A comprehensive comparison of the newly deciphered chimpanzee genome sequence and that of humans shows our closest living relatives share perfect identity with 96 percent of our DNA sequence, an international research consortium reported Wednesday.

In a paper published in the Sept.1 issue of the journal Nature, the Chimpanzee Sequencing and Analysis Consortium, including 67 researchers from the United States, Israel, Italy, Germany and Spain, described the landmark study comparing the genome of the chimp with that of human. Related papers will also appear in the journal Science.

"As our closest living evolutionary relatives, chimpanzees are especially suited to teach us about ourselves," said Robert Waterston, chair of the Department of Genome Sciences of the University of Washington School of Medicine. Waterston is a senior author of the study.

"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 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 percent identical. When DNA insertions and deletions are taken into account, humans and chimps still share 96 percent of their sequence, the researchers found.

At the protein level, 29 percent 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.

They 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 ions.

The researchers indicated the rapid evolution of these genes may have contributed to the special characteristics of primates, but further studies are needed to explore the possibilities.

The genomic analyses also showed that humans and chimps appear to have accumulated more potentially deleterious mutations in their genomes over the course of evolution than rodents.

While such mutations can cause diseases that may erode a species' overall fitness, they may have also made primates more adaptable to rapid environmental changes and enabled them to achieve unique evolutionary adaptations, the researchers said.

Despite the many similarities found between human and chimp genomes, the researchers emphasized that important differences exist between the two species.

About 35 million DNA base pairs differ between the shared portions of the two genomes, each of which, like most mammalian genomes, contains about 3 billion base pairs. In addition, there are another 5 million sites that differ because of an insertion or deletion in one of the lineages, along with a much smaller number of chromosomal rearrangements.

Most of these differences lie in what is believed to be DNA of little or no function. However, as many as 3 million of the differences may lie in crucial protein-coding genes or other functional areas of the genome.

Among these genetic changes are those that may be related to the human-specific features of walking upright on two feet, a greatly enlarged brain and complex language skills, the researchers said.

A few classes of genes appear to be evolving more rapidly in humans than in chimps. The single strongest outlier involves genes that code for transcription factors, which are molecules that regulate the activity of other genes and that play key roles in embryonic development.

A small number of other genes have undergone even more dramatic changes. 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.

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 animals against Alzheimer's disease.

The researchers also scanned the entire human genome for deviations from normal mutation patterns. Such deviations may reveal regions of "selective sweeps," which occur when a mutation arises in a population and is so advantageous that it spreads throughout the population within a few hundred generations and eventually becomes normal.

They found six regions in the human genome that have strong signatures of selective sweeps over the past 250,000 years.

Source: Xinhua