Segments of DNA called transposable elements, or “jumping genes,” can move from one location to another within a genome, creating genetic variation. The process of relocation is called transposition, which occurs through several methods. This article explores replicative transposition, a pathway that results in a copy of the element being inserted into a new position.
The “Copy and Paste” Mechanism
Replicative transposition is a “copy and paste” mechanism that duplicates a transposable element, leaving the original at its starting location while inserting a new copy elsewhere. This allows the element to proliferate within a host’s genome. The process is orchestrated by specialized enzymes that leverage the host cell’s molecular machinery.
The process begins when the enzyme transposase binds to specific DNA sequences at the ends of the transposable element. The transposase then makes single-strand cuts, or nicks, at each end of the transposon. This action exposes a reactive 3′-OH group on each strand of the element’s DNA. These exposed ends then attack the target DNA at the new insertion site.
Next, the transposase makes a staggered cut in the target DNA, creating short, single-stranded overhangs. The enzyme joins the nicked ends of the transposon to these protruding ends of the target DNA. This creates a branched intermediate structure linking the donor and target DNA molecules. Importantly, no double-strand breaks are made at the transposon ends, preserving the original DNA molecule.
Once the intermediate structure is formed, the host’s DNA replication machinery is recruited. The exposed 3′ ends of the cut target DNA serve as primers for DNA polymerase. The replication process fills in the single-stranded gaps using the transposon’s sequence as a template. This replication duplicates the transposable element.
Distinguishing from “Cut and Paste” Transposition
The “copy and paste” method differs from conservative, or “cut and paste,” transposition. In the “cut and paste” pathway, the transposase enzyme makes double-strand breaks to completely excise the element from its original position. This excised element is then moved and inserted into a new target site, leaving a break in the donor DNA that the cell must repair, which can lead to mutations.
The primary distinction between the two pathways is the final copy number of the transposable element. Replicative transposition results in a net gain, with one copy at the origin and a new copy at the target site. In contrast, the conservative method maintains a constant copy number by simply relocating the existing element.
This difference in outcome stems from the initial action on the donor DNA. Replicative transposition begins with single-strand nicks, preserving the original transposon for duplication. Conservative transposition starts with double-strand breaks that sever all connections to the donor site.
Formation of a Cointegrate
In many bacteria, replicative transposition forms a temporary structure called a cointegrate. This structure represents the fusion of the donor and target DNA molecules into a single, larger circular molecule. The cointegrate is held together by two newly synthesized copies of the transposable element.
The cointegrate’s creation is a direct result of the replication that follows the initial strand transfer. The linked donor and target DNA provides the template for the cell’s replication forks to copy the transposon sequence. This process generates the fused molecule containing two complete replicons and two copies of the transposon.
The cointegrate is not permanent and must be resolved to complete the process. This resolution is accomplished by a second enzyme called resolvase, which is often encoded by the transposon. Resolvase recognizes a specific DNA sequence, the resolution site (res), located within each of the duplicated transposons.
The enzyme then catalyzes a site-specific recombination event between the two res sites. This reaction separates the cointegrate back into two individual circular DNA molecules. The final products are the original donor DNA and the target DNA, which now carries its own copy of the transposable element.
Genomic and Evolutionary Consequences
Replicative transposition has significant consequences for the host genome. The most direct result is an increase in the copy number of the transposable element. This proliferation can rapidly spread genes contained within the transposon, such as antibiotic resistance genes in bacteria.
When a transposable element inserts into a new location, it can have mutagenic effects. If the insertion occurs within a functional gene, it can disrupt the gene’s coding sequence or regulatory regions, inactivating the gene or altering its expression. This process, known as insertional mutagenesis, is a source of spontaneous mutation.
Beyond single-gene disruptions, replicative transposition can provoke large-scale genomic rearrangements. The cointegrate intermediate can be a source of chromosomal deletions, inversions, and fusions. Homologous recombination between two copies of a transposon at different locations can also lead to major structural changes.
By creating new copies of elements, causing mutations, and rearranging the genome, this process generates new genetic material and combinations. This raw genetic variation provides novel traits upon which natural selection can act. These impacts position replicative transposition as a driver of genetic diversity and evolutionary change.