The Genetic code | Code vs Codon | Triplet code system

When biologists began to suspect that the instructions for protein synthesis were encoded in DNA, they recognise a problem: There are only four nucleotide bases to specify 20 amino acids.If each kind of nucleotide base were translated into an amino acid, only four amino acids could be specified, one per nucleotide base. The two-nucleotide sequence AG, for example, could specify one amino acid, and GT could specify another. Since there are four possible nucleotide bases in each position, this would give us 16 (that is, 4 * 4, or 42) possible arrangements—still not enough to code for all 20 amino acids. Triplets of nucleotide bases are the smallest units of uniform length that can code for all the amino acids. If each arrangement of three consecutive nucleotide bases specifies an amino acid, there can be 64 (that is, 43) possible code words— more than enough to specify all the amino acids. Experiments have verified that the flow of information from gene to protein is based on a triplet code: The genetic instructions for a polypeptide chain are written in the DNA as a series of non-overlapping, three-nucleotide words. The series of words in a gene is transcribed into a complementary series of non overlapping, three-nucleotide words in mRNA, which is then translated into a chain of amino acids.