Cell cycle in Eukaryotes

Question 1

What is the cell cycle

Answer 1

1. Repeated process by which the cell duplicates itself and divides into two daughter cells
2. Continuous cell divisions since beginnings of life on earth
3. The cell cycle is controlled
4. embryos show co-ordinated waves of cell division

Question 2

What can the loss of cell cycle control result in

Answer 2

1. Loss of cell cycle control is a key step in cancer formation
2. Identifying regulators of the cell cycle is important:
3. tumour suppressors or proto-oncogenes
4. predict people at risk
5. potential drug targets

Question 3

What are the basic events in the cell cycle

Answer 3

1. Duplicate cytoplasmic components (cell growth)
2. Chromosome duplication
3. Chromosome segregation
4. Cytoplasmic segregation

Question 4

What happens in Prophase

Answer 4

1. The chromosomes begin to condense.
2. The centrosomes begin to assemble at the poles of the cell.
3. Nuclei starts to breakdown- no longer nuclear envelope

Question 5

What happens in Prometaphase

Answer 5

1. Microtubules from centrosomes start to project and radiate out into cells

Question 6

What happens in Metaphase

Answer 6

1. The chromosomes align along the plane that bisects the cell and become attached to the microtubule fibres of the mitotic spindle.

Question 7

What happens in Anaphase

Answer 7

1. Segregation occurs; the sister chromatids are pulled apart by the mitotic spindle to the two opposite poles of the cell.

Question 8

What happens in telophase

Answer 8

1. The chromatids de-condense and a new nuclear membrane forms around each new set of chromosomes.

Question 9

How is the cell cycle regulated

Answer 9

1. Controlled by checkpoints
2. If problem- there is a pause for it to be rectified or for cell to exit cell cycle and not divide
3. Checkpoints there to detect problem in External and Internal environment

Question 10

What happens in the first checkpoint

Answer 10

1. Check if favourable internal environment
2. Right number of organelles
3. Correct amount of duplication etc

Question 11

What is checked at the second checkpoint

Answer 11

1. All DNA needs to have been duplicated

Question 12

What is checked at the third checkpoint

Answer 12

1. Make sure anaphase is triggered at correct time

Question 13

What may happen without the first checkpoint

Answer 13

1. Start checkpoint
2. Inappropriate cell proliferation
3. lack of nutrients
4. → cell death
5. → malformed tissues
6. → cancer

Question 14

What may happen without the G2/M checkpoint

Answer 14

1. Division without complete DNA duplication
2. daughter cells lack full complement of chromosomes (aneuploidy)
3. → cell death
4. → cancer

Question 15

What may happen without the Metaphase/Anaphase transition checkpoint

Answer 15

1. Division with incomplete chromosome attachment
2. daughter cells with uneven amounts of DNA (aneuploidy)
3. → cell death
4. → cancer

Question 16

How is progression through the checkpoints controlled?

Answer 16

1. Cyclins and cyclin-dependent kinases
2. Cyclin-dependent kinases (cdk) phosphorylate target proteins
3. Cyclins are regulatory proteins which bind cdks and allow them to become active

Question 17

The level of what changes throughout the cell cycle

Answer 17

1. Cdk levels do not change throughout the cell cycle.
2. It is the levels of cyclins that go up or down; this in turn controls the activation status of cdks.
3. Transcription levels of many other genes are also controlled during the cell cycle.

Question 18

Describe levels of G1/S cyclins and complexes through progression of cell cycle

Answer 18

1. G1/S-cyclins levels rise during late G1 Phase and form G1/S-Cdk complexes.
2. The G1/S-Cdk complexes will reach high levels that that allow progression through the Start checkpoint.
3. G1/S-cyclins levels they start off low in G1, rise and remain high until the end of M phase where they drop to low levels again.

Question 19

Describe levels of S-cyclin and complexes through progression of cell cycle

Answer 19

1. S-cyclin levels start to rise following progression through the Start checkpoint.
2. S-Cdk complexes reach high levels at the start of S phase triggering downstream events.
3. S-cyclin levels begin to drop following the G2/M checkpoint and reach basal levels at the metaphase-anaphase transition checkpoint.

Question 20

Describe levels of m-cyclin and complexes through progression of cell cycle

Answer 20

1. M-cyclin levels begin to rise in G2.

2. M-Cdk complexes are formed but remain inactivated until the end of G2.

Question 21

What is APC/C

Answer 21

1. APC/C initiates cell progression through the metaphase-anaphase transition checkpoint.
2. Anaphase promoting complex

Question 22

How are Cdk levels regulated

Answer 22

1. Phosphorylation
2. Binding of inhibitory proteins
3. Proteolysis
4. Localisation
5. Feedback loops
6. Transcription

Question 23

How is cdk regulated by binding of inhibitory proteins

Answer 23

1. Cdk regulation- binding of inhibitory proteins
2. Bind to entire complex- cyclin-cdk complex
3. Not active if bound

Question 24

How is cdk regulated by phosphorylation

Answer 24

1. reaction in which a phosphate group is covalently coupled to another molecule
2. Cdk needs a kinase to add a phosphate group to allow it to activate
3. Other regions on cdk other phosphate groups can be added which can be inhibitory and put it in inactive state even with binding of cyclin and activating phosphate

Question 25

How is cdk regulated by proteolysis

Answer 25

1. Degradation of protein by hydrolysis at one or more of its peptide bonds 2. Ubiquitination- Ubiquitin tag proteins are transported to proteasomes where they are degraded 3. Can then be recycled 4. Ubiquitin ligase added ubiquitin onto cyclins 5. Cyclin is targeted for degradation not cdk 6. But if cyclin is degraded it cant bind to cyclin dependent partner- cdk

Question 26

How is cdk regulated by localisation

Answer 26

1. If proteins are in different local areas of cell can’t physically interact 2. E.g. nucleus and cytoplasm

Question 27

How is cdk regulated by feedback loops

Answer 27

1. Feedback and feed-forward networks are responsible for many of the complex biological behaviours we see in signalling. 2. Feedback is observed when the output from a given node follows a path of links that returns to regulate the node of origin. 3. Distinct from negative and positive regulation – regulation is a result of the pathway activity