In a final step, a team led by Daniel G. Gibson, Hamilton O. Smith and Dr. Venter report in Thursday’s issue of the journal Science that the synthetic DNA takes over a bacterial cell just as the natural DNA did, making the cell generate the proteins specified by the new DNA’s genetic information in preference to those of its own genome.

The team ordered pieces of DNA 1,000 units in length from Blue Heron, a company that specializes in synthesizing DNA, and developed a technique for assembling the shorter lengths into a complete genome. The cost of the project was $40 million, most of it paid for by Synthetic Genomics, a company Dr. Venter founded.

But the bacterium used by the Venter group is unsuitable for biofuel production, and Dr. Venter said he would move to different organisms. Synthetic Genomics has a contract from Exxon to generate biofuels from algae. Exxon is prepared to spend up to $600 million if all its milestones are met. Dr. Venter said he would try to build “an entire algae genome so we can vary the 50 to 60 different parameters for algae growth to make superproductive organisms.”

On his yacht trips round the world, Dr. Venter has analyzed the DNA of the many microbes in seawater and now has a library of about 40 million genes, mostly from algae. These genes will be a resource to make captive algae produce useful chemicals, he said.

Some other scientists said that aside from assembling a large piece of DNA, Dr. Venter has not broken new ground. “To my mind Craig has somewhat overplayed the importance of this,” said David Baltimore, a geneticist at Caltech. He described the result as “a technical tour de force,” a matter of scale rather than a scientific breakthrough.

“He has not created life, only mimicked it,” Dr. Baltimore said.

Dr. Venter’s approach “is not necessarily on the path” to produce useful microorganisms, said George Church, a genome researcher at Harvard Medical School. Leroy Hood, of the Institute for Systems Biology in Seattle, described Dr. Venter’s report as “glitzy” but said lower-level genes and networks had to be understood first before it would be worth trying to design whole organisms from scratch.

In 2002 Eckard Wimmer, of the State University of New York at Stony Brook, synthesized the genome of the polio virus. The genome constructed a live polio virus that infected and killed mice. Dr. Venter’s work on the bacterium is similar in principle, except that the polio virus genome is only 7,500 units in length, and the bacteria’s genome is more than 100 times longer.