Messenger RNA contains a triplet sequence of nitrogen bases which code for the specific amino acids, used to make polypeptide chains. Each of the sets of three bases is known as a codon or genetic code.
Translation starts with a chain initiation codon (start codon). The most common start codon is AUG which is read as methionine or, in bacteria, as formyl methionine. The three stop codons have been given names : UAG is amber, UGA is opal, sometimes also called umber and UAA is ochre. Stop codons are also called “termination” or “non-sense” codons.
Characteristics of Genetic Code
• Triplet code : Three adjacent nitrogen bases constitute a codon which specifies the placement of one amino acid in a polypeptide.
• Start signal: Polypeptide synthesis is signaled by AUG or methionine codon and GUG — Valine codon. They have dual function.
• Stop signal: Polypeptide chain termination is signaled by three termination codons — UAA, UAG and UGA. They do not specify any amino acid and are hence also called non-sense codon.
• Universal code : The genetic code is applicable universally i.e., the codon specifies the same amino acid from a virus to a tree or human being.
• Non-ambiguous codon : One codon specifies only one amino acid and not any other.