DNA consists of two chainlike molecules (polynucleotides) that twist around each other to form the classic double-helix. The cell’s machinery forms polynucleotide chains by linking together four nucleotides. The nucleotides which are used to build DNA chains are adenosine (A), guanosine (G), cytidine (C), and thymidine (T). DNA houses the information required to make all the polypeptides used by the cell. The sequence of nucleotides in DNA strands (called a ‘gene’) specifies the sequence of amino acids in polypeptide chains.
Clearly a one-to-one relationship cannot exist between the four nucleotides of DNA and the twenty amino acids used to assemble polypeptides. The cell therefore uses groupings of three nucleotides (called ‘codons’) to specify twenty different amino acids. Each codon specifies an amino acid.
Because some codons are redundant, the amino acid sequence for a given polypeptide chain can be specified by several different nucleotide sequences. In fact, research has confirmed that the cell does not randomly make use of redundant codons to specify a particular amino acid in a polypeptide chain. Rather, there appears to be a delicate rationale behind codon usage in genes.
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