Transposition in biology refers to a process where specific segments of DNA move from one location to another within an organism’s genome. These mobile DNA segments are often called “jumping genes.” This dynamic movement reshapes the genetic landscape, influencing how genes are organized and expressed. It contributes to the fluidity and adaptability of genetic material.
Types of Mobile Genetic Elements
The movement of DNA segments is facilitated by specialized DNA sequences known as mobile genetic elements. These elements fall into two main categories: DNA transposons and retrotransposons, each employing a distinct strategy for relocation.
DNA transposons, sometimes called “cut-and-paste” elements, are segments of DNA that move directly from one genomic location to another. They exist as DNA throughout their entire transposition cycle.
Retrotransposons, in contrast, operate through an “RNA intermediate” mechanism, often described as “copy-and-paste.” These elements are first transcribed into an RNA molecule. This RNA copy then serves as a template for synthesizing a new DNA copy, which is subsequently inserted into a new genomic site. This process leads to an increase in the number of retrotransposon copies within the genome.
How Transposition Occurs
The movement of these genetic elements involves distinct enzymatic processes. DNA transposons utilize a “cut-and-paste” mechanism, initiated by an enzyme called transposase. This enzyme recognizes the ends of the transposon sequence and makes cuts in the DNA strands, excising the transposon from its original location. The excised transposon then seeks a new target site within the genome.
Once a new insertion site is identified, transposase facilitates the integration of the transposon into the host DNA. This involves creating staggered cuts in the target DNA, allowing the transposon to be ligated into the gap. The host cell’s DNA repair machinery then fills in the small single-stranded gaps, completing the insertion.
Retrotransposons employ a “copy-and-paste” strategy that involves an RNA intermediate. The retrotransposon DNA is first transcribed into an RNA molecule by the host cell’s machinery. This RNA molecule then serves as a template for the synthesis of a new DNA copy, a process catalyzed by an enzyme called reverse transcriptase. Reverse transcriptase can synthesize DNA using an RNA template, a reversal of the typical flow of genetic information.
The newly synthesized DNA copy of the retrotransposon is then integrated into a new location within the host genome. This integration is facilitated by another enzyme, integrase. The result is a new copy of the retrotransposon at a different genomic location, while the original copy remains in place.
Role in Genetic Variation and Evolution
Transposition influences genetic variation, shaping genomes and driving evolutionary change. The insertion of a mobile genetic element into a gene can disrupt its function, potentially leading to a loss of protein production or altered protein activity. An insertion near a gene can also influence its regulation, either increasing or decreasing its expression levels. These insertions can contribute to gene duplication events, providing raw material for the evolution of new gene functions.
The repeated copying and insertion of mobile elements, particularly retrotransposons, contribute to the overall size and structure of a genome. In humans, for example, retrotransposons like LINEs and SINEs make up a substantial portion of the genome, far exceeding the coding regions. Their presence can facilitate large-scale genomic rearrangements, such as deletions, inversions, and translocations, by providing sites for recombination between different genomic regions. This genomic plasticity can lead to the emergence of novel traits and contribute to the formation of new species over long evolutionary timescales.
While transposition can be a force in evolution, uncontrolled or misplaced insertions can also have detrimental effects, contributing to genetic disorders and diseases. If a mobile element inserts into a gene that is already known to cause a particular disease, it can lead to the development of that condition. Such insertions have been linked to various human diseases, including certain types of hemophilia and muscular dystrophy, by disrupting gene function. These elements play a role as both architects of genomic diversity and potential agents of instability.