CBG Lecture 8: Mutation and Repair Flashcards
how are mutations in DNA introduced
spontaneous
introduced by mutagens: virus/UV/chemicals
name some mutagens
virus
uv
chemical
per how many bases does polymerisation make a mistake 1
10^4 or 10^5
of the bases that polymerisation makes a mistake on, how many does exonuclease proofreading fail to correct
1 in 100 of 10^4 mistakes
what is the overall error rate of polymerisation
10^-9 per nucleotide
give some examples of where mutations can arise
tautomery during replication/transcription mispairing of aa to tRNA by aaRS mutations in DNA by mutagens damage to mRNA eg. truncation misfolding of proteins by prions
how is /dna prone to mutations
oxidative deamination by free radicals
depurination - spontaneous hydrolysis
thymine dimerisation by UV light
how does thymine dimerisation occur
what does it make
by UV light
cyclobutane dimer
what happens to cytosine in oxidative deamination
how does it happen
cytosine + free radicals -> uracil
what happpens to a depurinated adenine
adenine + spontaneous hydrolysis -> hole
how is hole made
by spontaneous hydrolysis and depurination of adenine
how is thymine dimerised
what does it make
thymine is dimerised by UV light and makes a cyclobutane dimer
what do uncorrected lesions lead to
mutation
name a potent mutagen
5-bromouracil
in what way can smaller lesions be repaired
base excision by a glycosylase, endonuclease, polymerase, ligase
in what way can large lesions be repaired
nucleotide excisions
name subtypes for a mutation with point change in a single base
missense
nonsense
silent
what is a missense mutation
change in codon (normally second base) leads to change in aa
give an example of a missense mutation disease
glu -> vale
(GAG->GUG)
sickle cell anaemia (Hb)
give an example of a nonsense mutation
GGA -> UGA
gly -> STOP
chloride channel : cystic fibrosis
what is a nonsense mutation
change in codon, making it a STOP codon
what is a silent mutation
change in base, no change in aa
what is an indel mutation
insertion or deletion of base
what is a frameshift mutation
when an INDEL occurs causing the reading of the bases to no longer be read the same
what does a frameshift mutation often cause
protein truncation as 3/64 triplets will be STOP codons
why is it that many lesions are easily repaired
because DNA is double stranded
what does DNA glycosylase do? how?
flips out and breaks the glycosidic bond between DNA and uracil in a U:G lesion
what process is repair often coupled with? why
repair is often coupled to transcription, ensuring that the most used genes (those actively transcribed) are given the most attention by repair enzymes
why is it that daughter DNA strands are often distinguishable from the parente strand for a while after replication
because of the lag in post-replicative modification of the DNA eg. by methylation so the daughter strand (which is more likely to contain the error) will be more likely to be targeted for correction
what do prokaryotes use to deal with mistranscribed, truncated mRNA
tmRNA - transfer messenger RNA
what does tmRNA encode
has an mRNA like domain encoding the degradation tag ANDENYALAA
name some functions of tmRNA
deals with mistranscribed truncated mRNA
recycles the stalled ribopsome
adds a proteolysis inducing tag to the unfinished pp
facilitates the degradation of aberrant mRNA
at which two points in translation does fidelity need to be maintained
- ribosome decoding of codon to anticodon
2. tRNA aminoacylation of tRNA to amino acid
what is the effect of streptomycin
causes pyrimidines to be misread during translation
what is the effect of mupirocin
new antibiotic that inhibits IleRS in MRSA: inhibits aminoacyl synthetases
describe tRNAs
large and easily distinguished by aaRS
readily distinguishable by their anticodons
what are aaRS
alanyl-tRNA synthetase
helps attach the aa to the tRNA
have proofreading esterase
what is the error rate of tRNA
10^-4 per tRNA
what AS’s do aaRS’s have
aminoacylation active site - large enough to accommodate aa
esterase proofreading active site smaller
in what way is genetic code robust
the mutations that do make a difference to the encoded mutations very rarely replace a hydrophobic amino acid with a very polar one or vv
in what way is the genetic code redundant
several codons code for the same amino acid because the third base of the codon is usually irrelevant
what is the difference in binding eneryies between valine and isoleucine
difference in one CH2
difference in binding energy is 12kJmol-1
name 2 enzymes that have separate active sites for the process and and for proofreadin
DNAP
aaRS
why is it good for an enzyme to have proofreading performed by a separate active site
ensures the two processes are independent and that the probability of error is minimised since it will make the probability the product of two independent processes
name some confusing amino acid pairs
valine : isoleucine:leucine
alanine + glycine
aspartamine + glycine
serine and lysine
what happens to errors that do slip through
degradation mechanisms to fix or remove them
what do aaRSs do
catalyse the esterification of a specific amino acid to its compatible cognate tRNA
in which number base of a codon does a mutation make little difference to codon
first base
