This paper examines the contested "biological" meaning of the genetic effects of radiation amid nuclear fear during the 1950s and 1960s. In particular, I explore how the question of irreversibility, a question that eventually led to the discovery of DNA repair, took shape in the context of postwar concerns of atomic energy. Yale biophysicists who opposed nuclear weapons testing later ironically played a central role in the discovery of DNA excision repair, or "error-correcting codes" that suggested the reversibility of the genetic effects of radiation. At Yale and elsewhere, continuing anticipation of medical applications from radiation therapy contributed to the discovery of DNA repair. The story of the discovery of DNA repair illustrates how the gene was studied in the atomic age and illuminates its legacy for the postwar life sciences. I argue that it was through the investigation of the irreversibility of the biological effects of radiation that biologists departed from an inert view of genetic stability and began to appreciate the dynamic stability of the gene. Moreover, the reformulation of DNA repair around notions of information and error-correction helped radiobiologists to expand the relevance of DNA repair research beyond radiobiology, even after the public concerns on nuclear fallout faded in the mid-1960s.