mutations

what is a mutation?

permanent change in the base sequence of a DNA molecule

what is a mutagen?

an agent capable of increasing the frequency of mutation

what are the two kinds of spontaneous mutation that can arise?

tautomerisation and indels

what is tautomerisation

a change to a base side chain with different hydrogen bonding

what is an indel mutation?

an insertion or deletion resulting from DNA slipping

what are the four ways mutagens can induce mutation?

affect on base pairing, mimicking DNA bases, absence of pairable base, change in base pairing conformation

what kind of mutation does EMS cause?

affect on base pairing

basic mechanism of EMS induced mutation

changes guanine conformation resulting in 2 hydrogen bonds instead of 3

what kind of mutation does 2-amino purine cause?

mimics DNA bases

basic mechanism of how 2-amino purine causes mutation

mistakenly gets incorporated in place of adenine but can change base pairing as it likes to pair with T or C

what kind of mutation does aflatoxin produce?

absence of pairable base

basic mechanism of how aflatoxin causes a mutation

forms apurinic sites that result in a ‘blank’ that a random base is inserted into

what is an apurinic site

a position where an adenine or guanine is lost from the DNA

what kind of mutation does UV light cause

change in base pairing conformation

mechanism for how UV light causes mutation

adjacent thymines turn into photodimers and translesion DNA polymerases skip over these

what are the two methods used for reducing the frequency of spontaneous mutations?

DNA proofreading and mismatch repair

how does DNA proofreading work?

DNA polymerase 1 and 3 have exonuclease function which can recognise when an incorrect base is added and remove it

what is a possibility if a mistake isn’t picked up in DNA proofreading?

wrong amino acid being coded for

what do organisms that don’t have DNA proofreading capabilities have?

a higher frequency of mutation

what are the advantages of mismatch repair

detects mismatched bases, determines which bases are wrong, can remove the wrong base

what are the factors involved in mismatch repair? (6)

MutS, MutH, MutL, DNA helicase UvrD, DNA polymerase 3 and ligase

what is the function of MutS in mismatch repair system

detects the mismatch through distortions in the newly synthesised DNA

what is the function of MutH in mismatch repair system

cuts the newly synthesised strand at the closest unmethylated point

how does MutH determine the difference between the new strand and the template strand (in MMR)

methylation

what is the role of DNA polymerase 3 and ligase in mismatch repair system

DNA polymerase 3 fills in the gap and ligase is used to seal the nicks to generate a continuous backbone

where does repair of mutagen-induced and spontaneous mutation have to occur?

pre-mutagenic lesion

what is the process for repairing damage caused by EMS?

alkylproteins allow transfer of alkyl group from the DNA base onto the protein

what is the process for repairing apurinic sites (caused by aflatoxin)

endonuclease removes the base-sugar bond at apurinic site, exonucleases remove the section containing the defective DNA, DNA synthesis to fill the gap

what are the two mechanisms for repairing photodimers?

photolyase enzyme and nucleotide excision repair

how is the photolyase enzyme used to repair photodimers?

it recognises and binds to the photodimer and is able to split the bond when exposed to white light

how is nucleotide excision repair used to repair photodimers?

pyrimidine dimers are induced, these are recognised and the DNA on either side is nicked allowing removal, DNA polymerase fills the gap and ligase seals the nick