Mapping a clan of mobile selfish genes(have already identified over 40 Alu elem...)

Bennett and Devine tested Alu elements by putting each of 89 family representatives on a small circle of DNA next to a gene that allows human cells to resist a poisonous drug. They then introduced the DNA circles into cells in culture dishes.

If the Alu element could jump, carrying the drug-resistance gene onto the cells' chromosomes, the cells survived the drug. The authors conclude that around 10,000 Alu elements are still capable of jumping around, with 37,000 having at least a low level of activity. The youngest ones were all capable of moving around, and the oldest ones were all inactive.

"These results mean that Alu is by far the most abundant class of jumping genes and poses the greatest transposon-mediated threat to our genomes," Devine says.

The term retrotransposons comes from how they replicate: first, the DNA is transcribed (copied) into RNA, and the RNA is reverse-transcribed into DNA again. Depending on the type of cell, if an Alu element is located near genes that have been shut off, the Alu element is less likely to get transcribed.

That means the number of Alu elements that do move around is probably slightly lower. The team has constructed a database of Alu elements to compile additional information about each family.

Devine says an enzyme that is part of the normal machinery of the cell transcribes Alu elements, but they actually depend on another type of repetitive element, called L1, to make the enzyme that can reverse-transcribe them.

Scientists think Alu elements "hijack" part of the cell during the copying process. In the cell, Alu RNA is thought to resemble another type of RNA that guides protein production. The team's tests indicate that Alu elements that can best mimic that RNA, called the signal recognition particl
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