JUMPING GENES
Genetic units that can move or be "transposed" within the genome are referred to as jumping genes. A mobile
genetic element, also known as a 'transposon' can become integrated at many different sites in the genome, either by moving from place to place or by producing copies of itself that can be inserted elsewhere in the genome Unlike other processes that reorganize DNA, transposition does not require extensive areas of homology between transposon and its destination site. Jumping genes were first discovered by Barbara McClintock during her studies on maize genetics (a discovery for which she was awarded the Noble Prize in 1983)
The simplest types are known as insertion sequences (S) typically consisting of some 700-1500 base pairs. It contains genes for those enzymes which are required for
its transposition and bounded at both ends by identical or very similar sequences of nucleotides in reversed
orientation known as inverted repeats (about 15-25 base pairs) which vary among IS elements giving each IS its own
characteristic inverted repeats. Between inverted repeats a gene which codes for enzyme transposase is present which is required for transposition.
Transposons, which consist of a central region containing extra genes, flanked on both sides by IS elements that are
identical or similar in sequence.
N Kleckner has suggested that a primordial transposon might arise by the modification of a gene encoding an
enzyme for the creation and repair of DNA breaks. All that would be needed is for the enzyme to develop a modest
degree of specificity, perhaps by recognizing a particular DNA sequence of six or eight nucleotide pairs. Such a sequence might occur by chance in inverted orientation on either side of the gene, creating a situation in which the
gene's product could interact with each of these flanking sequences. By "cutting and pasting" the DNA, this modified
enzyme could then transpose the entire unit to a new position in the genome. Such a unit would therefore behave as a primordial transposon. The wide distribution of transposable elements suggests that they have played a significant role in the evolution. One hypothesis is that they are natural tools for genetic engineering. Their ability to copy, transpose and rearrange other DNA sequences (e.g, antibiotic resistance genes) can be a benefit for the organisms carrying them. It is also believed that transposons have played an important role in plasmid evolution.
