Transposable elements or transposons are short sequences of DNA that has the ability to move from one location to another in the genome of a single cell. They were first observed and identified by Barbara McClintock about 50 years ago, during the genetic study of maize. She noticed insertions, deletions and translocations caused by these mobile genetic elements, which resulted in a change in the colour of corn kernels. The simplest types of transposons code for the enzyme transposase which helps them to jump to a new location in the genome, a phenomenon called transposition. Hence, it is also called as 'jumping gene'.
Transposition can increase or decrease the amount of DNA in a genome and its attachment to a new location can affect the genes nearby, causing mutations. These are more abundant in eukaryotic genomes than in prokaryotes. In eukaryotes, transposons can move within a chromosome or between two chromosomes and in prokaryotes the transposition can occur either within chromosomal DNA, between chromosomal DNA and plasmid or between two plasmids. Different types of transposons have adopted special mechanisms for jumping and the role of the enzyme transposase makes the movement of these mobile DNA fragments easy to control, and this makes it an important tool for genetic studies.
Transposons constitute a major fraction of the total genome of eukaryotes, which is proved by their C-value and are often considered as junk DNA because they don't provide any benefit to the host. They insert to the new location by DNA recombination and this does not demand for an extensive base pair complementarity. The function of transposons is only to make their multiple copies in the genome, and so it is also named as 'selfish DNA'. It is interesting to note that there are autonomous and non-autonomous transposons too. While the former can move on its own, the latter needs the help of other transposable elements for their transposition. This dependence of non-autonomous transposons is due to their lack of genes that codes for transposase or reverse transcriptase enzymes that are indispensable for the 'jumping' process.
There are mainly three types of transposable elements. Insertion Sequences (IS) contains inverted repeats and transposase gene. Composite transposon contains two IS elements and antibiotic resistance gene(s) and non-composite transposons have inverted repeats, transposase gene and antibiotic resistance gene(s). There are two modes by which transposition takes place - conservative and replicative. In conservative transposition, the transposase enzyme makes a blunt cut at the two ends of transposon in the donor DNA and a staggered cut at the target sequence. The transposon is ligated into the target. Then, the enzymes DNA polymerase I and ligase fills the gap formed due to sticky ends. The whole process results in the duplication of the target DNA and the donor DNA is destroyed. In replicative method, nicks are made at the ends of transposon and target, after which the donor and target DNA are linked via single strands of transposon. The transposon is replicated when the gaps are filled by DNA polymerase I and ligase and the recombination between the two internal resolution sites are done by the enzyme resolvase. Here, the transposon does not leave the host DNA.
Depending upon the mode of transposition, transposons are categorized into three classes. Class I transposons are called retro transposons which moves by 'copy and paste' mechanism, their speciality being the involvement of an RNA intermediate. First, the retro transposons transcribe to RNA and this RNA intermediate is not only translated, but also transcribed back into DNA using the enzyme reverse transcriptase. This reverse transcribed DNA is then inserted into a new location in the genome. These are very similar to retroviruses like HIV, hinting their evolutionary origin. Class II are DNA transposons that achieve transposition through a 'cut and paste' mechanism with the help of the enzyme transposase, without involving an RNA intermediate. As a result, the inverted repeats of the transposon are flanked by short direct repeats, making the insertion site of the transposon easily identifiable by transposase in future. Class III is Miniature-inverted-repeats-transposable elements (MITEs) which are too small to code for any protein. Its transposition method is still unknown and these are commonly found in the genome of human, plants and animals.
The discovery of transposons opened a new gateway to the further studies on its applicability in genetics. Transposons are used to induce mutations in a gene, where the mutant allele is identified by the presence of the transposon itself. The ability of a transposable element to insert itself into a specific site in a genome of an organism gives it a high potential to be used for genetic modification and tremendous research is advancing in the development of transposons, for their successful application in gene therapy.
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