We are trying to understand what the minimum set of genes is. And while M. genitalium has 470 genes, M. pneumoniae has the same 470 genes, plus 200 extra ones. They did this using transposons, which are genes that act specifically to cut up other genes. One by one, they cut out mycoplasma genes to see which ones the organism could live without. This was harder than it sounds.
Organisms have backup systems.
Nonetheless, they got M. genitalium down to about 300 essential genes. This could form the basis for trying to put together an artificial M. genitalium, Venter reckons. Perhaps it could be built base pair by base pair from the nucleotides that make up DNA, which in turn makes up the genes. DNA is made up of nucleotides, which join together in pairs, called base pairs, in ladder-like strands twisted into a double helix. The cells could be burst open and the raw genetic material taken out to construct the new bacterium.