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Thursday, April 24, 2003

NATURAL SELECTIONS

Happy b'day double helix


This week is the anniversary of what some have called the most important intellectual innovation in human history, the discovery of the structure of DNA. From a paper originally published in Nature on April 25, 1953, DNA has made it into the pantheon of chemical structures instantly known to all members of the public, scientist and nonscientist alike.

News photo
James Watson (left) and Francis Crick stroll through Cambridge University in England during the 1950s. The two scientists won a Nobel Prize in 1962 for the discovery of the double-helix structure of DNA.

Other molecules with the same instantly recognizable status might be H2O and CO2 other acronyms as well-known are AIDS and HIV. Not everyone will know that DNA stands for deoxyribonucleic acid, but most people know that it is the molecule that contains the instructions for building life. The other day in Dublin, I saw the letters DNA sprayed mysteriously on a wall (on the next street there was more acronym graffiti, this time its context more understandable: IRA). The point is that DNA is now almost universally known. It is, as one commentator has put it, "the Mona Lisa of science."

It was not always so. The original publication, by James Watson and Francis Crick, ended with a famous piece of understatement: "This structure has novel features which are of considerable biological interest."

But it took the best part of 50 years for DNA to really arouse the interest it deserves, as Bruno Strasser, of the Institute for the History of Medicine and Health at the University of Geneva, points out in this week's Nature.

"Only when the role of DNA in protein synthesis became clearer, in the late 1950s, did biochemists, for example, take a serious interest in it," Strasser writes. "The DNA double helix became a 'totem' for the widely diverse tribe of molecular biologists."

But it was the 1990s that saw the revival of interest in DNA.

"The key to understanding the renewed fame of the double helix at the end of the 20th century is the Human Genome Project, which began in 1990," Strasser writes. "It brought the double helix, and Watson and Crick's discovery of it, to center stage as never before."

The commemoration of the discovery of DNA may this week saturate people's interest, but it is worth trying to pay attention for several reasons. Strasser says it is important because it helps shape a collective memory that influences today's scientific agenda. Collective memories are "social groups' shared representations of the past," he writes.

"Historians now pay much attention to how collective memories (of the Holocaust and of the French Revolution, for example) shape identity, and how they are constructed and transmitted through commemoration and oral tradition. It is not surprising that a similar process exists in science."

The 1953 paper, says Strasser, sets a model for good practice, where creativity and hard evidence fashion scientific knowledge.

And there is another reason that the double helix is important: The discovery gave life to the Human Genome Project.

"Setting the double helix as the highest standard of scientific creativity helped to make the descriptive genome sequencing project look less tedious," says Strasser. After all, HGP researchers had to sequence 3 billion base pairs, the building blocks of DNA. In doing so they have discovered the code of some 21,000 to 39,000 genes (we don't know how many just yet).

Comparisons between the two also help lift Watson and Crick's paper "above criticisms that it was an unimaginative piece of applied science," Strasser argues.

The consequences of knowing how to read the stuff of life are felt continuously, and have far-reaching implications. Coincidently, in the same issue of Nature this week, Sean Carroll from the Howard Hughes Medical Institute and the Laboratory of Molecular Biology, University of Wisconsin, Madison, describes one such implication.

One of the greatest challenges of biology, Carroll says, is understanding what separates us from other primate species.

Human and chimplike species split from their common ancestors 5-7 million years ago. Though we share 98.6 percent of our DNA with chimps, there are still millions of letters of genetic code that separate us from them. But, as Carroll phrases the question, which of those are mere "ticks of the molecular evolutionary clock," and which are "the smoking guns of human genetic evolution?"

In other words, what makes us different from the great apes? What are the genes that make us human, and when did we get them? Where in our brains are the things that give us human qualities and abilities?

The fossil record gives a patchy account of human ancestry. We have fossils of Homo erectus and earlier humanlike apes, such as Australopithicus. We even have fossils nearly 7 million years old, from soon after the time the human lineage split from the chimps'. But, strangely and irritatingly, there are no fossils of the ancestors of chimpanzees.

Scientists can use what we do have to compare the anatomy of the different fossils. In this way they build a picture of how key traits, such as cranium size and teeth arrangement, have changed over time.

With the human genome project now complete, and the chimpanzee genome on its way, genetics will further improve the picture. Scientists can now identify regions of gene sequence that may be linked to particular traits. The hope is that soon, or soon-ish, we will be able to answer some of the questions posed above.

However, Carroll ends his paper with a caution.

"The sequencing of the chimpanzee genome will reveal no more directly about the origin of human traits than the sequence of the human genome tells us about how to construct a human baby. Headlines may claim more, but we would be well advised to describe this as just the beginning of a large, complex and profoundly interesting story."

The process of discovery that Watson and Crick started 50 years ago is still far from completion; the molecule called the Mona Lisa of science is still as enigmatic as that painting. With so much still to be revealed, who can say what questions we will be asking on its 100th anniversary, in April 2053.

Rowan Hooper welcomes comments at rowan.hooper@tcd.ie


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