What are codons?
The genetic code is made up of a total of 64 base triplets or codons. At least one codon encodes the information for each of the 20 amino acids used in the synthesis of proteins during translation. While one codon can code for only one amino acid, more than one codon can code for the same amino acid, which is described as the degeneracy of the code. The majority of amino acids are encoded for by more than one codon.
START and STOP codons
AUG is the most common START codon, which signals the beginning of translation. It codes for the amino acid methionine (Met) and directs the addition of Met to the growing polypeptide chain during protein synthesis. Of the 64 codons, only 61 code for amino acids and the remaining three codons are STOP codons that signal the end or termination of translation. UAA, UAG, and UGA are the three RNA STOP codons and TAG, TAA and TGA are the three DNA stop codons.
Traditionally, the genetic code was represented by RNA codons, as it is messenger RNA (mRNA) that directs translation. Codons in the mRNA are decoded by transfer RNA (tRNA) during protein synthesis.
RNA codons and the amino acids they encode are tabulated below:
As a result of advances in genomics and computational technology, genes are mostly now discovered at the DNA level, before conversion to mRNA and proteins and it has become increasingly popular to use DNA codons. The DNA codons are identical to the RNA codons, except for the one base thymine (T), which replaces uracil (U) in the RNA codons.
The DNA codons and the amino acids they represent are tabulated below:
As can be seen from the above tables, most of the amino acids are encoded by multiple codons. Asn, Asp, Cys, Gln, Glu, His, Lys, Phe, and Tyr have two codons; Ile has three codons; Ala, Gly, Pro, Thr, and Val have four codons; and Arg, Leu, and Ser have six codons. Only two amino acids - Met and Trp – are encoded by a single codon each.
Reviewed by Sally Robertson, BSc