codons and mutations

how many possible codons are there?

64

what does ‘degenerate code’ mean?

amino acids can be encoded by more than one codon

different codons that code for the same amino acid are synonymous

what is a start codon?

signals start of translation for ribosome

start codon: AUG (in all cells)

AUG codes for methionine

what is a stop codon?

marks the carboxyl terminus of polypeptide chains

UAA, UAG, UGA are all stop codons

how do you find the reading frame of an mRNA strand?

look for long open reading frames (100+ codons)

look for Shine-Dalgarno sequence in prokaryotes

look for Kozak sequence in eukaryotes

what is the shine-dalgarno sequence?

(5’) AGGA …. (3’)

what is the Kozak sequence?

(5’) ACCAUGG (3’)

what are the different types of mutations?

• synonymous mutation

• missense mutation

• nonsense mutation

• frameshift mutation

what is a synonymous mutation

• one nucleotide is change, codon is changed

• new codon codes for same amino acid

• NO EFFECT on function of protein

what is a missense mutation?

• nucleotide substitution results in new codon for different amino acid

• different amino acid may have an effect on protein function

what is a nonsense mutation?

• nucleotide substitution results in stop codon

• protein sequence is terminated early

• most likely causes loss of protein function

what is a frameshift mutation?

• addition or subtraction of a nucleotide

• shifts the reading frame, changing codons

• primary structure is changed, likely to cause complete loss of function