Genetic code.
<P>
   The  3  bases  of  a codon are read in the 5' - 3' direction along the
   mRNA.
<P>
   The 64 codons specify 20 different amino acids.
<P>
   The  codon  AUG not only stands for the amino acid methionine (Met but
   also functions as a "start" signal to begin translating the mRNA.
<P>
   Three codons are "stop" signals, marking the end of a polypeptide.