Researchers have assembled the entire genome of a living organism — a
bacterium — in what they hope is an important step to creating
The bug, Mycoplasma genitalium, has the smallest known genome of any
truly living organism, with 485 working genes. Viruses are smaller, but
they are not considered completely alive as they cannot replicate by
Bacteria can and do, and the team at the non-profit J. Craig Venter
Institute in Maryland has been working for years to try to build M.
genitalium from scratch.
"We consider this the second in significant steps of a three-step
process in our attempts to make the first synthetic organism," Craig
Venter, founder of the institute, told a telephone briefing.
"This entire process started with four bottles of chemicals."
M. genitalium has a fairly simple structure — all its DNA is
carried on a single chromosome. Chromosomes are the structures that
carry genetic material, and the entire code is called the genome. Other
genetic material called RNA is needed to convert this gene map into
something a cell can use to function.
Writing in the journal Science, the researchers said they used first
E. coli bacteria and then yeast cells to copy pieces of DNA and
assemble them into an artificial chromosome.
Next, the researchers say, they want to insert this artificial
chromosome into a cell and see if they can make their synthetic
chromosome "boot up" the cell.
They started by chemically making DNA fragments in the lab. Their
first step was to make the four building blocks of DNA — adenine,
guanine, cytosine and thiamine or the A, G, C and T that make up the
For M. genitalium, which can cause a sexually transmitted infection
in men and women, these four letters repeat in pairs 580,000 times.
It is not possible to build one long strand of DNA because it is too
brittle, so the sequence was broken down into pieces called cassettes.
Some work was outsourced to a lab that specializes in making circular
DNA fragments called plasmids.
Venter said the chromosome had been disabled so that it could not
live outside the lab and so that it could not take over some other
organism by mistake. The plan also underwent ethical review by a panel
at the University of Pennsylvania.
Venter’s hope is to eventually make synthetic microorganisms that
could be used for producing biofuels, cleaning up toxic waste or
pulling excess carbon dioxide out of the atmosphere.
In June 2007, the team announced that they had managed to change one
species of bacteria called Mycoplasma capricolum into another,
Mycoplasma mycoides, by replacing the genome of one with the genome of
Venter said he would like to use a synthetic chromosome in a similar way, to trick one organism into acting like another.
Another key question — what is the minimum number of genes needed
for life? Venter hopes to be able to make a streamlined, stripped-down
organism from scratch.
"We have 100 genes or so that we know we can’t knock out without
killing the cell that are of unknown structure," he told the briefing
— meaning his team still has a way to go in understanding which genes
are absolutely necessary for life.