7. mutations

Question 1

what is a mutagen

Answer 1

chemical that causes mutation

Question 2

examples of mutagens

Answer 2

environment: UV light, ionising radiation, chemical agents

Question 3

the effect of UV light on DNA

Answer 3

promotes formation of intrastrand thymine dimer distorting DNA

Question 4

the effect of chemical mutagen damage of DNA

Answer 4

Point mutation: transition (purine - another purine) and transversion (purine to something else)

Insertion/ deletion mutation

Question 5

What can nitrous acid cause and why

what substance is found in food that leads to this

Answer 5

transitions as it oxidatively deaminates aromatic primary amines

nitrate (nitrous acid’s conjugate base) found in meat preservatives as sodium nitrate to kill bacteria but converted to nitrate in stomach

cellular metabolism generates reactive oxygen species guanine to 8 oxylguanine binds to adenine

Question 6

what can alkylating agents cause

Answer 6

transversions

alkylation of purine N7- susceptible to hydrolysis

• loss of base
• gap filled by error-prone enzymatic repair replaced by prymidine

Question 7

how do instertion/ deletion mutations arise

Answer 7

intercalating agents

leading to distortion of DNA as distance between 2bp are doubled

they can also cause point mutations

Question 8

Benzo[a]pyrene

what is it

where is it found

what happens when the body tries to metabolise it

Answer 8

a polycylic aromatic hydrocarbon

found in cigarette smoke and dry heat cooking

caused by incomplete combustion of organic matter

• Covalently links to guanine nucleobase at N2
• This G “adduct” is misread as T
• Leads to G-C ► T-A

Question 9

what is a polar mutation

Answer 9

mutation of one gene that effects downstream genes/ operons

problem in polycistronic RNA

Question 10

The Ames Test

Answer 10

uses bacteria that cant make histidine

add mutagen and measure amount of histidine produced

Question 11

Mechanisms for DNA repair: direct reversal

Answer 11

1. pyrimidine dimers restored via photoreactivation catalysed by DNA photolayse (not in humans but in prok. and euk.)
   - splits the dimer by light excitaiton energy
2. alkyltransferase reverses base methylation caused by alkylating agents
   - remove methyl and ethyl group to own residues
   - humans have it (suicide enzymes)

Question 12

DNA repair: Base excision repair

Answer 12

bases removed and replaced

1.Glycosylases cleave the glycosidic bond of altered nucleotide

2. Leaves a deoxyribose residue with no attached base
   - Apurinic or apyrimidinic site
   - High toxic
3. The deoxyribose residue is cleaved on one side (5’) by AP endonuclease
4. The deoxyribose & adjacent nucleotides are removed by deoxyribose phosphate lyase
5. Gaps filled by DNA polymerase trims other side & ligase to seal it

Question 13

why are apurinic or apyrimidinic sites dangerous in base excision repair

Answer 13

cytotoxic

can irreversibly trap mammalion tropoisomerase I

lack glycosidic bond

• Can linearise & then cross-link to other cellular components
• So, remain tightly bound to glycosylase

Question 14

Nucleotide excision repair

Answer 14

corrects pyrimidine dimers & other lesions causing displacement of bases

• NER responds to helix distortions
• In humans: a major defense against carcinogens in tobacco smoke, sunlight

Question 15

Mismatch repair

Answer 15

if polymerase has not been accurate

preformed by Mut proteins

prok. recognise methylated strand (suggests correct strand)
euk. recognise fragments

Question 16

what do base excision repair and nucleotide excision repair act on and what is still susceptible

Answer 16

when lesion affects one strand

double stranded breaks

Question 17

what are double stranded breaks generated by

Answer 17

ionising radiation

free radicals

gene rearrangments

dividing cells can have 1 chromosome break

Question 18

what are the mechanisms the repair double stranded breaks

Answer 18

1) Nonhomologous End-Joining (NHEJ)
2) Recombination Repair

Question 19

Nonhomologous end joining

Answer 19

directly rejoins regions of DSBs

ends trimmed, filled and ligased

in euk. protein Ku is DNA sensor

nucleotide trimming can generate mutations (no use of template strands) but not as bad as DSBs

Question 20

Homologous end joining

Answer 20

DSBs may be non-mutagenically repaired through recombination

• Occurs via 2 Holliday Junctions
• DSBs are resected to produce single-stranded ends
• 3’ ends invades the other strand (mediated by Rad51)
• DNA polymerase fills in the gaps
• Ligase seals the joints