Chromosome 21 sequenced

Hattori et al. report the isolation of 518 large-insert bacterial clones which span the whole of the long arm of human chromosome 21. They assembled these clones into four large contigs, with just three gaps of less than 40 kb each. They sequenced 33,546,361 bp of DNA, achieving 99.7% coverage (at 99.995% accuracy) and leaving only seven small sequencing gaps. Comparison of multiple sequences indicated that there was one sequence difference for each 787 bp. The researchers applied a number of sequence comparison and gene prediction programs in an attempt to catalog all the genes on chromosome 21. They divided this catalog into five categories: known human genes (127 genes); novel genes with extensive similarities (13 genes); novel genes with regional similarities (17 genes); novel anonymous genes defined by gene prediction (68 genes); and pseudogenes (59). Hence, approximately 41% of the predicted genes have no known function. The authors also report extensive statistical analysis, including characterization of chromosomal duplications and repeats, and variations in gene sizes and gene densities. For example, they confirm that gene-poor regions have few Alu sequences and low GC content, and that many genes are associated with CpG islands.Information on mapping and sequencing chromosome 21 is available from the various centers involved: Human chromosome 21 project at the Max-Planck-Institut of Molekulare Genetik, Berlin; the Human Genome Research Group at the RIKEN Genomics Center, Japan; the Genome Sequencing Center at the Institute of Molecular Biology, Jena; Advanced Lifescience Information Systems (ALIS) at the Japan Science and Technology Corporation (JST); and the Department of Genome Analysis at the Gesellschaft für Biotechnologische Forschung (GBF), Germany.The authors note that chromosome 21 is relatively poor in genes. They identified only 225 genes, compared to almost twice that number reported for human chromosome 22. As these two chromosomes represent ap