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A mechanism that is identical to the one used by a large family of transposons that are collectively referred to as retroviral-like retrotransposons is used by retroviruses to insert and remove themselves from chromosomes. These components can be found in organisms as diverse as yeasts and mammals alike. The Ty1 element that can be found in yeast is one of the most well understood. The first step in its transposition is the transcription of the entire transposon, which results in the production of an RNA copy of the element that is longer than 5000 nucleotides. This process is analogous to that of a retrovirus. This transcript contains the genetic code for the enzyme that is responsible for reverse transcription, and it is translated into messenger RNA by the host cell. Through the use of an RNA/DNA hybrid intermediate, this enzyme creates a double-stranded DNA copy of the RNA molecule, thus accurately simulating the initial stages of infection caused by a retrovirus. The linear double-stranded DNA molecule then integrates into a site on the chromosome through the use of an integrase enzyme that is also encoded by the Ty1 DNA. This process is analogous to that of retroviruses. The Ty1 element bears a striking resemblance to retroviruses; however, unlike retroviruses, it does not have a functional protein coat; as a result, it is unable to move between cells outside of a single cell and the cells that it produces.
Recombination that is Specific to a Site
Site-specific recombination is an exchange that can take place between pairs of defined sequences that are either located on the same DNA molecule or on two different DNA molecules. This exchange can take place on the same DNA molecule or on two different DNA molecules. It’s possible that the DNA sequences will become integrated, cut out, or inverted as a result of the exchange. A site-specific recombinase is responsible for catalysing the recombination process. This enzyme can function on its own or in conjunction with accessory factors to mould the DNA target. DNA cleavage at the recombination site produces an intermediate that has the recombinase covalently linked to the ends of the DNA. Reversing this process reseals the DNA to form the recombinant and releases the recombinase from the DNA. During this recombination process, there is no need for replication or repair to take place.
Any DNA sequence can have mobile genetic elements inserted into it through a process called transpositional site-specific recombination.
Transposons are mobile genetic elements that, in general, have only modest target site selectivity and can, as a result, insert themselves into a wide variety of different DNA sites. They are also known as transposable elements. During the process of transposition, a specific enzyme, which is typically encoded by the transposon and is referred to as a transposase, acts on a specific DNA sequence located at each end of the transposon. This enzyme first detaches the transposon from the DNA that flanks it, and then it inserts the transposon into a new target DNA site. It is not necessary for there to be homology between the ends of the element and the insertion site.
Because the majority of transposons only move very infrequently (once every 105 cell generations for many elements in bacteria), it is frequently challenging to differentiate them from nonmobile parts of the chromosome. In most instances, it is unknown what abruptly triggered their movement in the first place.
The structure of transposons as well as the mechanisms by which they transpose genetic material allow for the possibility of categorising them into one of three broad classes, each of which will be examined in greater depth in the following sections. Those that fall into the first two of these classes translocate via reactions that are almost identically the same for breaking DNA and joining DNA. However, in the case of DNA-only transposons, the mobile element is always present in the form of DNA throughout the entirety of its life cycle. A transposase is responsible for cutting the translocating DNA segment directly from the donor DNA and joining it to the target site. In contrast, the movement of retroviral-like retrotransposons is governed by a mechanism that is less direct. Initially, the DNA sequence of the mobile element is transcribed into RNA by an enzyme known as an RNA polymerase. The RNA is used as a template while the enzyme reverse transcriptase transcribes this RNA molecule back into DNA. It is this DNA copy that is ultimately inserted into a new location in the genome. Rather than being referred to as a transposase, the transposase-like enzyme that is responsible for catalysing the insertion reaction is known as an integrase for historical reasons. The third kind of transposon also moves by producing a copy of its own DNA from an RNA molecule that has been transcribed from it. The mechanism that is involved, however, for these non retroviral retrotransposons is different from the one that was just described in the sense that the RNA molecule is directly involved in the transposition reaction.
Conclusion
It is believed that other repeated DNAs that do not encode an endonuclease or a reverse transcriptase in their own nucleotide sequence can multiply in chromosomes by a mechanism that is very similar to this one. This mechanism makes use of a variety of endonucleases and reverse transcriptases that are already present in the cell, including those that are encoded by L1 elements. For instance, the ubiquitous Alu element does not contain any endonuclease or reverse transcriptase genes; however, it has successfully replicated itself to become a significant component of the human genome.
