he discovery of DNA's double helix 50 years ago was the founding event of molecular biology. It was also the moment that forged the reputations of two of biology's most compelling figures and united their names, no doubt in perpetuity.

"We will remember Darwin and Mendel, Watson and Crick, all in the same category," says Dr. Philip Sharp, a Nobel Prize-winning biologist at the Massachusetts Institute of Technology.

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Dr. Sydney Brenner, one of the founders of molecular biology and a close colleague of Dr. Crick's, has the same assessment. "The DNA structure is one of those turning points," he says. They will be remembered as the biologists of the 20th century, there is no doubt about that, much as Darwin is remembered as the biologist of the 19th century."

Their relationship has not been seamless. Francis Crick and James D. Watson each speak affectionately of the other now, but with a certain reserve on Dr. Crick's part, and with a still competitive edge on Dr. Watson's. The intense meeting of minds that drove them to divine the structure of DNA — a feat that each has said he could not have done without the other — made for an enduring relationship. But it was one that the ever restless Dr. Watson kept straining.

Both have had long and fruitful careers since that defining moment. Dr. Crick presided over the golden age of molecular biology, the period until 1966 when the genetic code was worked out. Deciding the major problems of molecular biology were then solved, he turned to study of biology's hardest remaining issues: embryonic development, the origin of life, and the brain. Now at the Salk Institute, where he has worked since 1976, he still wrestles with the nature of consciousness, publishing his most recent article on the subject this month.

Dr. Watson also forged an unusual career path. He became a scientific impresario, building Harvard's biology department into a leading institution, and then doing the same at the Cold Spring Harbor Laboratory on Long Island, where he is now president. He created political support for the Human Genome Project and, becoming its first director in 1988, set the goals and strategy that are now bringing it to fruition.

But it is the discovery of the double helix at the center of all life that has shaped their relationship.

"Jim and I hit it off immediately," Dr. Crick wrote in his memoir, "What Mad Pursuit," "partly because our interests were astonishingly similar and partly, I suspect, because a certain youthful arrogance, a ruthlessness and an impatience with sloppy thinking came naturally to both of us."

Despite the shared nature of their success, Dr. Watson, 12 years younger than Dr. Crick and only 23 when they first met, long felt the need to prove his independence.

"Just because he was so dominant I didn't automatically accept everything he told me, or otherwise you would have just lost your self-confidence," Dr. Watson said in an interview last month. "Francis treated me like a younger brother. Very nice — you look after your younger brother. But younger brothers sometimes want to be equal to their older brothers." It was not until 1962, he says, with the Nobel Prize and the success of his Harvard laboratory that "I didn't feel I had no story of my own."

Dr. Watson was not above playing practical jokes on his "elder brother," some with a certain edge to them. Tiring of Dr. Crick's voluble explanations of the new double helix to a stream of visitors, Dr. Watson forged a letter, purportedly from Linus Pauling, inviting Dr. Crick to lecture on the helix at Dr. Pauling's lab at Caltech. The clue to the letter's fraudulence, overlooked by its victim, was that it asked Dr. Crick to speak for as long as possible, Dr. Watson says, still racked with laughter at the memory. Another joke, perhaps to symbolize their search for the naked truth, was the unattired woman whom Dr. Watson hired to jump out of a cake at a party celebrating Dr. Crick's 50th birthday.