Bio102: Introduction to Cell Biology and Genetics

Genetic Code and Mutation

Key Terms:

inborn error of
metabolism / One Gene-One
Enzyme / Start and Stop
Codons / mutation / mutagen
metabolic pathway / wildtype (wt) / codon / missense mutation / carcinogen
minimal media / transcription / redundancy / nonsense mutation / Ames test
auxotroph / translation / reading frame / frameshift mutation / reversion
Central Dogma / genetic code / mRNA / silent mutation

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) à RNA à 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 à CAC)

Nonsense mutations change an amino acid codon to a stop codon (e.g. UAC à UAA)

Insertions or deletions cause a change in the reading frame (e.g. UAC à UA)

Silent mutations make no change in the protein (e.g. UAC à 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.