Which one of the following pairs of codons is correctly matched with their function or the signal for the particular amino acid?
(1) GUU, GCU – Alanine
(2) UAG, UGA – Stop
(3) AUG, ACG – Start/methionine
(4) UUA, UCA – Leucine
(2) The group of nucleotides that specifies one amino acid is a code word or codon. The nucleotides of mRNA are arranged as a linear sequence of codons, each codon consisting of three successive nitrogenous bases. Three codons UAG, UAA and UGA are the chain stop or termination codons. They do not code for any of the amino acids.
In most organisms AUG codon is the start or initiation codon, ie, the polypeptide chain starts either with methionine or N-formylmethionine.
Leucine – UUA, UUG, CUU, CUC, CUA, CUG
Alanine – GUC, GCC, GCA, GCG
GUU – Valine
UCA – Serine