Transposons are sequences of DNA that can move around to different positions within the genome of a single cell, a process called transposition. In the process, they can cause mutations and change the amount of DNA in the genome. Transposons were also once called "jumping genes", and are examples of mobile genetic elements. Discovered by Barbara McClintock early in her career, the discovery earned her a Nobel prize in 1983. There are a variety of mobile genetic elements, and they can be grouped based on their mechanism of transposition. Class I mobile genetic elements, or retrotransposons, move in the genome by being transcribed to RNA and then back to DNA by reverse transcriptase, while class II mobile genetic elements move directly from one position to another within the genome using a transposase to "cut and paste" them within the genome. Transposons are very useful to researchers as a means to alter DNA inside of a living organism. Transposons make up a large fraction of genome sizes which is evident through the C-values of eukaryotic species.
Transposons are classified into two classes based on their mechanism of transposition. 10% of the human genome is made up of transposons.
Class I: Retrotranspositions 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 transcribed, 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 the evolutionary origins of such viruses.
There are three main classes of retrotransposons:
- Viral: encode reverse transcriptase (to reverse transcribe RNA into DNA), have long terminal repeats (LTRs), similar to retroviruses
- LINEs: encode reverse transcriptase, lack LTRs, transcribed by RNA polymerase II
- Nonviral superfamily: do not code for reverse transcriptase, transcribed by RNA polymerase III
Retroviruses were first identified 80 years ago as agents involved in the onset of cancer. More recently the AIDS epidemic has been shown to be due to the HIV retrovirus. In the early 1970s it was discovered that retroviruses had the ability to replicate their RNA genomes via conversion into DNA which became stably integrated in the DNA of the host cell. It is only comparatively recently that retroviruses have been recognized as particularly specialized forms of eukaryotic transposons. In effect they are transposons which move via RNA intermediates that usually can leave the host cells and infect other cells. The integrated DNA form (provirus) of the retrovirus bears a marked similarity to a transposon.
Class II: DNA transposons The major difference of class II transposons from retrotransposons is that their transposition mechanism does not involve an RNA intermediate. Class II transposons usually move by a mechanism analogous to 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 and the insertion sites of DNA transposons may be identified by short direct repeats (a staggered cut in the target DNA filled by DNA polymerase) followed by inverted repeats (which are important for the transposon excision by transposase).
Not all DNA transposons transpose through cut and paste mechanism. In some cases a replicative transposition is observed in which transposon replicates itself to a new target site.
The transposons which only move by cut and paste may duplicate themselves if the transposition happens during S phase of the cell cycle when the "donor" site has already been replicated, but the "target" site has not.
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.
The transposition cycle of retroviruses has other similarities to prokaryotic transposons, which suggest a distant familial relationship between these two types of transposon. Crucial intermediates in retrovirus transposition are extrachromosomal DNA molecules. These are generated by copying the RNA of the virus particle into DNA by a retrovirus-encoded polymerase called reverse transcriptase. The extra chromosomal linear DNA is the direct precursor of the integrated element and the insertion mechanism bears a strong similarity to "cut and paste" transposition.
Examples - The first transposons were discovered in maize (Zea mays), (corn species) by Barbara McClintock in 1948, for which she was awarded a Nobel Prize in 1983. She noticed insertions, deletions, and translocations, caused by these transposons. These changes in the genome could, for example, lead to a change in the color of corn kernels. About 50% of the total genome of maize consists of transposons. The Ac/Ds system McClintock described are class II transposons.
- One family of transposons in the fruit fly Drosophila melanogaster are called P elements. They seem to have first appeared in the species only in the middle of the twentieth century. Within 50 years, they have spread through every population of the species. Gerald Rubin and Allan Spradling pioneered technology to use artificial P elements to insert genes into Drosophila by injecting the embryo.
- Transposons in bacteria usually carry an additional gene for function other than transposition---often for antibiotic resistance. In bacteria, transposons can jump from chromosomal DNA to plasmid DNA and back, allowing for the transfer and permanent addition of genes such as those encoding antibiotic resistance (multi-antibiotic resistant bacterial strains can be generated in this way). Bacterial transposons of this type belong to the Tn family. When the transposable elements lack additional genes, they are known as insertion sequences.
- The most common form of transposon in humans is the Alu sequence. The Alu sequence is approximately 300 bases long and can be found between 300,000 and a million times in the human genome.
- Mu phage transposition is the best known example of replicative transposition. Its transposition mechanism is somewhat similar to a homologous recombination.
Transposons are mutagens. They can damage the genome of their host cell in different ways:
- A transposon or a retroposon that inserts itself into a functional gene will most likely disable that gene.
- After a transposon leaves a gene, the resulting gap will probably not be repaired correctly.
- Multiple copies of the same sequence, such as Alu sequences can hinder precise chromosomal pairing during mitosis and meiosis, resulting in unequal crossovers, one of the main reasons for chromosome duplication.
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 parasites that live within the genome of cellular organisms. In this way, they are similar to viruses. 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 similar to that of transposons.
Evidence exists that transposable elements may act as mutators in bacteria.
Applications Transposons were first discovered in the plant maize (Zea mays, corn species), which is named dissociator (Ds). 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 genotypes. 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.
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