Transposons in bacteria

Mobile genetic elements are classified according
to their effect and molecular structure:
simple insertion sequences (IS) and the more
complex transposons (Tn). A transposon contains
additional genes, e.g., for antibiotic resistance
in bacteria.
Transposition is a special type of recombination
by which a DNA segment of about 750 bp to
10kb is able to move from one position to
another, either on the same or on another DNA
molecule. The insertion occurs at an integration
site (1) and requires a break (2) with subsequent
integration (3). The sequences on
either side of the integrated segment at the integration
site are direct repeats. At both ends,
each IS element or transposon carries inverted
repeats whose lengths and base sequences are
characteristic for different IS and Tn elements.
The expression “direct” signifies that two copies
of a sequence are oriented in the same direction
(e.g., TTAG on each side of the integrated
transposon). Direct and inverted repeats are
evidence of the presence of amobile genetic element.
One E. coli cell contains on average
about ten copies of such sequences. They have
also been demonstrated in yeast, drosophila,
and other eukaryotic cells.