Protein control of cell division

Question 1

What does the cytoskeleton do?

Answer 1

Gives mechanical support and shape to cells.

Question 2

What does the cytoskeleton consist of?

Answer 2

Different protein structures including microtubules, which are found in all eukaryotic cells.

Question 3

What are microtubules?

Answer 3

Hollow cylinders composed of the protein tubulin. They radiate from the MTOC or the centrosome.

Question 4

What do microtubules do?

Answer 4

Control the movement of membrane-bound organelles and chromosomes.

Question 5

Cell division requires remodelling of the cytoskeleton, how is this done?

Answer 5

Formation and breakdown of microtubules involves polymerisation (growth) and depolymerisation (shrinkage) of tubulin.

Question 6

What do microtubules form and why?

Answer 6

Spindle fibres that are active during cell division.

Question 7

The cell cycle consists of?

Answer 7

Interphase and Mitotic (M) phase.

Question 8

Interphase?

Answer 8

Involves growth and DNA synthesis including G1 (growth phase); S phase (during which DNA is replicated) and G2 (further growth phase)

Question 9

What does the mitotic (M) phase involve?

Answer 9

Mitosis and cytokinesis.

Question 10

What happens in mitosis?
(Includes cytokinesis)

Answer 10

Chromosomal material is separated by spindle microtubules. This is followed by cytokinesis in which the cytoplasm is separated into 2 daughter cells.

Question 11

What does mitosis consist of?

Answer 11

Prophase, Metaphase, Anaphase and Telophase.

Question 12

What happens in prophase?

Answer 12

• DNA condenses into chromosomes each consisting of 2 sister chromatids.
• Nuclear membrane breaks down; spindle microtubules extend from the MTOC by polymerisation and attach to chromosomes via their kinetochores in the centromere region.

Question 13

What happens in metaphase?

Answer 13

Chromosomes are aligned at the metaphase plate.

Question 14

What happens in anaphase?

Answer 14

As spindle microtubules shorten by depolymerisation, sister chromatids are separated and the chromosomes are pulled to opposite poles.

Question 15

What happens in telophase?

Answer 15

• The chromosomes decondense and nuclear membranes are formed around them.
• Cytokinesis also takes place which involves the separation of the cytoplasm into 2 daughter cells.

Question 16

Progression through the cell cycle is controlled by?

Answer 16

Checkpoints

Question 17

What are checkpoints?

Answer 17

Mechanisms within the cell that assess the condition of the cell during the cell cycle and halt progression to the next phase until certain requirements are met.

Question 18

What is involved in regulating the cell cycle?

Answer 18

Cyclin proteins that accumulate during cell growth.

Question 19

How does progression through the cell cycle get controlled?

Answer 19

Cyclins combine with and activate CDKs. Active cyclin-CDK complexes phosphorylate proteins that regulate progression through the cycle. If sufficient phosphorylation is reached, progression occurs.

Question 20

G1 check point - what happens there to halt the cell cycle?

Answer 20

Rb acts as a tumour suppressor by inhibiting the transcription of genes that code for proteins needed for DNA replication.

Question 21

G1 check point - what happens there to continue the cell cycle?

Answer 21

Phosphorylation by G1 cyclin-CDK inhibits the Rb. This allows transcription of genes that code for proteins needed for DNA replication.

Question 22

What happens at the end of the G1 check point?

Answer 22

Cell cycle progresses from G1 to S phase.

Question 23

What happens at the G2 checkpoint?

Answer 23

Success of DNA replication and any damage to DNA is assessed.

Question 24

DNA damage triggers the activation of several proteins including p53 that can:

Answer 24

• Stimulate DNA repair
• Arrest the cell cycle
• Cause cell death (Apoptosis)

Question 25

What does the metaphase checkpoint control?

Answer 25

Progression from metaphase to anaphase

Question 26

What happens at the metaphase checkpoint?

Answer 26

Progression is halted until the chromosomes are aligned correctly on the metaphase plate and attached to the spindle microtubules.

Question 27

What does an uncontrolled reduction in the rate of the cell cycle cause?

Answer 27

Degenerative disease.

Question 28

What does an uncontrolled increase in the rate of the cell cycle cause?

Answer 28

Tumour formation.

Question 29

What is a photo-oncogene?

Answer 29

A normal gene usually involved in the control of cell growth or division which can mutate to form tumour-promoting oncogene.

Question 30

What is apoptosis triggered by?

Answer 30

Cell death signals that can be internal or external.

Question 31

Why is apoptosis essential during development of an organism?

Answer 31

To remove cells no longer required as development progresses or during metamorphosis.

Question 32

When might cells initiate apoptosis?

Answer 32

In the absence of growth factors.

Question 33

Example of external death signal?

Answer 33

Production of death signal molecules from lymphocytes.

Question 34

How do external death signal molecules work?

Answer 34

Bind to a surface receptor protein and trigger a protein cascade within the cytoplasm.

Question 35

Example of an internal death signal?

Answer 35

DNA damage.

Question 36

How does the internal death signal resulting from DNA damage work?

Answer 36

Causes activation of p53 tumour suppressor protein.

Question 37

What do both types of death signal result in?

Answer 37

Activation of caspases that cause destruction of the cell.