Comparing the chimp and human genomes

31/8/05. By NIH/National Human Genome Research Institute

The first comprehensive comparison of the genetic blueprints of humans and chimpanzees shows that our closest living relatives share perfect identity with 96 per cent of our DNA sequence.

In a paper published in the 1 September 2005 issue of the journal Nature, the Chimpanzee Sequencing and Analysis Consortium described its landmark analysis comparing the genome of the chimp (Pan troglodytes) with that of human (Homo sapiens).

The chimp sequence draft represents the first non-human primate genome and the fourth mammalian genome described in a major scientific publication. "As our closest living evolutionary relatives, chimpanzees are especially suited to teach us about ourselves," said the study's senior author, Robert Waterston (University of Washington School of Medicine in Seattle). "We still do not have in our hands the answer to a most fundamental question: What makes us human? But this genomic comparison dramatically narrows the search for the key biological differences between the species."

The DNA used to sequence the chimp genome came from the blood of a male chimpanzee named Clint at the Yerkes National Primate Research Center in Atlanta. Clint died last year from heart failure at the relatively young age of 24, but two cell lines from the primate have been preserved.

The consortium found that the chimp and human genomes are very similar and encode very similar proteins. The DNA sequence that can be directly compared between the two genomes is almost 99 per cent identical. When DNA insertions and deletions are taken into account, humans and chimps still share 96 per cent of their sequence. At the protein level, 29 per cent of genes code for the same amino sequences in chimps and humans. In fact, the typical human protein has accumulated just one unique change since chimps and humans diverged from a common ancestor about 6 million years ago.

To put this into perspective, the number of genetic differences between humans and chimps is approximately 60 times less than that seen between human and mouse and about 10 times less than between the mouse and rat. On the other hand, the number of genetic differences between a human and a chimp is about 10 times more than between any two humans.

The researchers discovered that a few classes of genes are changing unusually quickly in both humans and chimpanzees compared with other mammals. These classes include genes involved in perception of sound, transmission of nerve signals, production of sperm and cellular transport of electrically charged molecules called ions. Researchers suspect the rapid evolution of these genes may have contributed to the special characteristics of primates, but further studies are needed to explore the possibilities.

More than 50 genes present in the human genome are missing or partially deleted from the chimp genome. The corresponding number of gene deletions in the human genome is not yet precisely known. For genes with known functions, potential implications of these changes can already be discerned.

For example, the researchers found that three key genes involved in inflammation appear to be deleted in the chimp genome, possibly explaining some of the known differences between chimps and humans in respect to immune and inflammatory response. On the other hand, humans appear to have lost the function of the caspase-12 gene, which produces an enzyme that may help protect other animals against Alzheimer's disease.

Adapted from a news release by the NIH/National Human Genome Research Institute .

Further reading

Chimpanzee Sequencing and Analysis Consortium: Initial sequence of the chimpanzee genome and comparison with the human genome. Nature 2005 437:69-87. Abstract

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