The detailed genome sequence of the male-making human Y chromosome is unveiled in this week's Nature (Vol. 423, No. 6942, 19 June 2003, pp. 825-837 and pp. 873-876). Many cases of male infertility are due to genetic defects, and the Y sequence should give insights into their precise cause. The chromosome is also an important evolutionary document, recording 300 million years of genetic change since it went its separate way from the X chromosome.
About 95% of the Y chromosome is a genetic pariah - it does not swap DNA with a partner chromosome when sperm and eggs are made. This region is 23 million DNA letters long and contains 78 genes, say David C. Page and colleagues. Some genes are switched on throughout the body's cells, whereas others are active only in the testes.
Most of the chromosome is taken up with repetitive DNA sequences. Such repetitive, isolated DNA is at risk from accumulating mutations and becoming a genetic junkyard. In a separate study, Page and colleagues reveal how the Y chromosome avoids this, and so protects genes active in the testes that lie in the repetitive regions. The Y chromosome, they suggest, swaps DNA with itself; that is, these repetitive sequences exchange sequence between the two arms of the chromosome. This happens at an astonishingly high rate - the difference between a man's Y chromosome and his father's averages about 600 base pairs.
Sequencing repetitive DNA is a difficult and laborious task - completing the Y chromosome sequence is a "nearly heroic" achievement, says Huntington Willard in an accompanying News and Views article.
David C. Page
The Whitehead Institute
MIT, Cambridge, MA
Tel: +1 617 258 5203
Tel: +1 919 668 4477
(C) Nature press release.
Message posted by: Trevor M. D'Souza