You might imagine that the DNA in your cells has a simple history. Even though it’s been recombined in every generation through sex and gained the occasional mutation, on the whole the genome has been stable and has been passed down reliably from your ancestors.
But that’s not the entire story. Nearly half of your genome is a wild drama: mobile, repetitive, disruptive, even viral. This half is the result of genetic material that can clip itself out of the DNA sequence, float off, and re-root somewhere else. These sequences can multiply and expand, inflating the genome from within. They can hop into the middle of another sequence and break it. They can also be fertile soil for new adaptations to grow.
These unruly genetic fragments are known as transposable elements, or transposons for short. Often called jumping genes for their ability to relocate in a genome, they may seem pathological — indeed, many have viral origins — or perhaps little more than junk. But transposons are increasingly understood to be a key feature of many genetic tool kits. Their connections to the evolution of everything from moths to wombs, and even to the fundamental biological processes that turn genes off and on, suggest that the relationship between host genome and transposon is best understood as a deep coevolutionary entanglement.
The Copy-Paste Parasites
The first hints of transposons’ existence were uncovered more than 80 years ago by the geneticist Barbara McClintock while she was studying color variation in corn kernels at the Cold Spring Harbor Laboratory in New York. She worked with a corn strain whose kernels were typically solid purple, but some were speckled, with purple pigment spattering a yellow base. She hoped to explain how genes produced this color variation.
McClintock’s explanation would challenge geneticists’ understanding of how the genome works. She discovered genetic elements that could move: They could excise themselves from one location and insert themselves into another on the same chromosome or a different one. Sometimes, these genetic acrobats would jump into the middle of a purple pigment gene and interfere with its function, producing a speckled cell. If it jumped out again, the pigment gene would be restored, making a purple cell.
Barbara McClintock’s description of mobile genetic sequences in 1944 was initially treated with skepticism. She was awarded a Nobel Prize in 1983.
Smithsonian Institution Science Service; Restored by Adam Cuerden
McClintock called these mobile genes “controlling elements” for their dominion over the expression of the color-producing genes; today we call them transposons for their ability to transpose themselves, or change positions, within a genome. Three decades later, in 1983, McClintock was awarded a Nobel Prize for her discovery, which showed that genes are not fixed in place.
Since then, researchers have uncovered transposons in organisms across the tree of life and described a whole taxonomy of subtypes that cluster into two main groups.
The transposons McClintock discovered are DNA transposons, so called because they travel as a DNA molecule. Transposons in this class jump by means of “cut and paste.” Enzymes called transposases bind to the ends of the DNA transposon and splice it free. The liberated bundle then touches down somewhere else in the genome, where native DNA repair processes paste it in.
A second class of transposons, known as retrotransposons, aren’t cut directly out of the genome. Instead, the DNA sequence is copied into RNA, a molecular strand that is flexible and mobile by nature. Liberated from the genome, the sequence in the RNA copy is then reverse-transcribed into DNA at a different location. By copying themselves instead of cutting, retrotransposons can easily flood a genome with many iterations of themselves. Over time, this has led retrotransposons to make up large proportions of a host genome in some cases; for example, nearly half of the human genome is retrotransposons.
