Transposable Genetic Elements
Transposable elements are special DNA sequences ranging upwards in length from several hundred base pairs. These sequences can spread within a cell by being copied into new DNA locations as well as within a population through infection, transformation, transduction, or conju-gation.
The phenomenon of transposing elements was first described in maize in the elegant genetic studies of McClintock. General interest in the subject was then stimulated by genetic and physical studies with MIEscherichia coli. The development of genetic engineering made the detection, characterization, and study of transposing elements much easier than before. Such elements have now been found in all organisms that have been carefully examined.
The cell is an ideal home for a parasitic sequence of nucleotides. There it should enjoy almost the same treatment as an integrated lambda phage genome. Furthermore, the copying of such a sequence into a new chromosomal location likely requires biochemical activities already present or easily synthesized in the host cell. Given such an environment, it is not surprising that parasitic sequences have evolved and now exist. A deeper question is why such sequences do not consti-tute most of the cell’s DNA. Most likely, the tendency of such sequences to proliferate is countermanded by evolution on a grander scale. A cell line or organism that is weighted down by an excessive number of unused DNA sequences would be at a survival disadvantage compared to others containing fewer such sequences.
Consequently, evolution will constantly select for cells without too many transposable sequences, and, overall, an unhappy balance will exist between the sequence and the host cell line. This is not to say that transposable elements are without value to their hosts. As we will see, the presence of repeated DNA sequences can facilitate chromosome rearrangements. This reshuffling of genetic material may greatly speed evolution and aid cells that contain at least a few repeated sequences.
In addition to being a burden upon DNA replication, the insertion of parasitic DNA sequences into the genome will inevitably damage crucial genes. We might therefore expect sophisticated parasitic sequences to devise ways to avoid killing genes or proteins by their insertion. Two methods could be used. A sequence might arrange to splice itself out of RNA when the region has been transcribed, or the sequence might code for a protein that splices itself out of translated protein. If the host RNA or protein product is rejoined, effects upon the host of the parasitic sequence will be minimal. Transposable sequences have been identified that perform either of these functions.