Human-chimp DNA difference trebled

22:00 23 September 2002
NewScientist.com news service
Andy Coghlan




We are more unique than previously thought, according to new comparisons of human and chimpanzee DNA.

It has long been held that we share 98.5 per cent of our genetic material with our closest relatives. That now appears to be wrong. In fact, we share less than 95 per cent of our genetic material, a three-fold increase in the variation between us and chimps.

The new value came to light when Roy Britten of the California Institute of Technology became suspicious about the 98.5 per cent figure. Ironically, that number was originally derived from a technique that Britten himself developed decades ago at Caltech with colleague Dave Kohne. By measuring the temperature at which matching DNA of two species comes apart, you can work out how different they are.

But the technique only picks up a particular type of variation, called a single base substitution. These occur whenever a single "letter" differs in corresponding strands of DNA from the two species.

But there are two other major types of variation that the previous analyses ignored. "Insertions" occur whenever a whole section of DNA appears in one species but not in the corresponding strand of the other. Likewise, "deletions" mean that a piece of DNA is missing from one species.
Littered with indels

Together, they are termed "indels", and Britten seized his chance to evaluate the true variation between the two species when stretches of chimp DNA were recently published on the internet by teams from the Baylor College of Medicine in Houston, Texas, and from the University of Oklahoma.

When Britten compared five stretches of chimp DNA with the corresponding pieces of human DNA, he found that single base substitutions accounted for a difference of 1.4 per cent, very close to the expected figure.

But he also found that the DNA of both species was littered with indels. His comparisons revealed that they add around another 4.0 per cent to the genetic differences.
Junk and genes

"We're not any more different than we were," says Britten. "But we see a bit more divergence than before because insertions and deletions are taken into account. It almost triples the difference."

The result is only based on about one million DNA bases out of the three billion which make up the human and chimp genomes, says Britten. "It's just a glance," he says.

But the differences were equally split between "junk" regions that do not have any genes, and gene-rich parts of the genome, suggesting they may be evenly distributed.

Britten thinks it will be some time before we know what it is about our genes that makes us so different from chimps. He thinks the real secrets could lie in "regulatory" regions of DNA that control whole networks of genes. "It'll be a while before we understand them," he says.

ournal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.172510699)

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