Synthetic Life: Two Down, One to Go

For the first time ever, scientists announced last week that they have finally successfully created an entire synthetic genome. Working diligently in the lab, scientists were able to stitch together the DNA of the smallest known free-living bacterium, Mycoplasma genitalium. The research is hailed as a groundbreaking event in genetic manipulation that will one day lead to the "routine" creation of synthetic genomes—possibly including chromosomes in larger animals like mammals.

This accomplishment marks the next big step in creating entire synthetic life forms. The new work is the second step in a three-step process, said research leader Hamilton Smith, a biologist and Nobel laureate at the J. Craig Venter Institute in Rockville, Maryland.
The first step was reported last year also by the same team at Venter's institute, with the successful transplantation of a genome from one species of bacteria into another, which effectively switched the organism’s identity.

"The third step, which we're working on now, is to take the chemically synthesized DNA, which is in the test tube, and get it into a bacterium where it can take over and produce a synthetic cell," Smith said.

The research includes a project to create a cell with "the smallest number of genes that can still confer life," Smith added.

The team chose M. genitalium because it contains about 485 genes, the smallest known of any organism capable of surviving on its own. The researchers believe that at least 400 genes or so are necessary for life. Once they have completely created a synthetic copy of the bacteria, scientists will begin to eliminate genes to determine which ones are essential.

"So, this is only the beginning," Smith said.

Whereas scientists can relatively easily assemble short sequences of DNA, synthesizing entire genomes is so far quite difficult. Completing the second step was not an easy feat.
The more base pairs of the four building blocks of DNA—adenine, cytosine, guanine, and thymine—are strung together, the weaker the strands become.

Previously the longest synthesized string contained 32,000 base pairs of DNA. The M. genitalium genome, however, contains 582,970 base pairs. First the researchers broke up the genome into 101 segments, called cassettes, each containing between 5,000 and 7,000 base pairs of genetic code.

The researchers then added “watermarks” to the code to differentiate the synthetic DNA from genomes of wild M. genitalium, partially to help prevent nefarious misuses. They also inserted a gene to block the ability of the synthetic genome to infect animal hosts.

Then they combined increasingly larger sections of the genome together in a test tube with linking and repair enzymes found in the bacterium Escherichia coli until they had four overlapping quarter genome sections.

However, they were unsuccessful at combining the quarters into halves in E. coli, so the team switched to brewers' yeast. The genome came together through a process the yeast uses to repair damaged DNA.

"That was pretty remarkable," Smith said.

Previously scientists had not known that a single yeast cell could pick up all the overlapping pieces and correctly assemble them. "Yeast will play a big part in the future in assembling large DNA molecules," Smith predicted.

Posted by Rebecca Sato.

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