BALTIMORE, MARYLAND--For almost 30 years, researchers had assumed that the DNA of humans and chimps is at about 98.5% identical. Now a closer look has revealed previously undiscovered nips and tucks in equivalent sections of DNA. The DNA sliced out of--or into--these genomes could explain some of the differences between humans and our closest primates cousins.
When biologists look for differences between genomes, they typically consider stretches of DNA shorter than 1000 bases. They pick out single bases that don't match the equivalent bases in another species--so-called single nucleotide polymorphisms. At a more global view of the genomic landscape, a few cytogeneticists have mapped out differences in how chromosomes appear under the microscope.
Now two research teams have spotlighted the middle ground, using so-called gene chips to evaluate millions of bases of DNA in a single experiment. The chips carry snippets of known genetic material that, when paired up by DNA in a test sample, tell researchers what genetic code is present. The two teams reported their results here last week at the annual meeting of the American Society of Human Genetics.
For their analysis, genomicists Kelly Frazer, David Cox, and their colleagues at Perlegen Sciences in Mountain View, California, assessed the resemblance between 27 million bases of the chimp's chromosome 22 and the equivalent human chromosome, 21, using chips densely packed with small pieces of DNA. This method enabled them to detect insertions and deletions ranging in length from 200 to 10,000 bases. They found 57 areas of rearrangement between human and chimp, Frazer reported. That translates into about 5700 such changes over the genome's 3 billion bases, she estimates.
Evan Eichler and Devin Locke, geneticists at Case Western University in Cleveland, Ohio, used a different type of array, fitted with much bigger pieces of human genetic material, to study insertions and deletions of up to about 150,000 bases. The chip covered 360 million bases, or 12% of the genome. They compared this DNA to DNA from Asian and African great apes and found 63 differences. Together, these insertions and deletions suggest that the genomes are not quite as similar as researchers had thought.
Although the true significance of the new differences between chimps and humans is unclear, "these kinds of things are really exciting," says Michael Conneally, a human geneticist at Indiana University Medical Center in Indianapolis. With this research, "we can really find out so much more about evolution."