From the chemical point of view, proteins are linear chains…

From the chemical point of view, proteins are linear chains of amino acids.  The sequence of amino acids can be written as a string of letters matching nucleic acid sequences using the 4 bases.  More than one nucleotide code word is required to code for all 20 amino acids of cell biology.  At least how many different two-letter sequence code words could be made using 4 different base pair letters?