Session 10 ILOs - DNA repair and cell cycle

Question 1

Describe the general outline of the different phases of the eukaryotic cell cycle: G1, S, G2 & M

Answer 1

G1 = Growth 1 - where the cell content duplicates itself
S = DNA replication - when the DNA replication occurs
G2 = Growth 2 - where the cell is double checked and repaired
M = Mitosis/Meiosis - where cell division/replication occurs

Question 2

Describe phase G0 in the cell cycle

Answer 2

G0 = resting phase

G0 phase comes off G1 stage and cells in this phase are not immediately preparing to divide. G0 can be a temporary or permanent phase depending on the cell type & properties

EXAMPLE: Some cells enter G0 temporarily until an external signal triggers the onset of G1 e.g. liver cells Whereas cells that never or rarely divide, such as mature cardiac muscle and nerve cells, remain in G0 permanently.

Question 3

Describe the variety of exogenous (external) and endogenous (internal) factors that influence the integrity of DNA (because DNA damage)

Answer 3

Exogenous (external) factors:
Ionising radiation
UV light
Mutagenic chemicals
Anti-cancer drugs
Alkylating agents
Free radicals

Endogenous (internal) factors (internal):
Free radicals
Replication errors

Question 4

Give examples of the types of DNA damage that can occur

Answer 4

1. Base change (apurinic site)
2. Dimers (additional chemical structures)
3. Interstrand cross links
4. Mismatch/insertion/deletion

Question 5

Describe in general terms what is meant by replication stress and the different ways in which it can be caused

Answer 5

Replication stress is the inefficient DNA replication thet leads to replication fork slowing, stalling or breakage

1. Replication machinery defects EXAMPLE: DNA polymerase mis-incorporation
2. Replication fork progression hindrance EXAMPLE: fork slippage (backwards or forwards)
3. Defects in response pathways EXAMPLE: regulation of origin firing

Question 6

Outline and explain how fork slippage causes mutations, including trinucleotide expansions leading to disorders such as Huntington’s disease

Answer 6

Fork slippage can be either backwards or forwards
Backwards slippage = leads to insertion mutation
Forwards slippage = leads to deletion mutation

Huntington’s disease: CAG repeats in the HTT gene, leads to polyglutamine repeats in the Huntington protein. This causes the mutant protein to aggregate in neurones because Huntington symptoms

Question 7

Explain and describe the DNA damage response

Answer 7

The DNA damage response includes the cellular pathways that sense, signal and repair DNA damage
Either DNA damage OR replication stress because because proteins to decide on the outcome, sensors, transducers & effectors decide one of 5 pathways:
1. Senescence - permanent cell cycle
2. Cell cycle transition - cell cycle should be stopped
3. Apoptosis
4. Transcription of genes needed
5. DNA repair needed

There are 3 outcomes of DNA damage response:

1. Senescence
2. Proliferation
3. Apoptosis

Question 8

Compare and describe in general terms, the DNA repair mechanisms base-excision repair, nucleotide-excision repair and mismatch repair

Answer 8

1. Base-excision repair
   Where the base has been converted, the wrong base is detected and removed, the sugar phosphate is removed and the new, correct nucleotide is added in
2. Nucleotide-excision repair
   Where there is a dimer (double base addition), the surrounding DNA is opened to form a bubble. The bubble is cut out, new undamaged DNA is synthesised (5’ -> 3’) and is inserted and fused by ligase
3. Mismatch repair
   A mismatch is identified in newly synthesised DNA and is cut out by exonuclease activity (including it’s neighbours), new undamaged DNA is synthesised (5’ -> 3’) and is inserted and fused by ligase

Question 9

Compare and describe in general terms, non-homologous end joining and homology-directed repair

Answer 9

Non-homologous directed repair is error prone and cuts out a section of DNA, ALWAYS leading to a mutation (however, sometimes this is the only option)
Homology-directed repair however uses the homology of other pair (homologous pair) to form the new DNA (perfect match!)

Question 10

Describe in general terms, what is meant by tumour heterogeneity

Answer 10

Means that tumours are not generally composed of all the same cell type, can have:

1. Intertumour heterogeneity = can have different tumours in different people
2. Intratumour heterogeneity = within a tumour, there can be multiple sub-clones

Question 11

Describe the complex relationship between DNA repair, mutations and cancer

Answer 11

DNA repair factors can repair mutations, which helps to prevent cancer from forming (stops pre-malignant cells) - however defects in DNA repair factors stimulate carcinogenesis. There are thousands of different DNA repair factors, which makes it more complex!
Using synthetic lethality strategies, you can use the cell’s DNA repair factors against it!

Question 12

Describe in general terms, what is meant by cancer evolution

Answer 12

Cancer evolution is where ‘early driver’ mutations, followed by mutation accumulation leads to malignant tumours and clonal expansion (multiple subclones) can lead to a heterogeneous tumour - made up of all the multicolour phases of tumours