Researchers track the movements of `footloose' DNA segments

An important new mechanism for driving evolution and genetic diversity has been revealed by researchers at the University of Pennsylvania Medical Centre.

The group showed that bits of an organism's genetic blueprint, DNA, can copy themselves from one part of the genome to another, inserting themselves at new locations. In particular these wandering strips of DNA can target genes, an event which could occasionally provide new functionality to a gene and so serve - or diminish - its value in an evolutionary sense.

"These findings suggest a new mechanism for shuffling important genetic sequences . . . that may lie downstream from these active mobile DNA elements," according to Dr Haig H. Kazazian, chairman of the Department of Genetics and senior author on the paper.

"From an evolutionary perspective, here is a way to create novel genetic combinations. While many such changes might prove lethal, some could improve function in individuals, leading to a selective advantage for those individuals."

These footloose DNA segments are known as retrotransposons. Up to 60 retrotransposons of one type called LINE-1s or L1s have been discovered in the human genome so far. Species such as the mouse are thought to have as many as 3,000 such elements.

The research group engineered an L1 to include a marker that would only be activated if the mobile L1 inserted itself in a working gene. The results showed that at least 6 per cent of of them targeted working genes.

In the journal Science the group describes experiments showing that L1s are also able to pick up and move DNA sequences adjacent to themselves during retrotransposition.

Duplication of an L1 is meant to end when a terminating DNA sequence is reached, a genomic stop sign. This sign is weak on L1 sequences, so extra DNA can stow away in a "readthrough".

"Overall, we found that not only can these retrotransposons jump into genes, but readthrough events that pick up flanking DNA are not uncommon," according to lead author, Dr John Moran.

These novel recombinations or mutations could prove a help or a hindrance to an organism in a changing environment.