Chromosomes reveal surprise human-chimp differences
18:00 26 May 04
NewScientist.com news service
Humans and their closest relatives, chimpanzees, may be more different than geneticists have realised.
“Previously, scientists have estimated that humans and chimps differ in about 1.5 per cent of the DNA letters that spell out their genomes. However, these estimates have been based on studies of only small subsets of the two genomes, because the chimp genome has not been sequenced precisely enough to allow a large-scale, base-by-base comparison.
That has now changed, thanks to the International Chimpanzee Chromosome 22 consortium, a team of researchers based in Asia and Europe that has sequenced a single chimpanzee chromosome in unprecedented detail.
The group then compared this sequence against its human counterpart, chromosome 21. They found that the two differ at only 1.44 per cent of the DNA bases that the two chromosomes have in common - a minuscule difference that confirms earlier estimates.
However, each gene contains hundreds or thousands of bases. This means even the tiny difference seen is enough to change the amino acid sequence of 83 per cent of the proteins generated by the 231 genes on the chromosome.
Lost and found
"Simple math probably says the 80 per cent is not surprising," says Asao Fujiyama, a consortium member at the Japanese National Institute of Informatics in Tokyo.
Also, because most of these proteins differ by only a handful of amino acids, they should still be very similar in function.
However, the group also found nearly 68,000 places where genetic material -usually just a few bases - had been gained or lost in one of the species. These changes were enough to cause major differences in the structure of over 20 per cent of the proteins - a much larger difference than previously suspected.
"Since chimps are our closest kin, we thought that protein structures would be highly conserved," says Fujiyama. It is not yet clear, though, how these changes help explain the unique biology of humans.
"We don't have a good understanding of how much of the difference really matters to what makes us human," says Tarjei Mikkelsen, a bioinformaticist at the Broad Institute in Cambridge, Massachusetts, who is involved in analysing the chimpanzee genome.
Journal reference: Nature (vol 429, p 382)