| Transposon |
Articles about Transposon |
Information AboutTransposon |
| CATEGORIES ABOUT TRANSPOSON | |
| molecular biology | |
| mobile genetic elements | |
|
TYPES OF TRANSPOSONS Transposons are classified into two classes based on their mechanism of transposition. Class I: Retrotransposons See Also: Retrotransposon Retrotransposons work by Copying Themselves And Pasting Copies back into the genome in multiple places. Initially retrotransposons copy themselves to RNA (transcription) but, in addition to being translated, the RNA is copied into DNA by a Reverse Transcriptase (often coded by the transposon itself) and inserted back into the genome. Retrotransposons behave very similarly to retroviruses, such as HIV , giving a clue to their evolutionary origins. There are three main classes of Retrotransposons: Viral superfamily: similar to retroviruses, have long terminal repeats (LTRs), encode reverse transcriptase (to reverse transcribe RNA into DNA) LINES: encode reverse transcriptase (to reverse transcribe RNA into DNA), lack RTLs, transcribed by RNA polymerase II Nonviral superfamily: do not code for reverse transcriptase, transcribed by RNA polymerase III Class II:DNA Transposons Class II transposons move by Cut And Paste , rather than copy and paste, using the Transposase enzyme. Different types of transposase work in different ways. Some can bind to any part of the DNA molecule, and the target site can therefore be anywhere, while others bind to specific sequences. Transposase makes a staggered cut at the target site producing sticky ends, cuts out the transposon and ligates it into the target site. A DNA polymerase fills in the resulting gaps from the sticky ends and DNA ligase closes the sugar-phosphate backbone. This results in target site duplication. Both classes of transposon may lose their ability to synthesise reverse transcriptase or transposase through mutation, yet continue to jump through the genome because other transposons are still producing the necessary enzyme. EXAMPLES
TRANSPOSONS CAUSING DISEASES Transposons are Mutagen s. They can damage the genome of their host cell in different ways:
Diseases that are often caused by transposons include Hemophilia A and B, Severe Combined Immunodeficiency , Porphyria , predisposition to Cancer , and Duchenne Muscular Dystrophy . Additionally, many transposons contain promoters which drive Transcription of their own Transposase . These promoters can cause aberrant expression of linked genes, causing disease or Mutant Phenotypes . EVOLUTION OF TRANSPOSONS The evolution of transposons and their effect on genome evolution is currently a dynamic field of study. Transposons are found in all major branches of life. They may or may not have originated in the Last Universal Common Ancestor , or arisen independently multiple times, or perhaps arisen once and then spread to other kingdoms by Horizontal Gene Transfer . While transposons may confer some benefits on their hosts, they are generally considered to be Selfish DNA Parasite s that live within the genome of cellular organisms. In this way, they are similar to Virus es. Viruses and transposons also share features in their genome structure and biochemical abilities, leading to speculation that they share a common ancestor. Since excessive transposon activity can destroy a genome, many organisms seem to have developed mechanisms to reduce transposition to a manageable level. Bacteria may undergo high rates of gene deletion as part of a mechanism to remove transposons and viruses from their genomes while Eukaryotic Organisms may have developed the RNA Interference (RNAi) mechanism as a way of reducing transposon activity. In the nematode '' Caenorhabditis Elegans '', some genes required for RNAi also reduce transposon activity. Transposons may have been co-opted by the vertebrate immune system as a means of producing antibody diversity. The V(D)J recombination system operates by a mechanism of similar to that of transposons. Evidence exists that transposable elements may act as mutators in bacteria and other asexual organisms. TRANSPOSONS IN SCIENCE Transposons were first discovered in the plant Maize . Likewise, the first transposon to be molecularly isolated was from a plant ( Snapdragon ). Appropriately, transposons have been an especially useful tool in plant molecular biology. Researchers use transposons as a means of mutagenesis. In this context, a transposon jumps into a gene and produces a mutation. The presence of the transposon provides a straightforward means of identifying the mutant allele, relative to chemical mutagenesis methods. Sometimes the insertion of a transposon into a gene can disrupt that gene's function in a reversible manner; transposase mediated excision of the transposon restores gene function. This produces plants in which neighboring cells have different Genotype s. This feature allows researchers to distinguish between genes that must be present inside of a cell in order to function (cell-autonomous) and genes that produce observable effects in cells other than those where the gene is expressed. Transposons are also a widely used tool for mutagenesis in ''. SEE ALSO REFERENCES
EXTERNAL LINKS |
|
|