First female DNA sequenced
In a world first, geneticists at the Leiden University Medical Centre (LUMC) have determined the DNA sequence of a woman. This special achievement is expected to provide considerable insight into human genetic diversity. The sequenced DNA belonged to Dr Marjolein Kriek, a clinical geneticist at LUMC. 'If anyone could properly consider the ramifications of knowing his or her sequence, it is a clinical geneticist,' commented Professor Gert-Jan B van Ommen, leader of the LUMC team and director of the Center for Medical Systems Biology (CMSB) of the Netherlands Genomics Initiative. Up until now, only four individual genomes have been sequenced, all male, including that of Dr James Watson, who has been credited with discovering the structure of DNA in 1953 along with Francis Crick. This key discovery was made in the Pauling 'tradition', by playing with molecular models. The decision to sequence the female DNA is timely, believes Professor van Ommen 'While women don't have a Y-chromosome, they have two X-chromosomes. As the X-chromosome is present as a single copy in half the population, the males, it has undergone a harsher selection in human evolution. This has made it less variable. We considered that sequencing only males, for 'completeness', shows insight into X-chromosome variability. So it was time, after sequencing four males, to balance the genders a bit.' Using the latest technology, approximately 22 billion base pairs - the 'letters' of the DNA language - were read. That is approximately eight times the size of the human genome. 'This high coverage is needed to prevent mistakes, connect the separate reads and reduce the chance of occasional uncovered gaps,' says Dr Johan den Dunnen, project leader at the Leiden Genome Technology Center. 'The sequencing itself took about six months. Partly since it was run as a 'side operation' filling the empty positions on the machine while running other projects. Would such a job be done in one go, it would take just ten weeks,' continues Dr den Dunnen. A further six months will be needed to analyse the DNA. DNA was first isolated in Europe by the Swiss physician Friedrich Miescher in 1989. This research was continued by Phoebus Levene; it was he who first suggested that DNA consisted of a string of nucleotide units linked together through the phosphate groups. While on the right track, he was mistaken in believing that the chain was short with the bases repeating in a fixed order.
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