The creation of the "synthetic cell", described as a "landmark" by one British expert, is a 15-year dream come true for maverick genetics entrepreneur Dr Craig Venter.

It has major implications for genomics, including the manufacture of artificial organisms designed for specific tasks such as making vaccines or cleaning up pollution.

But experts recognise that as Mary Shelley demonstrated in her famous novel, there are potential dangers too. Synthetic life could, for instance, pave the way to terrifying biological weapons.

Dr Venter's researchers explain in the journal Science how they effectively "re-booted" a simple microbe by transplanting into it a set of genetic code sequences that were built from scratch.

The genome was copied from the blueprint contained in Mycoplasma mycoides, a simple bacterium that infects cattle and goats.

After first constructing short strands of DNA, the scientists used yeast cells as natural factory assembly lines.

The sequence was built in a step-by-step process. DNA repair systems in the yeast attached the pieces together, gradually lengthening the strands to finish up with a chromosome more than a million "letters" of genetic code long.

The final test came when the completed chromosome was transplanted into another bacterium, Mycoplasma capricolum, replacing its native DNA.

After a failed first attempt, the scientists brought the cells to life. Driven by the new genome, the bacteria took on the appearance and behaviour of M. mycoides, generating different proteins and multiplying.