The genetic code

The information on DNA is encoded as triplets of bases, called codons. Each triplet can code for one amino acid in a polypeptide chain. So for example, if GCA codes for the amino acid alanine and TAC codes for glycine, then the code GCAGCATACGCA would code for a polypeptide with the sequence ala-ala-gly-ala. As there are over twenty different amino acids in nature, a triplet code allows coding of up to 64 amino acids. Such a code is termed redundant and in reality each amino acid is coded for by several different codes. This minimises mutation rates as, for example, if GCC also codes for alanine, then a mutation from CA to CC would have no effect on primary structure. Each of the codons is translated in sequence as the code is non-overlapping, but first the genetic information must be transcribed, and then transferred out of the nucleus. It is transferred as an RNA molecule, a single-stranded polynucleotide containing the base uracil instead of thymine, and the five-carbon sugar ribose.