Lecture 5 Flashcards
What did Jeffrey Miller do to determine what different mutagens do?
Jeffrey Miller used the lac I gene of E coli and had a screen which could easily identify mutations within the gene as well as the nature of them. He checked the mutations caused by ethylmethylsulphonate, UV light and aflatoxin.
What do most mutagens cause, what are some examples?
Most mutagens increase the frequency of substitution mutations, but some result in indels. They work through a variety of chemical/molecular mechanisms such as ethylmethylsulphonate (GC to AT and TA to CG) UV light(majority GC-AT, but lots of others too) and aflatoxin(lots of GC-TA).
How does Ethylmethylsulphonate lead to mutations?
ethylmethylsulphonate does lots of GC to AT changes and TA to CG, very few others, ethyl group binds to oxygen on guanine or thymine, leading to only two hydrogen bonding sites and hence changing base pairing to GT and confusing base pairing.
How does aflatoxin B cause mutations?
aflatoxin B generates an apurinic site (sites without purine) by binding to the DNA backbone of Guanine (not at hydrogen pairing zone), this destabilises electron sharing removing the purine, this leaves a “blank” where the purine should be. which can lead to mutations as a base may be inserted opposite to this blank (typically adenine), changing the base pair sequence.
Why do apurinic sites lead to more DNA errors?
Most DNA in cells is made by high-fidelity DNA polymerases that have proof-reading activity, these can’t synthesise past “damaged” DNA such as apurinic sites, instead special error prone DNA polymerases do this, they are much more likely to incorporate the wrong base even when the DNA is not damaged.
What do all mutagens require and what is a way they can mutate besides, binding to amino acids or removing a pairing?
Some mutagens mimic normal bases and become incorporated into the DNA, later pairing with e wrong bases. All of these mechanisms require DNA replication to become mutations.
What is a pre mutagenic lesion and what does this mean for DNA repair?
A pre mutagenic lesion is a change to the DNA that may lead to a mutation, this needs at least one round of DNA replication is needed to become a mutation, DNA repair must occur before this occurs and the pre mutagenic lesion becomes a mutation and hence DNA repair systems repair the DNA before it is replicated.
How are ethylmethylsulphonate and apurinic site lesions repaired?
Ethylmethylsulphonate lesions are repaired by alkyltransferases which reverse the binding of the ethyl group. Apurinic sites is done by AP endonuclease, it recognises the site and cuts the DNA strand that contains it, defective and some adjacent DNA are then removed by excision enzymes and the gap is filled in by DNA synthesis ( a type of excision repair).
How does ultraviolet light cause mutations?
Radiation causes mutation, e.g ultraviolet light causes adjacent thymidine(typically) bases can become covalently cross linked, these fail to base pair properly during DNA replication and hence error prone DNA polymerase must take over for these parts.
Why are mutation mechanisms in bacteria important for humans?
DNA sequencing shows molecular mutation in humans is the same as bacteria, higher eukaryotes have equivalent repair systems as in bacteria.
