Cell Cycle

Question 1

What is the cell cycle?

Answer 1

The cell cycle is the orderly sequence of events in which a cell duplicates its content and divides in two.

Question 2

Is cell division rate uniform?

Answer 2

No.
Some cells are shed every 20 hours such as intestinal cells whereas others like hepatocytes can liver for years.
Some never divide like cardiac myocytes.

Question 3

What phases make up the cell cycle?

Answer 3

G1 phase -> S phase -> G2 phase -> Mitosis

Question 4

What phases comprise interphase?

Answer 4

G1 phase -> S phase -> G2 phase

Question 5

What is G0?

Answer 5

G0 is the quiescent phase where the cell has not entered the cell-cycle. This comprises most cells.

Question 6

What drives a cell to enter the cell cycle?

Answer 6

In the presence of appropriate nutrients, cells can respond to extracellular factors, growth factors, that stimulate entry from G0 to G1.

Signals can be amplified or modulated by other pathways.

Question 7

What causes the expression of c-Myc and what does it do?

Answer 7

Growth factor signalling pathways induce the expression of c-Myc, a transcription factor which stimulates the expression of cell cycle genes. c-Myc promotes G0 to G1 transition.

Question 8

What could overexpression of c-Myc cause, what does this make it?

Answer 8

Overexpression could cause tumour growth, making it an oncogene.

Question 9

What are CDKs?

Answer 9

Cyclin-dependant kinases.

Question 10

What do CDKs do?

Answer 10

Cdks are important in phosphorylation and dephosphorylation, which is key in signaling events by phosphorylating/dephosphorylating serine, threonine, or tyrosine residues. This allows for greater control of events.

Question 11

Where are CDKs found?

Answer 11

Cdks are present in proliferating cells but only active when a cyclin is bound. Their concentrations fluctuate/cycle during mitosis, hence the name cyclin.

Question 12

What does the phrase protein kinase cascades refer to?

Answer 12

The fact that proteins in the cell cycle are often regulated by a kinase, which itself is regulated by another kinase.

Question 13

What does the protein kinase cascade allow?

Answer 13

Signal amplification, diversification, and regulation.

Question 14

What controls the cell cycle?

Answer 14

Cyclically activated and expressed proteins.

Question 15

How are CDKs activated?

Answer 15

1. A cyclin is produced which binds to the CDK, forming an inactive cyclin-CDK complex.
2. Protein kinases (CDK activating kinase CAK and CDK inhibitory kinase Wee1) will phosphorylate the cyclin-CDK complex at both the inhibitory phosphate site and the activating phosphate site.
3. An activating protein phosphatase removes the inhibitory phosphate, activating the complex.

Question 16

What does the formation of the active cyclin-CDK complex lead to?

Answer 16

A positive feedback loop of more activating protein phosphatases causes an explosive increase in the number of inactive M-CDKs being activated.

Question 17

How can activated M-CDKs be turned off?

Answer 17

Ubiquitination of the cyclin-CDK complex destroys the cyclin, inactivating the complex.

Question 18

How do the cyclins give direction and timing to the cell cycle?

Answer 18

They are sequentially active and degraded.

Each cyclin stimulates the synthesis of the genes required for the next phase and the next cyclin.