The transposase recognizes sequences at the inverted terminal repeats and excises the element at those sites. Ac produces functional transposase; Ds cannot produce the functional enzyme due to deletions. The inverted terminal repeats are the same in both Ac and Ds elements, and transposase recognizes them and transposes both Ac and Ds elements. If there is no Ac element in the the genome, no functional transposase is produced and thus no Ds element transposition takes place. To summarize,
1) Autonomous elements encode functional transposase; capable of self-transposition.
2) Nonautonomous elements cannot encode functional transposase; transpose only when an autonomous element is present.
When an Ac or Ds element inserts into a new site, the site is cut in staggered fashion; on either end of the cut site single-stranded portions of eight nucleotides are generated. DNA polymerase adds nucleotides to fill in the single-stranded DNA gaps. This leads to the creation of short direct repeats of eight nucleotide pairs in the DNA sequences that flank the inverted repeats of the element. Or, the direct repeats are created at the either ends of the inserted transposable element. During the next transposition event, when the transposase excises the transposable element, some of the repeated sequence is left behind as a “footprint” where the transposable element once resided.
This is the last post describing “The Ac-Ds Transposon System”.