Transposon

Transposons, also called transposable elements or jumping genes, are stretches of deoxyribonucleic acid (DNA) that can move around an organ­ism’s chromosome. These “transpositions” occur at a very low frequency. A transposon can contain one gene or a set of genes, and transposons are found in both eukaryotes and prokaryotes. The transposon encodes en­zymes that cut the transposon from the DNA sequence and reinsert it else­where. This cutting and pasting requires short DNA segments at either end that are inverted repeats of each other called insertion sequences. These in­sertion sequences are duplicated by the transposon enzymes at the insertion site, also called the target site. No particular DNA sequence serves as the target site for transposons. However, during insertion each transposon du­plicates a set number of nucleotides at the chromosomal target site.

Prokaryote transposons may replicate DNA as well as cut and paste it. Transposons in eukaryotes do not replicate DNA. They move either by cut­ting and pasting, or by creating a ribonucleic acid (RNA) intermediate. These so-called retroposons are thought to be related to retroviruses whose genetic material is RNA. Retroposons are also thought to have created the repetitive Alu sequences that make up a very large fraction of human chro­mosomes.

Although transposition occurs at a low frequency, evolution has pro­vided ample time in which to transpose elements. In addition to the Alu se­quences in humans, about 3 percent of the fruit fly Drosophila melanogaster genome is made up of transposable element DNA.

In the 1940s, Barbara McClintock first discovered mobile genetic ele­ments in corn that caused differences in gene expression, resulting in ker­nels containing dots of different colors against a background predominant color. Because transposons can be inserted anywhere in a chromosome, they can cause genetic mutations by disrupting whole genes, which they do in pigment genes in corn. They can also disrupt expression of genes down­stream of the target site by inserting between the regulatory and the ex­pressed parts of a gene. If two transposons end up flanking a gene, the ends can work together as one large transposon, duplicating that gene within the genome. Gene duplication is a mechanism of evolution. One copy of the gene can mutate further, perhaps resulting in a new function, while the other is retained.

References

Alberts, Bruce, et al. Molecular Biology of the Cell, 4th ed. New York: Garland Pub­lishing, 2000.