Lean Gene Machine; December 2005; Scientific American Magazine; by Steven Ashley; 3 Page(s)
Some 25,000 genes code for the proteins required to build each human being, a figure representing only 1 or 2 percent of our entire genome. The remainder is "junk DNA"--base-pair sequences that do not directly code for proteins. But where organisms must operate extremely efficiently to endure hostile habitats, evolution has greatly optimized the DNA, shaving the genome down to what some regard as the minimum genetic requirement to create and develop life.
A new champion in this special bantamweight class has emerged. An oceanic bacterium, Pelagibacter ubique (or SAR11), one of the smallest self-replicating cells known, has only 1,354 genes, investigators report in the August 19 Science. "SAR11 has almost no wasted DNA," says Stephen J. Giovannoni, the microbiologist who led the collaboration between research groups at Oregon State University and Diversa Corporation (a prospector for useful natural pharmaceuticals and enzymes). Stripped down to the bare essentials for living, the bacterium's gene sequence has almost none of the clutter that most genomes have amassed over time: no duplicate entries, no viral genes, no introns or noncoding sequences. (Many organisms--namely, parasites and symbionts--have smaller genomes, but they rely on others to perform bodily housekeeping tasks.)