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Unformatted text preview: NEWS & VIEWS Vol 437 | 1 September 2005 50 The chimpanzee and us Wen-Hsiung Li and Matthew A. Saunders Publication of the draft DNA sequence of the chimpanzee genome is an especially notable event: the data provide a treasury of information for understanding human biology and evolution. What genetic changes make us so different from the chimpanzee, our closest relative? Scientists have been trying to answer this challenging question for decades, and publica- tion of the draft of the chimpanzee genome (page 69 of this issue) 1 is a significant step for- ward. The species studied is the common chimpanzee, Pan troglodytes ; its only ‘sister’ species is the pygmy chimpanzee or bonobo, Pan paniscus (Fig. 1). The draft tells us that the DNA sequence of our genome and that of the chimpanzee differ by only a few per cent. This still amounts to tens of millions of differences because each genome contains some 3 billion nucleotides. One way to determine what the important differences are is to identify evolu- tionary changes that are specific to us, Homo sapiens . Another is to look for signatures of positive natural selection in the sequences of the two genomes. Both of these approaches, and other comparative analyses, are described in the draft-genome paper 1 and the compan- ion papers (pages 88–104) 2–4 . The assembly of a complete genome requires multiple rounds of sequencing. The chim- panzee genome draft represents a sequencing coverage of about 3.5 times, lower than that in the initial publication of other genomes, such as those of human, mouse and rat. Nonethe- less, the draft is extremely useful for showing general differences between the chimpanzee and human genomes. The new data show that they differ by only 1.23% in terms of nucleotide substitutions. This is identical to a previous estimate from a mere 53 regions, each of about 500 base pairs, randomly chosen from the genome 5 . The sequence divergence varies among genomic regions, presumably because of regional variations in mutation rate, selective constraints and the rate of sequence exchange (recombination) between chromosome pairs during cell division. The highest divergence is found for the Y chromosome and the lowest for the X chromosome. This is expected, because the Y chromosome is present only in males, which have a higher germ-line mutation rate than females, whereas the X chromosome is carried in both females and males. Natural selection is commonly thought to operate mainly at the protein level. For this reason, nucleotide changes in protein-coding regions are usually classified into two groups: ‘synonymous changes’ (which do not cause any change in amino acids) and ‘non-synonymous changes’ (which do cause amino-acid changes)....
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- DNA, Human genome, chimpanzee genome