Answer:
DNA: Three nucleotides encode an amino acid. Proteins are built from a basic set of 20 amino acids, but there are only four bases. Simple calculations show that a minimum of three bases is required to encode at least 20 amino acids.
RNA: You can think of the sequences of bases in the coding strand of DNA or in messenger RNA as coded instructions for building protein chains out of amino acids. There are 20 amino acids used in making proteins, but only four different bases to be used to code for them. Obviously one base can't code for one amino acid.
