PEPTIDE BIOSYNTHESIS. Peptide/protein = string of amino acids in specific sequence, specified by sequence of nucleotides in the gene (DNA). mRNA. DNA. DNA bases RNA basesRNA Codons amino acids A UUUU (phenylalanine) UUA (leucine) T A UGG (tryptophan)
= string of amino acids in specific sequence, specified by sequence of nucleotides in the gene (DNA)
DNA bases RNA basesRNA Codonsamino acids
A UUUU (phenylalanine)
T A UGG (tryptophan)
C GAUG (start)
……TAC, AAA, AAT, ACC, ATT…..
…..AUG, UUU, UUA, UGG, UAA…..
…start phenylalanine--leucine--tryptophan stop
Sequences of 3 bases in RNA code for a single amino acid
There are 64 possible ‘triplets’ that can be formed from the 4 different bases, but there are only 20 amino acids (AA)
In most cases, more than one type of triplet codes for a given AA
For example, CAA and CAG both code for the same AA, glutamine
A substitution of a G for the second A in CAA would constitute a neutral mutation, since the triplet would still code for glutamine (i.e., CAA becomes CAG). Thus, natural selection will not act directly to alter the frequency of the ‘mutant’ codon
Such neutral mutations may serve as useful genetic markers
Figure 15.7A, B