Cell Replication

Question 1

What is the cell cycle?

Answer 1

An orderly sequence of events which a cell duplicates its contents and divides in time

Question 2

At what rates do cell divide?

Answer 2

Embryonic (vs adult) = fast
Low complexity (vs high complexity) = fast
High turnover needed = fast
Terminally differentiated cells e.g. neurones = don't divide
Tumour cells = fast

Question 3

What are the different phases of the cell cycle?

Answer 3

Interphase: G1 and S (DNA replication) and G2
Mitosis: M , most vulnerable phase off the cycle. Cells more easily killed, DNA damage can’t be repaired, gene transcription silences

Question 4

What are things a cell must achieve within a cell cycle?

Answer 4

Replicate 3 billion bp
Double in size
Tear itself apart in a controlled fashion

Question 5

What is the G0 phase?

Answer 5

The quiescent phase
In the absence of a stimulus, cells go into this phase
Cells are not dormant however are non-dividing e.g. skeletal muscle, neurones

Question 6

What are cell cycle checkpoints?

Answer 6

Cells need to monitor the external environment e.g. nutrients
The cell may need to pause e.g. for DNA repair or to undergo apoptosis

Question 7

What are the different checkpoints for different phases?

Answer 7

G1: is the environment favourable?
G2: Is the DNA replicated? Is all DNA damage repaired?
M: Are all the chromosomes properly attached to the mitotic spindle?

Question 8

How do cells leave G0?

Answer 8

There is a signalling cascade
Growth factors stimulate entry from G0 to G1
There is a signal amplification and then signal integration/ modulation by other pathways

Question 9

What is the key role of C-MYC?

Answer 9

Growth factor signalling pathways induce the expression of C-MYC
C-MYC promotes G0 to G1 transition
C-MYC is an oncogene- its over expressed in many tumours

Question 10

What is the role of cyclin dependent kinases? (CDKs)

Answer 10

The stimulate the synthesis of genes needed for next phase

They are important in key signalling events

Serine/Theanine/Tyrosine can be phosphorylated

CDKs are present in proliferating cells throughout the cell cycle

They’re only active when a cyclin is bound

Question 11

What are the different cyclins that bind to CDKs?

Answer 11

Cyclin A, B, D and E
They’re expressed at different specific points in the cycle
They’re synthesised then degraded
Their concentration fluctuates within the cell cycle

Question 12

How are different cyclins activated?

Answer 12

1.Growth factor induces C-MYC
2.C-MYC induces cyclin D
3. Cyclin D binds to CDK 4/6
and stimulates expression of cyclin E
4. Cyclin E binds to CDK 2 and stimulated expression od cyclin A
5. Cyclin A binds to CDK2 and stimulates expression of cyclin B
6. Cyclin B binds to CDK1 and induces mitosis

Question 13

What is a protein kinase cascade?

Answer 13

Frequently the protein regulated by a kinase is another kinase and so on. This leads to:
signal amplification
diversification
opportunity for regulation

Question 14

What is cell cycle control based on?

Answer 14

Cyclically activated and expressed proteins e.g. CDKs

Question 15

How is CDK activity regulated?

Answer 15

By interaction with cyclins and phosphorylation

Phosphorylation and dephosphorylation control a lot of the activity of the kinases

Question 16

How is a CDK activated by phosphorylation?

Answer 16

1. Cyclin is produced and binds to CDK to give cyclin-CDK complex
   This complex is inactive
2. Variety of protein kinases will phosphorylate the CDK
3. Activating protein phosphatase will remove the inhibitory phosphate and lead to an active cyclin- CDK complex

Question 17

How does positive feedback drive the cell through the cell cycle?

Answer 17

1. Phosphorylation of M-CDK by CDK activating kinase (CAK) and CDK-inhibitory kinase (Wee1)
2. Removal of inhibitory phosphate by phosphatase gives active M-CDK which drives cell through M phase
3. Active M-CDK can phosphorylate cdc25 which activates production of more M-CDKs
4. This is positive feedback and drives cell from G2 to M phase

Question 18

What 3 steps are needed to drive CDK progression?

Answer 18

1. phosphorylation of CDK
2. Dephosphorylaton of CDK
3. Cyclin binding

Question 19

How are cyclins turned off?

Answer 19

Cyclins are turned off by ubiquitination
Cells stick ubiquitin molecules onto proteins it wants to degrade
Cyclin gets degraded into amino acids

Question 20

What is retinoblastoma (Rb) ?

Answer 20

A tumour suppressor and a molecular break
If Rb is inactive then there are problems with cell cycle progression
Its found in al nucleated cells

Question 21

How does retinoblastoma affect cell proliferation?

Answer 21

Active Rb sequesters a TF (transcription factor) in an active form
The TFs cannot turn on genes needed for cell cycle progression e.g. DNA polymerase

Question 22

How is retinoblastoma inactivated?

Answer 22

Activation of intracellular signalling leads to production of G1-CDK and G1/S-CDK complexes
They can phosphorylate Rb causing its inactivation and release of TF
Target genes such as DNA polymerase and thymidine kinase can now be activated

Question 23

What is the role of P53?

Answer 23

P53 arrests cells with damaged DNA in G1
X-rays induce a break in dsDNA
P53 recognises the break and activates protein kinases that phosphorylate p53 and activates it- P53 is stablised
Active P53 binds to regulatory region of P21 gene which leads to transcription of p21 mRNA

Question 24

What happens to P53 when its not needed?

Answer 24

P53 is degraded in absence of DNA damage

This means when damage is detected it can be very rapidly repaired

Question 25

What is the role of P21?

Answer 25

P21s are inhibitors of cyclin-CDK complexes

This breaks the cell cycle

Question 26

What are some examples of oncogenes?

Answer 26

EGFR/ HER2: mutationally activated and over expressed in breast cancers
Herceptin is used for treatment of HER2+ metastatic breast cancer

Ras: mutationally activated in many cancers

Cyclin D1: over expressed in 50% of breast cancers

C-MYC: over expressed in many tumours

Question 27

What are examples of tumour suppressors?

Answer 27

Rb: loss of function mutations in 80% of small cell lung cancers

P53: loss of function mutations in over 50% of all human cancers