Control of cell proliferation Wk2

Question 1

What is cell proliferation?

Answer 1

The process by which a cell grows and divides to produce 2 daughter cells. This is required in fertilisation (single cell zygote) and embryogenesis (growth and tissue formation - produces 10(13) cells) - had to be highly regulated or birth defects appear

Question 2

What is regulation 1 in cell proliferation?

Answer 2

Cell numbers must be controlled - highly regulated for proteins/growth factors

• Control involves cell communicaiton
• Growth factor signals and cell-cell contracts positively regulate cell proliferation e.g. if and when cells will divide

Question 3

What is regulation 2 in cell proliferation?

Answer 3

New cells must be identical - most cell division is symmetrical

• The processes of cell growth and division (e..g cell cycle) must be tightly controlled
• Cell cycle control involves “checkpoints” and feedback control
• Maintenance of genome integrity is crucial

Question 4

What are the stages of the cell cycle?

Answer 4

G0 = Cell will stay in this phase until an external signal stimulates them
G1 = Growth + preparation for DNA replication. This is just a period of growth
S = synthesis of DNA (replication)
G2 = Growth phase
M = Mitosis phase - chromosome segregation, cell division (PMAT)
C = cytokinesis - cytoplasmic division

Question 5

What’s the first checkpoint in cell cycle?

Answer 5

G1 = “Restriction Point” - no growth factors (G0)

• Ensures that cell is large enough to divide, that nutrients are available to support daughter cells.
• If a cell receives a go ahead signal point, it continues with cell cycle
• If the cell does not receive the go ahead signal, it exits the cell cycle and switches to a non-dividing state called G0
• Most cells in the human body are in G0

Question 6

What’s the second checkpoint in cell cycle?

Answer 6

G2 checkpoint
(Any damaged DNA)
- Ensures that DNA replication (S phase) has completely successfully

Question 7

What’s the third checkpoint in cell cycle?

Answer 7

Metaphase checkpoint
(Cell is in good position to split). Incomplete DNA replication/damaged DNA and spindle attachment
- Ensures that all of the chromosomes are attached to the mitotic spindle

Question 8

Give an overview of proliferation

Answer 8

• Only starts when appropriate signals are received: produced by adjacent cell, other place in body etc.
• Replication can be stopped at any of the control points
  = intact genome

Question 9

What helps in cell-cycle regulation?

Answer 9

• Cell cycle is regulated by enzymes
• Cyclin dependent kinases (Cdks)
  Regulate cell cycle checkpoint transitions
  Are themselves regulated by feedback
• Growth factors - G1Cdk

Question 10

What do kinases do?

Answer 10

An enzyme which acivates/deactivates a protein by phosphorylating them.

• Give the go ahead signals at G1 and G2 checkpoints
• The kinases that drive these checkpoints must themselves be activated
• Activating molecules are cyclins, proteins that cyclically fluctuate in concentration in the cell cycle. These kinases are cyclin-dependent kinases (Cdk)

Question 11

What do cyclins do?

Answer 11

• Cyclins accumulate during G1, S and G2 phases of the cell cycle
• By the G2 checkpoint, enough cyclin is available to form M-Cdk (“Maturation Promoting Factor”) complexes which initiates mitosis.
• Later in mitosis, M-Cdk switches itself off by initiating a process which leads to destruction of cyclin.
• Cdk persists within the cell as an inactive form until it associates with new cyclin molecules synthesised during interphase of the next round of the cell cycle
• Different cyclin - Cdks are required at different stages of the cell cycle

Question 12

Explain the retinoblastoma protein “restriction point”

Answer 12

• The restriction point is regulated by the retinoblastoma protein (pRB)
• In the absence of growth factors, pRB binds to transcription regulators of genes for cell proliferation, so preventing continuation of cell cycle.
• Growth factors target surface receptors activating Cdks which phosphorylate pRB causing it to release its binding of transcription factors.
• These activate genes for cell proliferation
• pRB is a tumour suppressor protein
• Loss of function mutation = loss of control and cell cycle continues

Question 13

Explain how p53 regulates DNA damage checkpoint (G1/S)

Answer 13

• DNA damages cause an increase in protein p53.
• This activates the protein p21, an inhibitor of Cdk (will not proceed with cell cycle)
• Prevents continuation into S phase, giving time for SNA repair
• If DNA damage is severe, p53 induces the cell to kill itself by apoptosis.
• p53 = tumour suppressor protein
• Loss of function leads to replication of damaged DNA = genome instability/resitance to apoptosis

Question 14

What are the consequences of checkpoint failure?

Answer 14

• Proliferation of cells in absence of growth factors
• Replication of damaged DNA
• Segregation of incompletely replicated chromosomes
• Divisions of cells with wrong number of chromosomes
• Genone instability = increased rate of mutation

Question 15

Explain signal/growth factor

Answer 15

1. Activates receptor
2. Ras protein (‘G-protein’ - GTP binding)
3. Kinase 1 activated (kinase cascade)
   4, Kinase 2 activated (kinase cascade)
4. Kinase 3 activated (kinase cascade)
5. Into nucleus
6. Gene regulatory proteins (transcription factors)
7. Response -expression of genes required for proliferation