7. DNA damage and repair

Question 1

With which type of cancer is ethanol particularly associated?

Answer 1

Breast

Question 2

Recall an example of an occupational carcinogen

Answer 2

Tar smoke in laying roads

Question 3

Recall 2 medical examples of carcinogens

Answer 3

X Rays

Chemotherapy

Question 4

Recall the 4 main mechanisms of DNA damage

Answer 4

1. DNA adducts
2. Base dimers + chemical crosslinks
3. Strand breaks
4. Base hydroxylation

Question 5

What is the most common class of environmental pollutants? Recall how they are produced

Answer 5

Polycyclic aromatic hydrocarbons - produced by burning fossil fuels + tobacco

Question 6

Summarise the normal metabolism of B[a]P

Answer 6

2 step oxidation:

1. CYP450 –> epoxide
2. Epoxide hydrolase –> 2 unreactive alcohol molecules

Question 7

Recall where the metabolism of B[a]P most frequently is dysfunctional

Answer 7

Production of diol epoxide = hugely reactive with DNA to produce adduct

Question 8

Why are unstable products of metabolism attracted to DNA?

Answer 8

Since it is nitrogenous = good source of electrons to stabilise molecule by binding with it

Question 9

Where is faulty B[a]P metabolism most likely to cause cancer?

Answer 9

Anywhere in body

Question 10

Recall three examples of polycyclic aromatic hydrocarbons

Answer 10

1. Benzo[a]pyrene
2. Aflatoxin B1
3. 2-napthylamine

Question 11

How is aflatoxin B1 formed and where is it most commonly found?

Answer 11

Aspergillus flavus mould

Poorly stored grains and peanuts

Question 12

Where is a DNA adduct caused by aflatoxin B1 most likely to cause cancer?

Answer 12

Liver

Question 13

Where is a DNA adduct caused by 2-napthylamine most likely to cause cancer?

Answer 13

Bladder

Question 14

Recall the normal metabolism of 2-napthylamine

Answer 14

1. CYP1A2 –> reactive metabolite

2. Detoxified by glucuronidation (sugar addition)

Question 15

Why is 2-napthylamine particularly associated with bladder cancer?

Answer 15

Acidic urine pH removes the sugar from the metabolite of 2-napthylamine, thus toxifying it

Question 16

How does solar radiation cause damage?

Answer 16

UV light reacts with thymine to produce a crosslink, when the cell tries to repair this it introduces a mutation

Question 17

How does ionising radiation lead to DNA damage?

Answer 17

Generates superoxide/ hydroxyl free radicals –> very reactive with DNA

Question 18

What are the consequences of oxygen radicals?

Answer 18

1. Double and single strand breaks
2. Apurinic and apyrimidinic sites
3. Base modification

Question 19

What is the normal function of p53

Answer 19

TSG that detects and responds to DNA damage:

Mild damage –> repair, overwhelming damage –> apoptosis

Question 20

How is P53 activated?

Answer 20

Released from MDM2

Question 21

Recall the 4 main mechanisms for DNA repair

Answer 21

1. Direct reversal of DNA damage
2. Base excision repair
3. Nucleotide excision repair
4. During or post-replication repair

Question 22

Give an example of direct reversal of DNA damage

Answer 22

Pyrimidine dimers produced by UVB exposure can be fixed by a family of enzymes called photolyases

Question 23

What are the main enzymes involved in identifying sites for base-excision repair

Answer 23

DNA glycoylases

Apurinic/ apyrimidinic endonucleases

Question 24

Summarise the mechanism of nucleotide excision repair

Answer 24

XP proteins assemble at damage site and stretch of nucleotides either side of adduct is cut out, repair polymerases fill gap

Question 25

How might a mutation be fixed in the DNA?

Answer 25

Wrong base re-inserted during DNA repair

Question 26

Recall the 5 steps of testing for potential damage of a chemcial/ drug

Answer 26

1. Structural alerts
2. In vitro bacterial
3. In vitro mammalian
4. In vivo mammalian
5. Investigation of in vivo mammalian

Question 27

How is in vitro bacterial testing of a chemical for potential to damage DNA carried out?

Answer 27

Ames test:
GM bacteria to not be able to produce histidine
Incubate with rat liver enzymes
Mix with active chemical
How many colonies produced? Cannot grow without histidine so all growth is due to mutation that permits histidine production

Question 28

What is the purpose of in vitro mammalian testing of a chemical for potential to damage DNA?

Answer 28

Adds complexity of chromosome: may protect of potentiate the chemical’s ability to damage DNA

Question 29

What is the most common way to carry out in vivo mammalian testing of a chemical for potential to damage DNA?

Answer 29

Bone marrow micronucleus test in mice

Question 30

How is in vivo mammalian testing of a chemical for potential to damage DNA carried out?

Answer 30

Lifetime bioassay of a mammal - looking for epigenetic effects