7. mutations Flashcards
what is a mutagen
chemical that causes mutation
examples of mutagens
environment: UV light, ionising radiation, chemical agents
the effect of UV light on DNA
promotes formation of intrastrand thymine dimer distorting DNA
the effect of chemical mutagen damage of DNA
Point mutation: transition (purine - another purine) and transversion (purine to something else)
Insertion/ deletion mutation
What can nitrous acid cause and why
what substance is found in food that leads to this
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
what can alkylating agents cause
transversions
alkylation of purine N7- susceptible to hydrolysis
- loss of base
- gap filled by error-prone enzymatic repair replaced by prymidine
how do instertion/ deletion mutations arise
intercalating agents
leading to distortion of DNA as distance between 2bp are doubled
they can also cause point mutations
Benzo[a]pyrene
what is it
where is it found
what happens when the body tries to metabolise it
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
what is a polar mutation
mutation of one gene that effects downstream genes/ operons
problem in polycistronic RNA
The Ames Test
uses bacteria that cant make histidine
add mutagen and measure amount of histidine produced
Mechanisms for DNA repair: direct reversal
- pyrimidine dimers restored via photoreactivation catalysed by DNA photolayse (not in humans but in prok. and euk.)
- splits the dimer by light excitaiton energy -
alkyltransferase reverses base methylation caused by alkylating agents
- remove methyl and ethyl group to own residues
- humans have it (suicide enzymes)
DNA repair: Base excision repair
bases removed and replaced
1.Glycosylases cleave the glycosidic bond of altered nucleotide
- Leaves a deoxyribose residue with no attached base
- Apurinic or apyrimidinic site
- High toxic - The deoxyribose residue is cleaved on one side (5’) by AP endonuclease
- The deoxyribose & adjacent nucleotides are removed by deoxyribose phosphate lyase
- Gaps filled by DNA polymerase trims other side & ligase to seal it
why are apurinic or apyrimidinic sites dangerous in base excision repair
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
Nucleotide excision repair
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
Mismatch repair
if polymerase has not been accurate
preformed by Mut proteins
prok. recognise methylated strand (suggests correct strand)
euk. recognise fragments
Homologous end joining
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
