Treasure trove of genome junk

February 25, 2000

Huge tracts of genetic material that have been written off as "junk" DNA may be relics of missed evolutionary revolutions.

Margaret Kidwell, professor of ecology and evolutionary biology at the University of Arizona, has reassessed the role of transposable elements, "jumping" gypsy genetic material that makes up more than a third of the human genome. Her research has found that far from being dangerous or at best useless, these genes have probably played an important role in driving evolution.

Transposable elements are abundant sequences of DNA that can copy themselves and "jump" from one part of the genome to another. Some can even leap from one host lineage to another, a bit like a pathogen.

They have long been regarded as genetic nuisances that clutter genomes. Occasionally, they can land in the middle of a useful gene, creating a mutation with a catastrophic impact on the health of the individual.

But it is this ability to mutate its host that makes the transposable element so useful, Professor Kidwell said. "In the end, mutations are the raw material of evolution."

Most mutants are rapidly weeded out by natural selection. But there is a slim chance that one might emerge that is better able to adapt to changes in the environment than its normal cousins. Hence the random intervention of a transposable element can facilitate evolution and further genetic diversity.

"It may be very rare that a beneficial mutation will arise, but that mutation could be so significant that it is out of all proportion to its frequency," Professor Kidwell said. It has happened before: a fundamental part of the vertebrate immune system resulted from a transposable element called RAG gatecrashing an early vertebrate's genome.

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