7 - Mutation, mutagens and DNA repair

Question 1

what is a mutation

Answer 1

permanent change in the DNA of a cell

• change in gene position/number
• change in nucleotide sequence

Question 2

somatic tissue mutations

Answer 2

• not passed on from 1 generation to the next

- passes on to all cells descended from the original mutant

Question 3

germ-line mutation

Answer 3

passed on to offspring

Question 4

causes of mutations

Answer 4

spontaneous (through DNA replication) and induced

Question 5

examples of induced mutations

Answer 5

chemical:
- base analogues
- modifying agents
- intercalating agents
physical:
- ionising radiation
- UV

Question 6

spontaneous mutations

Answer 6

can occur due to inherent instabilities in DNA

• nucleotides can change to other conformations e.g. isomers and tautomers
• During DNA replication an incorrect base is inserted to form mismatched pair

Question 7

induced mutations

Answer 7

occur when an outside agent (mutagen) damages DNA

Question 8

what are the 3 classes of chemical agents that act on DNA

Answer 8

1. base analogues
2. modifying agents
3. intercalating agents

Question 9

base analogues

Answer 9

resemble baes, but pair incorrectly when incorporated into DNA
• 5 bromouracil is incorporated into DNA as though it were thymine
• Once incorporated it tends to rearrange into a form that resembles cytosine.
• Upon DNA replication, can result in a point mutation converting a AT bp to a GC bp

Question 10

Deaminating agents

Answer 10

chemicals that remove the amino group from adenine/cytosine
•DEAMINATION of C to U
• Replication of DNA containing deaminated C results in A being inserted
• Daughter strand base pairing changed from CG to TA.
• Occurs spontaneously at low rates
• Uracil not usually found in DNA
• Uracil DNA glycosylase has specific role in removing U from DNA to prevent mutations

Question 11

Alkylating agents

Answer 11

chemicals that add hydrocarbon to nucleotide bases
• Addition of an ethyl group at the O6 position alters the base-pairing characteristics
• Results in a point mutation converting a GC bp to an AT bp.

Question 12

intercalating agents

Answer 12

• Insert between bases and distort DNA helix
• Interfere with replication
• Tend to cause frameshift mutations
• E.g. ethidium bromide

Question 13

Ionising radiation

Answer 13

• ionises water in cells to produce reactive oxygen species - damages bases and breaks phosphodiester backbone of DNA
• Unrepaired DNA Double Strand Breaks (DSBs) are lethal to cells as they inevitably lead to loss of genetic material.
• Aberrant repair of DSBs can result in deletions, duplications inversions and translocations.
• Eukaryotic cells have evolved multiple mechanisms to repair double strand breaks.

Question 14

ultraviolet radiation

Answer 14

• UV irradiation is absorbed specifically by the pyrimidine bases cytosine and thymine.
• Covalent bonds can form between adjacent T or C nucleotides – pyrimidine dimers
• Blocks DNA synthesis leaving a gap opposite the site of damage. • Gap filled by special alternative polymerases, which are more error-prone as they don’t carry the same proofreading capabilities as the main DNA polymerases.

Question 15

nucleotide excision repair

Answer 15

• Broad specificity repair system - recognises distortions in the DNA helix.
• UV in sunlight causes mutations in skin cells.
• Individuals with the rare hereditary disorder called xeroderma pigmentosum are deficient in NER.
• XP characterised by development of skin cancer at an early age but only on those parts of body exposed to sun.

Question 16

Benzpyrene

Answer 16

• converted to benzpyrene diol epoxide in the body – potent mutagen and carcinogen
• benzpyrene is a pro-carcinogen

Question 17

Ames test

Answer 17

A procedure using bacteria to identify potential carcinogens