Intending to pursue a career in surgery, François Jacob (1920–2013) was forced to interrupt his studies in June 1940 and joined the Free French Forces in London. As an officer in the Armed Forces Medical Service, he served in North Africa and took part in the Normandy landings in 1944. After the Liberation, he resumed his medical studies, but his war injuries forced him to abandon his career as a surgeon. In 1950, he joined André Lwoff’s department at the Pasteur Institute and completed a Ph.D. in biology. He became a laboratory director there in 1956, then director of the cellular genetics laboratory in 1960.
His early work, conducted with Élie Wollman, was compiled in the book *Sexuality of Bacteria* (1959). By studying the processes of conjugation and recombination, they laid the foundations of bacterial genetics.
He also characterized lysogeny, one of the two pathways of viral reproduction, through the study of bacteriophages and their genetic interactions with bacteria.
Published in 1961, his work with Jacques Monod established that gene activity is finely tuned to the cell’s needs. They demonstrated the existence and coordinated action of regulatory sequences, repressors, and messenger RNA. This discovery profoundly transformed molecular biology and paved the way for the modern study of genetic regulatory networks. The operon model they proposed in 1961 transformed the understanding of the genome, which was henceforth no longer viewed as a mere collection of genes, but as a set of regulated and coordinated programs.
After 1961, François Jacob devoted himself in particular to the study of bacterial cell division. Together with Sydney Brenner, he proposed the replicon hypothesis in 1963, according to which the replication of a genetic element is controlled by a specific system. This model profoundly shaped the study of chromosome replication. It led to research into replication origins, initiation proteins, and the mechanisms that control the initiation of replication.
François Jacob subsequently worked on the genetics of mammalian cells, and then on the mechanisms of embryonic development and cell differentiation in mice.
He was elected professor at the Collège de France to the Chair of Cell Genetics (1964–1991).
He was awarded the Nobel Prize in Physiology or Medicine in 1965, jointly with André Lwoff and Jacques Monod, “for their discoveries concerning the genetic control of enzymatic and viral synthesis.”
In 1970, he published La Logique du vivant (The Logic of Life), a history of heredity that immediately established him as a leading historian of science, particularly admired by his colleague Michel Foucault. This work illustrates a central idea in François Jacob’s thinking: living organisms are organized into functional systems, within which genetic elements and regulatory factors interact to ensure the replication, transmission, and expression of hereditary information. The operon and the replicon are part of the scientific context that informs this line of thought.
Entry written by Ronan Le Guen (Collège de France) and Olivier Espéli (CIRB).
