Scientists have created a living organism whose DNA is entirely hu man-made — perhaps a new form of life, experts said, and a milestone in the field of synthetic biology.

Researchers at the Medical Research Council Laboratory of Molecular Biology in Britain reported on Wednesday that they had rewritten the DNA of the bacteria Escherichia coli, fashioning a synthetic genome four times larger and far more complex than any previously created.

The bacteria are alive, though unusually shaped and reproducing slowly. But their cells operate according to a new set of biological rules, producing familiar proteins with a reconstructed genetic code.

The achievement one day may lead to organisms that produce novel medicines or other valuable molecules, as living factories. These synthetic bacteria also may offer clues as to how the genetic code arose in the early history of life.