Transposable  elements ("jumping
   genes")  are  DNA sequences that can move from one chromosome locus to
   another.
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<lI> Transposons  move by means of a DNA intermediate. First the DNA is
copied to a mobile fragment, which is inserted elsewhere.
<lI> Retrotransposons move by means of an RNA intermediate. The segment
is  first  trancribed to RNA, then reverse transcriptase makes the
DNA mobile fragment, followed by insertion.
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   Often this results in gene duplication.