Lecture 2 - Eukaryotic Cell Division

Question 1

What happens if growth factors are not available during G1?

Answer 1

The cells enter a quiescent stage of the cycle called G0

Question 2

What happens if growth factors are available during G1?

Answer 2

Mitogens stimulate Cyclin expression and thus Cdk activity, thereby allowing cell cycle entry and progression

Question 3

Principal mitogen found in mammalian serum

Answer 3

PDGF. It is released from platelets during blood clotting. Potent mitogen for many different cell types.

Question 4

Give an overview of signal transduction pathway involving mitogens

Answer 4

1. Cell surface receptor is engaged by the mitogen ligand
2. Receptor becomes activated
3. This leads to activation of specific sets of intracellular proteins called signalling proteins
4. At one point, every pathway will diverge and the upstream protein will activate other enzyme

RELAY RACE

Question 5

Four distinct forms of signal transduction

Answer 5

1. Contact-dependent
2. Paracrine
3. Synaptic
4. Endocrine

Question 6

Manner in which mitogens and growth factors signal in

Answer 6

Paracrine. They are soluble factors that bind to cell surface receptors on target cells

Question 7

What can mitogens trigger?

Answer 7

1. Cell survival
2. Grow and divide
3. Differentiate
4. Apoptosis

Question 8

What does the protein Myc do?

Answer 8

1. It increases the expression of many delayed-response genes including some that lead to increased G1-Cdk activity
2. This triggers the phosphorylation of members of the Rb family of proteins

Question 9

What do the Rb family of proteins do?

Answer 9

They are important in restraining cell division in tissue

Question 10

What does the phosphorylation of Rb proteins cause?

Answer 10

1. Inactivates Rb proteins, which frees the gene regulatory protein E2F
2. This activates the transcription of G1/S genes, including the genes for a G1/S-cyclins and S-cyclin
   The resulting G1/S-Cdk and S-Cdk activities further enhance Rb protein phosphorylation, forming a positive feedback loop

Question 11

What do E2F proteins do?

Answer 11

1. Activate the transcription of G1/S genes
2. Also bind to specific DNA sequences in the promoters of a wide variety of genes
3. Also stimulate the transcription of their own genes, forming a positive feedback loop

Question 12

Role of p53 in stimulating cell cycle arrest in G1

Answer 12

1. When DNA is damaged, first kinase at the damage site is either ATM or ATR, depending on the type of damage.
2. Additional protein kinases , Chk1 and Chk2, are then recruited and activated
3. This results in the phosphorylation of the transcription regulatory protein p53
4. This blocks its binding to Mdm2, which normally binds to it and promotes its ubiquitylation and destruction in proteasomes
5. As a result, p53 accumulates to high levels and stimulates transcription of many genes, including the one for the CKI potein p21
6. p21 binds and inactivates G1/S-Cdk and S-Cdk complexes, arresting the cell in G1

Question 13

Overview of Apoptosis pathway induced by intrinsic cell damage

Answer 13

1. DNA damage is sensed causing signalling pathway to activate that results in the release of Cytochrome C from the mitochondria
2. Free Crytochrome C activates a protein called Apaf1 resulting in assembly of Apoptosome complex
3. Apoptosome complex then activates the proteolytic enzyme Caspase 9
4. Results in the subsequent cleavage and activation of downstream executioner caspases that cleave multiple substrates such as genomic DNA

Question 14

How do cells in G0/G1 phase pass the Restriction Point checkpoint to enter S phase?

Answer 14

1. Mitogens stimulate Ras -> MAPK pathway
2. This induces expression of the Myx transcription factor
3. Myx induces expression of G1 phase cyclin D
4. Cyclin D complexes with and activates