Transposons, also known as jumping genes, are segments of DNA in a single cell that can move in the genome. The process of transposon movement is called transposition. The effects of transposition are mutations and the lengthening or shortening of the genome. Transposons were once called junk DNA because they were seemingly useless. However, research has shown that these mobile segments of DNA are significant in the development of organisms.
Types of Transposons
Transposable elements longer and more complex than insertion sequence, called transposons,also move about in the bacterial genome. In addition to the DNA required for transposition, transposons include extra genes that go along for the ride, such as genes for antibiotic resistance. In some bacterial transposons, the extra genes are sandwiched between two insertion sequence. It is as though two insertion sequence happened to land relatively close together in the genome and now travel together, along with all the DNA between them, as a single transposable element. other bacterial transposons do not contain insertion sequences; these have different inverted repeats at their ends.
Transposons are not unique to bacteria and are important components of eukaryotic genomes as well.
Retrotransposons (Class I transposons) move to different areas of the genome through:
1. Transcribing to RNA
2. Reverse transcribing to DNA (via reverse transcriptase)
3. Inserting into a different area of the genome
Retrotransposons are prevalent in plants: the genome of maize and the genome wheat are composed of roughly 50-80% and 70% retrotransposon respectively. Roughly 40% of the human genome is composed of retrotransposons. The mechanism by which retrotransposons work is similar to that of retroviruses, suggesting that there is a relationship between the two.
The three subtypes of Class I transposons are:
1. Viral: encodes the reverse transcriptase enzyme which is used to reverse transcribe to DNA (step 2 of transposition) Behaves most like retroviruses in that it is characteristic of having long terminal repeats (LTRs).
2. Nonviral: does not encode the reverse transcriptase. Use RNA polymerase III
3. long interspersed elements (LINEs): encodes the reverse transcriptase enzyme. Uses RNA polymerase II
LTRs: Observed in retroviruses, long terminal repeats are segments of DNA that repeat from one hundred to a thousand times.
LINEs: Segment of DNA which codes for reverse transcriptase and also may code for an endonuclease.
DNA transposons (Class II transposons) can move to different areas of the genome through two mechanisms:
Mechanism 1: Transposase enzymes can bind to either a specific site of DNA or anywhere. Transposase cuts the DNA at the site it binds to and produces "sticky ends" and also cuts the transposon and ligates it to the specific site. Then DNA polymerase and DNA ligase run through the site to effectively enclose the transposon to the designated area.
DNA transposons that move through the genome via Mechanism 1 can be duplicated during the cell cycle.
Mechanism 2: The second mechanism by which Class I transposons move through the genome is called replicative transposition. In replicative transposition, the DNA transposon replicates itself into a new area of the genome.
DNA transposons are prone to eventually losing the ability to produce the necessary enzymes transposase or reverse transcriptase. However, other types of transposons will produce these enzymes, prolonging the ability of DNA transposons to move through the genome.
Transposons Are Mutagens
When transposons insert into the genome, they may potentially insert into a region encoding for a gene. This causes mutation because the gene will be inhibited. Insertion into introns, exons, and DNA sequences adjacent to the gene encoding region may also cause dysfunction of the gene. In addition to mutation due to insertion, mutation may occur if a sequence is copied several times such that it interferes in cell mitosis and meiosis. The irreparable gap produced when the transposon is removed may cause mutation as well.
Diseases resulting from mutations of this sort include Hemophilia, porphyria, and Duchenne muscular dystrophy. Transposition may also result in cancer.
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