S10 DNA Repair And Cancer

Question 1

What happens if a cell recognises damaged DNA?

Answer 1

DNA repair mechanisms work to fix the DNA

However if these mechanisms don’t work properly, mutation occurs

Question 2

What happens if a cell doesn’t recognise damaged DNA?

Answer 2

Mutation occurs

Question 3

What are 5 exogenous sources of DNA damage?

Answer 3

1. Ionising radiation
2. Alkylating agents
3. Mutagenic chemicals
4. Anti-cancer drugs
5. Free radicals

Question 4

What are two endogenous sources of DNA damage?

Answer 4

1. Replication errors

2. Free radicals

Question 5

How can DNA be damaged?

Answer 5

1. Apurinic site (base missing)
2. Deamination (amino group removed)
3. Mismatches
4. Pyrimidine dimer (bases on same strand bind)
5. Double strand breaks
6. Intercalating agent (chemicals fixing between strands)
7. Interstrand crosslink (prevents unwinding of DNA)
8. Bulky adduct (cancer causing chemical bound to DNA)

Question 6

What is DNA replication stress?

Answer 6

Inefficient replication leads to replication fork slowing, stalling and breakage

Question 7

What 3 things can lead to replication stress?

Answer 7

1. Replication machinery defects
2. Replication fork progression hindrance
3. Defects in response pathways

Question 8

Give some examples of replication machinery defects.

Answer 8

Things could go wrong with:

1. DNA polymerase
2. DNA helicase
3. Topoisomerase (breaks DNA to remove unwinding tension and then repairs)

Question 9

What happens if the wrong base is incorporated into a strand?

Answer 9

The mismatch is removed by 3’ to 5’ exonucleases an then DNA polymerase continues as normal

Question 10

What replication fork progression hindrance issues could arise?

Answer 10

1. Ribonucleotide incorporation
2. Transcription/DNA-RNA hybrids
3. DNA lesions
4. Fragile sites/oncogene-induced stress
5. DNA secondary structure
6. Repetitive DNA

Question 11

What can repetitive DNA lead to?

Answer 11

Fork slippage

Question 12

What are the two fork slippage scenarios?

Answer 12

1. New strand loops out, an extra nucleotide is added to new stand (due to slow DNA polymerase) - backward slippage
2. Template strand loops out, new strand is missing a nucleotide (due to fast DNA polymerase) - forward slippage

Question 13

Can fork slippage be slippage of a base triplet? What is it called?

Answer 13

Yes - trinucleotide repeat disorder/expansion

Question 14

What is an example of a trinucleotide repeat disorder?

Answer 14

Huntington’s disease

Question 15

What is Huntington’s disease?

Answer 15

Occurs due to extra CAG repeats (35-121 rather than 6-39) in HTT gene, meaning the protein doesn’t fold properly so aggregates in the neurones and leads to neurone degeneration.

The normal HTT gene protein function is unknown.

Question 16

What defects in response pathways can occur?

Answer 16

Each of the replication fork hindrance issues have response pathways however things can go wrong with these.

Question 17

What is the DNA damage response pathway?

Answer 17

1. DNA damage occurs due to replication stress
2. Sensors pick up signals
3. Transducers send signals to effectors
4. Effectors: senescence (permanent cell cycle arrest), cell apoptosis, transcription, DNA repair or cell cycle transitions occur
5. Outcomes are: proliferation (repair DNA and maintain function) apoptosis or senescence (the last two if DNA damage levels are very high/persist)

Question 18

Where are the cell cycle checkpoints?

Answer 18

G1 checkpoint - as enter S phase

G2 checkpoint - as enter mitosis

Mitosis checkpoint - around metaphase

Question 19

What does the G1 checkpoint check?

Answer 19

Is the environment favourable?

Question 20

What does the G2 checkpoint check?

Answer 20

Is all the DNA replicated? Is all the DNA damage repaired?

Question 21

What does the mitosis checkpoint check?

Answer 21

Are all chromosomes attached to the mitotic spindles?

Question 22

What is base excision repair?

Answer 22

Detection of incorrect base, incorrect base is removed and the hole is filled with the correct base by DNA polymerase and sealed with ligase.

Question 23

What is an example of when base excision repair is needed?

Answer 23

When deamination occurs converting cytosine to uracil

Question 24

What is nucleotide excision repair?

Answer 24

Detection of incorrect/affected nucleotide, surrounding DNA and affected part opens forming a ‘bubble’ which is cut out by enzymes and DNA polymerase then replaces the cut out DNA and ligase seals the backbone.

Question 25

What is an example of when nucleotide excision repair is needed?

Answer 25

When UV radiation produces a thymine dimer (two adjacent thymine’s chemically bonded)

Question 26

What is mismatch repair?

Answer 26

A mismatch is detected, the new DNA strand is cut and the misplaced nucleotide and neighbours are removed, the DNA is replaced with correct nucleotides by DNA polymerase and DNA ligase seals the backbone.

Question 27

How is a double strand break repaired?

Answer 27

1. Homologous-directed repair - use info of the sister chromatid to repair (preferred way - less chance of mutations)
2. Non-homologous end joining - rejoined with ligases

Question 28

What stimulates carcinogenesis?

What prevents carcinogenesis?

Answer 28

Replication stress

DNA repair mechanisms

Question 29

What is intracellular-tumour heterogeneity?

What does this mean for cancer treatment?

Answer 29

There are many sub clones within a tumour (different cell types).

Need to target with multiple drugs to have an effect on all the different sub clones.

Question 30

What are synthetic lethality strategies?

Answer 30

You can block a specific pathway that leads to cell death