the centrosome.

Question 17

What is the other name for the centrosome?

Answer 17

The microtubule organising centre.

Question 18

What are centrosomes made up?

Answer 18

Centrioles and a mass of proteins called the pericentriolar material. This pericentriolar material surrounds the centrioles.

Question 19

What are centrioles?

Answer 19

Complex molecular scaffolds that form rings that act as nucleators of the microtubules.

Question 20

When are centrioles replicated?

Answer 20

S/G2.

Question 21

What are the 6 phases of mitosis?

Answer 21

Prophase, prometaphase, metaphase, anaphase, telophase and cytokinesis.

Question 22

What happens in prophase?

Answer 22

Chromosome condension - cohesin holds the chromatids together and condensins (similar structure to cohesin) that further loop chromatin into tight bundles. The centrosomes move apart and form the mitotic spindle.

Question 23

What happens in prometaphase?

Answer 23

The condensed chromosomes are attached to microtubules and the nuclear envelope breaks down.

Question 24

How does the nuclear envelope breakdown?

Answer 24

There is phosphorlyation of lamins by cyclin B-CDK1. Fragments form vesicles that contain lamin B and the nuclear pore complexes disassemble.

Question 25

What happens in metaphase?

Answer 25

Sister chromatids line up on the metaphase plate.

Question 26

What is dynamic instability of microtubules?

Answer 26

They grow slowly and shrink rapidly.

Question 27

What orients the mitotic spindle?

Answer 27

The physical force from the dynamic microtubules.

Question 28

What happens if orientation of the mitotic spindle goes wrong?

Answer 28

Cancer may result.

Question 29

What is kinetochore?

Answer 29

A protein complex linking chromatin and microtubules.

Question 30

What force pushes/pulls chromatids?

Answer 30

The force of microtubule dynamics.

Question 31

What do sensor proteins do?

Answer 31

They monitor whether microtubules are attached or not.

Question 32

What is the spindle assembly checkpoint?

Answer 32

The last checkpoint in which cells commit before dividing.

Question 33

What does the mitotic checkpoint complex do?

Answer 33

It inhibits anaphase promoting complex when kinetochores are exposed.

Question 34

What happens when all of the kinetochores are attached to spindle?

Answer 34

APC is released and activated which targets securin for degradation. Separate is free to cleave cohesion.

Question 35

How is the kinetochore system an example of negative feedback?

Answer 35

One exposed kinetochore stops APC and the signal is amplified through kinases.

Question 36

What happens in anaphase?

Answer 36

Cohesin is degraded and the sister chromatids move to opposite poles. This process is very fast.

Question 37

What happens in telophase?

Answer 37

The nuclear envelopes reform and assemble around individual chromosomes. Microtubules bundle and push the nuclei apart and contractile rings begin to form of the midline which will become the cleavage furrow.

Question 38

What happens in cytokinesis?

Answer 38

Contractile rings cinches and pinches and the actin-myosin fibres slide against one each other. The midbody forms at the scission point.

Question 39

What happens if there is failure of cytokinesis?

Answer 39

Binucleate cells.

Question 40

What destroys cyclin CDK complexes after the DNA is replicated?

Answer 40

APC/C

Question 41

What happens if Emi1 is removed from APC/C?

Answer 41

This protein activates the APC/C, so replication origins continue firing and the cell never enters M phase.