Image: DOE HUMAN GENOME PROGRAM GENES ARE ENCODED IN THE DNA by four bases, the letters of the genetic alphabet (A, T, G, C). Chromosomes contain the DNA; they are located in the nucleus of a cell.

Hang on to your seats. The race to sequence the human genome--now in its final laps--is speeding up. Some three weeks ago, the Maryland company Celera Genomics--a relative newcomer to the track, headed by Craig Venter--appeared to lurch ahead of the favored contestant, the publicly funded Human Genome Project. On April 6, Celera announced that after only seven months of work, they had deciphered close to all 3,000,000,000-odd base pairs, or letters of the genetic alphabet, in the human genome.

It was news that made some spectators ready to switch bets. But, in fact, the race is still too close to call. The Human Genome Project, which has steadily worked at cracking the human genome since 1990, is also near completion of a "working draft." And Celera's announcement doesn't necessarily mean they have got the prize yet.

Celera's researchers used a "shotgun approach." They took one person's DNA (the molecule that contains all genes), chopped it up into little pieces and determined the sequence of base pairs in each piece. Now, however, they have to put the pieces back together. Only when they complete this jigsaw puzzle will they have sequenced the genome. But using giant computers that crunch numbers day and night, Celerea may finish this assembly any week from now.

Image: ARTEMIS PHARMACEUTICALS THE FRUIT FLY Drosophila melanogaster has been used by geneticists for almost 100 years. Now its genome sequence is fully known.

Regardless of who wins this sprint, the next race--to make sense of the genome--will be a marathon with many runners. This task will fall to a whole generation of biological, medical and information researchers. Just the first step, which researchers call annotation, could very well take many months. From the entire DNA sequence, they will need to find the 5 percent of it that actually contains genes. Looking for these estimated 80,000 to 140,000 genes among nonsense strings of base pairs will be like looking for needles in a haystack.

Then scientists will have to sort out what the individual genes do. And this is where yeast, worms and flies can help: Although they may not look it, these so-called model organisms share a lot of genetic information with humans. "I think about 40 percent of all yeast genes have a human counterpart or have a gene which is similar in function in humans," says Marc Cockett, executive director of Functional Genomics at Bristol-Myers Squibb. And these organisms are a lot easier to study than people: they breed faster, and their genes are easy to manipulate. Compared with animals like mice, they are cheaper to raise. Thus, by finding out how their genes work,we can learn a lot about our own genes.

So far geneticists have boarded three such species on the "Ark of Genomes," sequencing their genomes completely: Baker's yeast (Saccharomyces cerevisiae), the nematode worm Caenorhabditis elegans and the fruit fly Drosophila melanogaster. (Celera and the Berkeley Drosophila Genome Project just jointly published the fly genome last month in Science). Soon, the mouse--which from a genetic point of view is nearly human--will join this ship: its genome is sought in another race between the publicly funded mouse genome sequencing project and Celera. And the zebrafish, which is transparent when it is young and so lets researchers watch its organs grow, will probably be next in line.

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