Ancient DNA defence 'still at work'

DNA contains all the information needed for life

By Dr David Whitehouse BBC News Online science editor

A complex DNA protection system that evolved hundreds of millions of years ago so that the first primitive organisms could protect themselves is still at work today, according to American researchers.

This ancient self-defence mechanism turned primeval genetic invaders' tricks against them and incorporated them into useful cell functions.

The repair mechanism ensures that any foreign genetic material that has been inserted into the cell's DNA by a virus is neutralised before being passed on to the next generation.

Scientists believe that a similar mechanism inspects and protects the dna of higher animals such as humans and that the details of the process may be useful in fighting viruses.

Genetic master record

"This is exciting research that provides new understanding of how cells control the activity of their genes," says Joseph Gall, a cell biologist at the Carnegie Institution in Baltimore.

"This work shows once again that basic research on a seemingly obscure topic - how a minute pond organism reproduces - can throw light on important medical issues such as viral infection."

Professor Martin Gorovsky of the University of Rochester discovered the DNA repair mechanism while looking at a single-celled organism called Tetrahymena, which contains two nuclei, the region of a cell where DNA is stored.

The researchers looked at how the cell transfers its genetic code from one of its nuclei to one in its offspring monitoring each step as the cell inspected its dna and passed it on.

They discovered a hitherto unrecognised system of checks used to ensure the organisms DNA has not become contaminated before it goes to the next generation.

Tetrahymena houses different versions of its dna in each of its two nuclei. The smaller nucleus (called the micronucleus) does nothing more than keep the cell's full genome safe. It seems that Tetrahymena uses the smaller of its two nuclei as a master record of its dna so that it always has a safe set of genes for the cell's offspring. Spare set

The other nucleus, called the macronucleus, uses "working" dna to regulate the cell's life functions.

Gorovsky's team believes that in evolutionarily ancient times, cells had to fight against a variety of assaults just as they must today: viruses attacked cells, injecting their DNA to disrupt normal cell functions; and transposons, bits of nomadic genetic material that insert themselves into the cell's DNA causing havoc.

To survive, cells evolved a correction system that recognized the invading DNA and either eliminated or silenced it.

This new information could help scientists devise a way to develop drugs that would recognise invading viruses and neutralise their effects.