Scientists have found the essence of life - at least on a genetic level - and it comes down to about 300 genes.

US researchers think this is the minimum set of molecular instructions required to build a living organism.

It would clearly be creating a new species of life that does not exist Dr Craig Venter

It has been suggested that this could be tested by trying to synthesise an artificial bacterium in the lab - for scientists to create life from non-living chemicals.

The idea is currently the subject of an ethical review and the scientists involved say no attempt will be made to proceed with the daring experiment until there has been a full and public debate.

The prospect of "scientists playing God", as some will undoubtedly see it, is bound to provoke some fierce arguments.

Tiny organism

A team from The Institute for Genomic Research (Tigr) in Maryland pared-down the tiniest-known living organism, a bacterium called Mycoplasma genitalium, to its essential genes.

M. genitalium is the smallest-known bacterium" border="0" src="/olmedia/555000/images/_557525_bac1500.jpg" width="150" height="180"> M. genitalium is the smallest-known bacterium

"The analysis suggests that 265 to 350 of the 480 protein-coding genes of M. genitalium are essential under laboratory growth conditions, including about 100 genes of unknown function," the Tigr scientists have reported in the journal Science.

The existence of 111 unknown but essential genes suggests that biologists do not yet understand everything about basic life functions, they add.

M. genitalium lives in the human genital tract and lungs, causes no known disease, but has fewer genes than any other known living thing. Humans have between 80,000 and 140,000 genes, but M. genitalium has just 480.

Dr Craig Venter, founder of Tigr and now head of the Celera Genomics Corporation, said the study was redefining life in terms of the genome, the collection of all a creature's genes.

Essential genes

"Will we eventually get to a molecular definition of life? I hope that will happen, yes," he told the BBC.

M. genitalium has a close relative, Mycoplasma pneumoniae, that has the same 480 genes as M. genitalium, plus 200 extra ones. "So we decided these genes were not essential to life," Venter said.

Craig Venter: We still have much to learn about the biological cell

One by one, the team disrupted the genes in M. genitalium to see which ones the organism could not live without. They did this using transposons, which are stretches of DNA that insert themselves into genes. The number found to be essential was not exact, but close enough, the Tigr team said.

Most interesting is the large number of genes that are necessary, but about which the researchers have no idea what they do.

"Our results imply that of the 111 genes of unknown function that have not been disrupted in our experiments, the majority are essential," the researchers wrote.

"The presence of so many genes of unknown function among the essential genes of the simplest known cell suggests that all the basic molecular mechanisms underlying cellular life may not yet have been described."

Much to learn

The study also established that some genes were only essential in certain circumstances, when, for example, particular nutrients were denied to the microbe. Dr Venter told the BBC: "We realised that life is context sensitive. It does not exist on its own. It has to interact with its environment."

The researchers have suggested the next step would be to create an artificial bacterium, based on the essential genes. The first step in such an experiment would be to build an artificial chromosome to carry the genes.

"We are not going to carry out this experiment until there has been a broader debate on the issue," Dr Venter said. "Technically we would need to synthesise a genome and see if it led to a living organism.

"It would clearly be creating a new species of life that does not exist. I think if we could get down to the point of truly understanding and having one of the formulas for life - and you have to understand that there are thousands if not millions of different formulas - it would be a profound breakthrough."

Dr Venter said the technology could lead to custom microbes that have myriad practical and commercial implications such as to clean up toxic messes or to create renewable energy by splitting water into hydrogen and oxygen.

M. genitalium image by Frantz, Albay and Bott from University of North Carolina, Chapel Hill