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Bio102: Introduction to Cell Biology and Genetics
Genetic Code and Mutation
Key Terms:
inborn error of
metabolism
metabolic pathway
minimal media
auxotroph
Central Dogma
One Gene-One
Enzyme
wildtype (wt)
transcription
translation
genetic code
Start and Stop
Codons
codon
redundancy
reading frame
mRNA
mutation
mutagen
missense mutation
nonsense mutation
frameshift mutation
silent mutation
carcinogen
Ames test
reversion
Lecture Outline:
Archibald Garrod describes “inborn errors of metabolism”
blocks a metabolic pathway, causing accumulation of an intermediate in the pathway
Beadle and Tatum experiment
induced mutations in Neurospora crassa with x-rays
found mutants that were arginine (an amino acid) auxotrophs, so they needed added arg to live
some could live with ornithine or citrulline, precursors in the arginine metabolic pathway
deduced that a gene corresponds to one enzyme, thus the One Gene-One Enzyme Hypothesis
Central Dogma: information moves from DNA (gene)  RNA  Protein
Genetic Code describes which three-nucleotide codon on the mRNA corresponds to which amino acid
code is universal and redundant (so one amino acid may be encoded by >1 codon)
always starts with AUG codon (=Met amino acid)
each codon is then translated in-frame
ends with either UAA, UGA or UAG (which do NOT correspond to an amino acid)
Mutation is a change in the DNA. May effect the protein sequence.
Missense mutations change one amino acid (e.g. UAC  CAC)
Nonsense mutations change an amino acid codon to a stop codon (e.g. UAC  UAA)
Insertions or deletions cause a change in the reading frame (e.g. UAC  UA)
Silent mutations make no change in
the protein (e.g. UAC  UAU)
Ames test is used to find mutagenic
chemicals.
Measures mutation rate of a reversion
that allows Salmonella bacteria
to no longer by a histidine
auxotroph.