Transposons, often called “jumping genes,” are mobile genetic elements: segments of DNA that can change their position within a genome. This movement can sometimes alter a cell’s genetic identity or genome size. Transposons are widespread, found in nearly all organisms from bacteria to humans. They can constitute a significant portion of an organism’s genetic material, making up almost half of the human genome and even more in some plants.
How Transposons Move
Transposons employ distinct mechanisms to relocate within a genome. The two primary methods are “cut-and-paste” and “copy-and-paste” transposition.
The “cut-and-paste” mechanism is characteristic of DNA transposons. The transposon is excised from its original location by an enzyme called transposase, often encoded by the transposon itself. Transposase recognizes specific DNA sequences at the ends of the transposon, cuts it out, and reinserts it into a new, often random, genomic location. The host cell’s DNA repair machinery then fills any gaps, resulting in a short duplication of the target site DNA.
In contrast, retrotransposons utilize a “copy-and-paste” mechanism. This process involves an intermediate step where the retrotransposon’s DNA sequence is first transcribed into an RNA molecule. This RNA copy then undergoes reverse transcription, where an enzyme called reverse transcriptase converts the RNA back into a DNA copy. This newly synthesized DNA copy is then inserted into a new genomic position, while the original retrotransposon remains at its starting location.
Major Classes of Transposons
Transposons are categorized into two major classes based on their transposition mechanism: Class I retrotransposons and Class II DNA transposons. These classifications reflect differences in how they move within the genome.
Class I transposons, or retrotransposons, move via an RNA intermediate. They are abundant in eukaryotic genomes and are divided into types such as Long Terminal Repeat (LTR) retrotransposons and non-LTR retrotransposons. Common examples of non-LTR retrotransposons include Long Interspersed Nuclear Elements (LINEs) and Short Interspersed Nuclear Elements (SINEs). LINEs are typically longer (1,000-7,000 base pairs) and encode their own reverse transcriptase, allowing autonomous transposition. SINEs are shorter (100-500 base pairs) and rely on enzymes provided by LINEs to move.
Class II transposons, or DNA transposons, move directly as DNA. These elements encode a transposase enzyme that facilitates their excision and insertion into new genomic sites. Unlike retrotransposons, they do not involve an RNA intermediate. Examples include mariner and hAT elements, found across various organisms. These elements have specific DNA sequences at their ends, recognized by the transposase for mobility.
The Biological Impact of Transposons
Transposons, once considered “junk DNA,” are now recognized for their roles in shaping genomes and biological processes. Their mobility can lead to both beneficial adaptations and detrimental effects.
They contribute to genome evolution by generating genetic variation. Transposon insertions can alter gene function, duplicate genes, or create new regulatory sequences, driving genomic diversity. This process has played a role in the rapid evolution of some species, including humans, by modulating gene expression.
The insertion of transposons can also cause diseases by disrupting or altering the function of genes. For example, their insertion into or near genes can lead to genetic disorders such as hemophilia A. Such insertions can interfere with gene expression or create chromosomal rearrangements, impacting an organism’s health.
Beyond disease, transposons are involved in gene regulation. They can provide new promoters or enhancers, influencing when and where genes are expressed. Their sequences can also contribute to non-coding RNAs that regulate gene expression at various levels.
Transposons have been harnessed as tools in genetic research. Their ability to move and insert DNA makes them useful for genetic engineering, such as gene tagging to identify gene functions or introducing foreign DNA into genomes. Systems like the Sleeping Beauty transposon are used in laboratories for insertional mutagenesis and gene therapy research.