Transposons.  The  Ac  sequence contains noncoding regions (Nc) and an
   open  reading  frame  (ORF)  which  encodes  a  transposase enzyme and
   enables the sequence to "jump". The Nc also contains inverted terminal
   repeats not shown here.
   Ds  elements  are  similar  to  Ac  but  contain a deletion within the
   transposase  gene,  making  them  dependent  on  the  Ac  element  for
   transposition.