The Science of Existence (140-1) Transposons


Transposable genetic elements (TEs) comprise a vast array of DNA sequences, all having the ability to move to new sites in genomes either directly by a cut-and-paste mechanism (transposons) or indirectly through an RNA intermediate (retrotransposons). ~ Nina Fedoroff

A transposon is a DNA sequence which can change its position within a genome; informally called a “jumping gene.” Transposable elements (TE) were discovered by American cytogeneticist Barbara McClintock in the early 1940s while studying maize.

Several major types of TE are recognizable in the genomes of a wide range of organisms; these differ in their transposition mechanisms. ~ English evolutionary biologists Deborah Charlesworth & Brian Charlesworth

In prokaryotes, transposons are essential in cataloging viral encounters, thereupon creating their adaptive immune system (pais) library. Transposons also comprise a large fraction of most eukaryotes’ genomes, as transposition often results in TE duplication. 67% of the human genome comprises transposable elements.

Most TEs are found in intergenic DNA or (to a lesser extent) in introns. In some regions of the genome, TEs can be very densely packed, with jmulitple elements inserted within one another. ~ Deborah Charlesworth & Brian Charlesworth

Transposons are generally considered junk DNA. They have also been characterized as “selfish.” Such blithe dismissal belies biological reality. Depending upon context and instigation, transposons may be beneficial or a bane.

Transposon insertions can have beneficial effects for their respective host organisms. ~ Thomas Eulgem

Transposons help cells adapt to stress and serve as cellular defense against viruses. Insects can quickly become resistant to pesticides thanks to transposons.

Transposable elements can drive evolution by creating genetic and epigenetic variation. ~ Japanese plant cytologist Tokuji Tsuchiya & American plant cytologist Thomas Eulgem

Conversely, these genetic gypsies can disable genes where they impose themselves, even triggering cancer, and contributing to neurodegenerative disorders such as schizophrenia and Alzheimer’s.

Transposons do not just jump. Instead, they usually leave a copy behind at their original location.

If the copy and paste were left unchecked, TEs could explode the genome. But the process is regulated.

After a certain number of copies are made, transposase – the enzyme that catalyzes jumping – reaches a critical threshold, and transposition ceases.

Transposons are not rogue genetic elements. Instead, they are often part of an intricate complex of epigenetic functioning. Transposons associate in families, and the jumps they make transpire through that affiliation. In plants, transposons play a role reprogramming the germline.

The activity of transposons does not only depend on themselves, but also on factors which the host cells produce. ~ Serbian geneticist Ana Marija Jakšic

In a specific adaptation which repeatedly occurred, fish adapted to freshwater from saltwater via transposons.

A single adaptive genetic innovation repeatedly allowed marine fish to colonize and diversify in freshwater. Transposons were responsible. ~ American biologist Jesse Weber et al

Transposons orchestrate the genetic expressions responsible for the prolonged pregnancy of placental mammals. This dramatic evolutionary divergence from marsupials transpired ~90 million years ago.

The evolution of pregnancy was associated with a large-scale rewiring of the gene regulatory network. Transposable elements are potent agents of gene regulatory network evolution. ~ American evolutionary biologist Vincent Lynch et al

Though the specifics are not well understood, transposons appear to have played a major role in evolution via cross-species jumps.

Jumping genes introduce themselves into other genomes. ~ Australian geneticist David Adelson