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Scientists say they have reached an important milestone in their quest to actually build a life-form. They have synthesized a bacteria's entire chromosome from its chemical building blocks.

Soon, they hope to activate those genes and create living, multiplying bacteria. Eventually, synthetic organisms may help produce fuels, chemicals and medicines.

NPR's Richard Harris has this story.

RICHARD HARRIS: There's a whole new field starting up called synthetic biology. The idea is to take biotechnology to the next level, by not just moving genes around but building biological machinery from scratch. Scientists have already assembled virus genomes from chemical building blocks. Now they're onto bacteria.

A research team at the J. Craig Venter Institute outside Washington, D.C., has set its sights on a bacterium called Mycoplasma genitalium. This simple organism has just a few hundred genes to begin with, and a complete genetic code that's about half a million letters long — more than a thousand times smaller than a human's. Venter says the idea was to synthesize this genome by assembling those half-million letters in gene-making machines.

Mr. J. CRAIG VENTER (Founder, J. Craig Venter Institute): And this entire process started with four bottles of chemicals containing what's represented by A, C, G and T.

HARRIS: That is the chemical building blocks of DNA. Gene machines turned out a hundred and one relatively small snippets of DNA, each representing about 1 percent of the genetic code of this bacterium. Venter's colleagues then put those snippets in bacteria, which multiplied like crazy and served as copy machines for those pieces.

Then gradually, using bacteria and finally yeast cells, they stitched together those 101 pieces in the correct order. They then read back the genetic code to make sure they had made a true copy of the natural chromosome. They reported that success in the online edition of Science Magazine.

But what they have not been able to do as yet is to put this chromosome into a cell and get the cell to use that genetic information to start functioning and reproducing. Venter says this is the next big hurdle in the research.

Mr. VENTER: There are multiple barriers for this. It's not just a slam dunk or we would be announcing it today.

HARRIS: But his lab has some ideas about how to do this. Venter says he'd be surprised and disappointed if he can't succeed sometime this year. And he has quite an impressive track record in science. Most famously, he was instrumental in the effort to sequence the human genome. Eventually, Venter wants to use this method to create radical new versions of this bacterium so he can ultimately understand how its genes all work. And if the Venter lab can do that, they can then start to build radically different organisms from the ground up.

Mr. VENTER: We could enter into a new design phase of biology by actually constructing chromosomes of a more specific nature for a more specific purpose.

HARRIS: Venter's lab, like others, is focusing now on designer bacteria that can help combat global warming by creating more environmentally friendly fuels from plant materials.

George Church, at Harvard Medical School, is also interested in engineering bacteria that can do this. But he is not convinced that you need to build them from scratch.

Professor GEORGE CHURCH (Genetics, Harvard Medical School): And in fact, most of the proposals on the table really involve somewhere less than a hundred changes.

HARRIS: And making individual changes to the bacteria's genes isn't too tough, he says. Church sees a great deal of potential in synthetic biology. But he says that technologies also have risks, so we need to be sure this one doesn't end up being used for nefarious purposes such as engineering more deadly germs.

Richard Harris, NPR News.

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