Triplet genetic code.
<P>
   For   each   gene,   one  DNA  strand  functions  as  a  template  for
   transcription to produce mRNA.
<P>
   The base-pairing rules for DNA synthesis also guide transcription, but
   uracil (U) takes the place of thymine (T) in RNA.
<P>
   In  translation,  the  mRNA  is  read  in  the  5' - 3' direction as a
   sequence of base triplets, or codons.
<P>
   Each  codon  specifies  an  amino  acid  to  be  added  to the growing
   polypeptide chain.