Photo: Ori Fragman, Hebrew University

Hordeum spontaneum, the plant studied for BARE-1 retroelements.
With genome sequences arriving almost as regularly as the morning paper, the public's attention is focused on genes--new genes to protect crops against pests; rogue genes that make bacteria resistant to antibiotics; faulty genes that, if fixed, could cure diseases such as muscular dystrophy.

What many people don't realize is that genes account for only part of an organism's DNA, and in many cases it's a tiny fraction. If humans have 50,000 nuclear genes, as some researchers think, then 98 percent of more than three billion DNA base pairs is something else. Much of that something else is repeated sequences that litter the genome.

Repetitive DNA comes in various sizes and classes. A lot of it is short, tandem repeats clustered at the centromeres--where spindle fibers attach--and the specialized chromosome ends called telo-meres. But a lot...

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