From:    Patrick O'Neil
To:      Joe Morlan                             93-12-10 18:57:04
Subject: (speciation == evolution)

  Transposon is the word refering to transposable elements which refers to
segments of DNA that move around within a genome by self splicing, moving to
another location in a genome, and inserting itself again.  Humans do not, as
yet, appear to have any active transposons within them though there is
evidence that there have been transposition events in the past (my own
personal feeling is that there will be found to be a few active elements in
humans).
  Some transposons have useful purposes, serving to improve genetic diversity
or regulate some genes.  Most have no apparent purpose and could be considered
genetic parasites (or just remnants of what was once a useful moveable element.
  The first transposons discovered were found in maize by Barbara McClintock,
Cold Spring Harbor Lab, in the 1950's.  She first refered to them as
"controlling elements" because they were first seen to inhibit expression of
other neighboring genes--in this case, this inhibition caused corn kernels of
varied color to be produced on the same ear of corn.
  Transposons, by and large, are very simple-they have LTRs (long terminal
repeats: repetitive DNA sequences at either end of the element that usually
contains a strong promoter which activates the interior genes and genes
immediatly downstream of the transposon) and a couple interior genes that
codes for enzymes that cut out and then reinsert the whole sequence.