How do Transposable Elements Work?

Transposable elements come in a wide variety of types and spread through genomes in a variety of ways. Different species typically contain distinct types of transposable elements. The genomes of fruit flies, yeast, and humans each contain distinctive kinds of parasitic sequences.

As an example of how these selfish genes work,. let's consider a type called a long interspersed nuclear element (LINE), found in humans and other eukaryotes. A LINE is similar to the retroviruses. Each LINE consists of a stretch of DNA that contains a gene that codes for the enzyme reverse transcriptase, a gene that codes for the enzyme integrase, and a single promoter that is recognized by RNA polymerase II. Your genome contains tens of thousands of LINEs, each between 1 kb and 5 kb long.

A LINE contains all the sequences required for it to make copies of itself and insert them into a new location in the genome. When a LINE is expressed, reverse transcriptase is translated from the LINE mRNA. This enzyme make a cDNA version of the LINE mRNA. (cDNA is DNA created from an RNA transcript). The second LINE product, integrase, then inserts the newly synthesized LINE DNA into a new location in the genome. In this way, the parasitic sequence reproduces. If the transposition event occurs in a reproductive cell that go on to form eggs or sperm, the copied LINE will be passed on to offspring. If the LINE happens to insert istself inside a gene or a regulartory sequencs, it causes a mutation that is almost certain to reduce the host's fitness.

Most of the LINEs observed in the human genome are evolutionary remnants no longer able to transpose. Specifically, most of the LINEs in your cells don't contain a promoter or the genes for either reverse transcriptase of integrase. To make sense of this observation, researchers hypothesize that the insertion process is usually disrupted in some way. Analyses of the human genome have revealed that only a handful of our LINEs appear to be complete and potentially active.

Virtualy every prokaryotic and eukaryotic genome examined to date contains at least come transposable elements. They vary widely in type and number; however, bacterial and archaeal genomes have relatively few transposable elements. This observation suggests that bacteria and archaea either have efficient means of removing parasitic sequences or can somehow thwart insertion events. This hypothesis has yet to be tested rigorously.

Research on transposable elements and lateral gene transfer (movement of DNA from one species to another) has had a huge impact on how biologists view the genome. May genomes are riddled with parasitic sequences, and others have undergone radical change in response to lateral gene transfer events. The general picture is that genomes are dynamic. Their size and composition can change dramatically over time.