On a foggy Saturday morning in 1953, a tall, skinny 24-year-old man fiddled with shapes he had cut out of cardboard. They represented fragments of a DNA molecule, and young James Watson was trying to figure how they fit together in a way that let DNA do its job as the stuff of genes.

Suddenly, he realized that they joined together to form the “rungs” of a long, twisted ladder, a shape better known nowadays as a double helix.

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His first reaction: “It’s so beautiful.”

But it was more than that. Discovering the structure of deoxyribonucleic acid, or DNA, was a breakthrough that would help open the way to a revolution in medicine, biology and other fields as diverse as crime-fighting, genealogy and ethics.

Watson died

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