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Two More Human Chromosomes Are Complete

By Edward R. Winstead

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Two more human chromosomes—one crowded with genes, the other not—are now essentially complete, bringing the total number of finished chromosomes to nine (out of 24). A chromosome sequence is “finished” when most of the gaps and errors in the DNA sequence from draft versions have been eliminated.

The newly sequenced chromosomes, 13 and 19, are available online, and scientific papers about them appear this week in Nature.

Work on chromosome 19 began nearly two decades ago and parallels the history of modern genomics. This chromosome was among those originally targeted by the U.S. Department of Energy for sequencing in the days before the human genome project because it contained genes thought to be associated with cancer and the repair of damaged DNA.

Today, nearly 100 genes on chromosome 19 have been linked to genetic traits or diseases, including insulin-resistant diabetes. But no genes have been found for at least 20 inherited inherited disorders associated with chromosome 19, and the new sequence will aid in the search for these genes.

The effort to finish the chromosome sequence was led by Jane Grimwood at the Stanford Human Genome Center in Palo Alto, California.

In contrast to chromosome 19, chromosome 13 has relatively few genes, despite being a mid-sized chromosome. Among other genes, the breast cancer gene BRCA2 resides on chromosome 13. A region of the chromosome has been linked to both schizophrenia and bipolar disorder, or manic depression.

To gain an evolutionary perspective on the chromosome, the researchers compared it to chromosomes in mice and rats, and found that the “gene poor” areas of the chromosome 13—stretches where there are relatively few genes—were highly similar to sequences in rodents.

It’s possible that these regions, even though they don’t contain many genes, have been conserved over the course of evolution because they are important to human health. With the chromosome sequence in hand, researchers can now explore this possibility.

Andrew Dunham of the Wellcome Trust Sanger Institute in Hinxton, the United Kingdom, led the chromosome 13 project. The other finished human chromosomes are 6, 7, 14, 20, 21, 22, and the Y.
Dunham, A. et al. The DNA sequence and analysis of human chromosome 13. Nature 428, 522-528 (April 1, 2004).
Grimwood, J. et al. The DNA sequence and biology of human chromosome 19. Nature 428, 529-535 (April 1, 2004).

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