Associated Press · Public domain

A virologist who discovered reverse transcriptase, the enzyme that lets RNA viruses copy themselves into a host's DNA, overturning the assumption that genetic information only ever flowed the other way. The finding, made at the University of Wisconsin–Madison in the early 1970s, reshaped molecular biology and later became essential to understanding how HIV replicates. He shared the 1975 Nobel Prize in Physiology or Medicine with Renato Dulbecco and David Baltimore for the work.