Humans have 48 chromosomes

Difference between humans and chimpanzees bigger than expected

Apparently small differences between chimpanzee chromosome 22 and human chromosome 21 have considerable consequences for the encoded proteins

Until now, it was assumed that humans and chimpanzees differ only slightly in terms of their genetic make-up. But now a team of scientists from Germany, China, Japan, Korea and Taiwan, when directly comparing chimpanzee chromosome 22 with its human counterpart, chromosome 21, has found that almost 68,000 base segments in the human chromosome have been changed, i.e. either added or lost . While most of these changes have little or no effect on the function of the encoded proteins, the researchers found significant structural differences in as many as 20 percent of them. If these differences are extrapolated to the entire genome, apes and humans could differ in several thousand genes - which would better explain the differences between the two species (Nature, May 27, 2004).

The chimpanzee Pan troglodytes is our closest relative, with whom we share a common ancestor who lived about five to six million years ago. For a long time, people have therefore tried to decipher the molecular basis of those evolutionary changes that have led to two organisms with clear differences in phenotype and behavior. By comparing the chimpanzee and the human genome, the aim is to find out which genetic changes have led to such clearly human characteristics as cognition and consciousness, upright gait or an altered susceptibility to disease. Up until now, it was assumed that the genomes of humans and chimpanzees correspond to 98.6 percent at the level of their building blocks (nuleotides).

While comparative genome research between relatively distant organisms (evolutionary distance between humans and mice: 60 million years) focuses on finding similarities between species, the chimpanzee genome tends to lead to the differences between apes and humans. Scientists of the "International Chimpanzee Genome Chromosome 22 Sequencing Consortium" have now analyzed and compared a small part of the genetic make-up of humans and chimpanzees, human chromosome 21 and its counterpart, chimpanzee chromosome 22, with unprecedented accuracy. They found that the number of minor discrepancies - when a single base was replaced by another - is only 1.44 percent, which would confirm previous estimates of 98.6 percent identity.

But the researchers also found almost 68,000 longer sections in the genome in which entire sequences of bases have been incorporated as an insert or lost as a deletion. However, this means that the amino acid sequence of the proteins encoded by the 231 genes discovered differs by 83 percent in humans and monkeys. However, most of these changes have little or no effect on the function of the proteins. However, with at least 47 proteins, or 20 percent, significant structural differences were found.

While many proteins are absolutely identical in chimpanzees and humans, some proteins show structural differences that can lead to different functions. If you take into account that chromosomes 21 and 22 only carry about one percent of the total genetic material, the differences between humans and chimpanzees could comprise several thousand genes. But this would mean that the genetic differences between humans and their closest relatives would be much larger and more complex than previously assumed, says Marie-Laure Yaspo, head of the Chomosome Project at the Max Planck Institute for Molecular Genetics.

Next, the researchers want to sequence selected regions of the sex chromosomes, i.e. compare the X and Y chromosomes of humans and monkeys. The X chromosome is of particular medical interest because of the large number of hereditary diseases associated with it, especially various forms of mental retardation.

The sequencing of chromosome 22 was coordinated by the Japanese RIKEN Institute in Yokohama and the Max Planck Institute for Molecular Genetics in Berlin, and the complete sequence was published in July 2003. This consortium then worked out a direct comparison of both chromosomes. In addition to the Berlin Max Planck scientists headed by Prof. Hans Lehrach, numerous other researchers from all over Germany were involved in the international research team, including from the Society for Biotechnological Research in Braunschweig, the Institute for Molecular Biotechnology in Jena and the Max Planck Institute for Evolutionary Anthropology in Leipzig. Your contributions to this genome project represent an important milestone in the work of the National Genome Research Network (NGFN) in Germany, which is funded by the Federal Ministry of Education and Research.

The partners in the "International Chimpanzee Genome Chromosome 22 Sequencing Consortium" are:

Chinese National Human Genome Center at Shanghai: Zhu Chen ([email protected])

Max Planck Institute for Molecular Genetics: Marie-Laure Yaspo ([email protected])
Institute for Molecular Biotechnology: Matthias Platzer ([email protected])

Society for Biotechnological Research: Helmut Blöcker ([email protected])
Max Planck Institute for Evolutionary Anthropology: Svante Pääbo ([email protected])

RIKEN Genomics Sciences Center: Yoshiyuki Sakaki ([email protected])
National Institute of Genetics: Naruya Saitou ([email protected])

Korea Research Institute of Basic Sciences and Biotechnology: Hong-Seog Park ([email protected])

National Health Research Institutes: Shih-Feng Tsai ([email protected])