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.

SEE ALSO Chromosome, Eukaryotic ; DNA ; Gene ; McClintock, Barbara ; Retrovirus

Mary Beckman

Bibliography

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