A transposable element (TE) is a sequence of DNA capable of changing its position or generating copies at new locations within a genome. Movement, called transposition, depends on molecular machinery encoded by the element or supplied by other elements and the host. TEs occur in diverse organisms, including bacteria and eukaryotes. The term also commonly includes inactive descendants of formerly mobile sequences. Although popularly called “jumping genes,” many elements do not encode a functional gene, and most copies in some genomes can no longer transpose. (pmc.ncbi.nlm.nih.gov)
Discovery and historical development
Barbara McClintock discovered mobile genetic elements through studies of maize in the 1940s. By combining breeding experiments with observations of chromosomes, she showed that particular genetic loci could change position and influence nearby genes. Her work on the Activator–Dissociation system helped explain unstable inheritance and variegated pigmentation in maize kernels. She described these findings in a landmark 1950 publication. (pmc.ncbi.nlm.nih.gov)
The discovery established that genome organization was not necessarily fixed across generations or cell divisions. Subsequent research identified mobile elements in many other organisms and clarified their molecular mechanisms. McClintock received the 1983 Nobel Prize in Physiology or Medicine for discovering mobile genetic elements. (pmc.ncbi.nlm.nih.gov)
Classification and mechanisms
The standard classification of eukaryotic TEs distinguishes two major classes according to their transposition intermediate.
Class I elements, or retrotransposons, move through an RNA intermediate. After transcription, a reverse transcriptase produces a DNA copy that becomes integrated at another genomic location. Because the original element generally remains in place, this is commonly described as “copy-and-paste” transposition. Major groups include long-terminal-repeat retrotransposons and non-LTR elements such as LINEs and SINEs. LTR elements generally use an integrase for insertion; non-LTR elements commonly couple DNA synthesis to insertion through target-primed reverse transcription. (pmc.ncbi.nlm.nih.gov)
Class II elements, or DNA transposons, mobilize through DNA rather than RNA. Many employ a “cut-and-paste” mechanism: a transposase recognizes the element’s ends, excises it, and inserts it elsewhere. This machinery consists of one or more enzymes. However, DNA transposition is not exclusively cut-and-paste; some elements use replicative mechanisms, including rolling-circle-like transposition in Helitrons. Many insertions generate short target-site duplications, which are distinct from the element’s own terminal repeats. (pmc.ncbi.nlm.nih.gov)
A separate distinction concerns autonomy. Autonomous elements encode the element-specific machinery necessary for their mobilization, while nonautonomous elements depend on machinery supplied by compatible autonomous elements. Autonomy does not mean independence from the host cell. Human Alu elements, for example, are nonautonomous retrotransposons that use machinery encoded by LINE-1 elements. (pmc.ncbi.nlm.nih.gov)
Distribution and bacterial mobility
TE abundance differs greatly among organisms. Recognizable TE-derived sequences account for approximately 45% of the human genome under conventional annotation, although estimates depend on methods and the identification of ancient, highly diverged copies. Abundance should not be confused with activity: accumulated mutations and deletions have disabled many copies, leaving a genomic record of earlier transposition. (pmc.ncbi.nlm.nih.gov)
In bacteria, insertion sequences are relatively simple elements that primarily encode transposition functions. More complex transposons can carry additional genes, including determinants of antibiotic resistance. Transposition can relocate such genes between chromosomes and plasmids. Their subsequent transfer between cells contributes to horizontal gene transfer, but intracellular transposition and intercellular transfer are distinct processes. (journals.asm.org)
Effects on genomes and evolution
A new insertion can produce a mutation by disrupting a coding sequence or changing gene expression. Insertions may introduce transcriptional signals, alter RNA processing, or affect neighboring regulatory regions. Excision and insertion can also rearrange adjacent DNA. Effects vary with the element, insertion site, and biological context; the presence of a TE does not itself establish a functional consequence. (pmc.ncbi.nlm.nih.gov)
Even inactive copies can influence genome structure. Similar TE sequences at different positions can provide substrates for homologous recombination, producing deletions, duplications, or inversions. Over evolutionary time, host organisms can also recruit TE-derived sequences as promoters, enhancers, or parts of cellular genes. Such recruitment contributes to evolution without implying that every insertion is adaptive. (link.springer.com)
Host control
Hosts restrict TE activity through multiple mechanisms. DNA methylation and repressive chromatin states can inhibit transcription, while small-RNA pathways target TE transcripts or help establish silencing. In animal germ cells, PIWI-interacting RNAs are important components of this defense. These mechanisms connect TE regulation with epigenetics and developmental changes in genome activity. (pmc.ncbi.nlm.nih.gov)
Silencing varies among tissues and developmental stages. Increased TE transcription does not necessarily mean successful transposition: an expressed copy may lack intact coding sequences, and additional restrictions can prevent completion of mobilization. Distinguishing transcription, protein production, and new insertion events is therefore essential when measuring TE activity. (pmc.ncbi.nlm.nih.gov)
Research methods and applications
Researchers identify TEs through sequence similarity, structural features, and comparisons among genomes. Their repetitive nature complicates DNA sequencing analysis because short reads may match many copies, making assignment to individual loci difficult. Expression measurements and insertion detection consequently require methods tailored to repetitive sequences. (link.springer.com)
Transposition machinery also has experimental uses. Engineered transposases underpin ATAC-seq, which inserts sequencing adapters into accessible chromatin to map genome-wide accessibility. This application uses transposase chemistry as an analytical tool rather than measuring natural TE movement directly. (research.stowers.org)