A boy from Nancy
François Jacob was born on June 17, 1920, in Nancy, in eastern France, the only child of Simon Jacob, a merchant, and Thérèse Franck. When he was three the family moved to Paris. He was an inquisitive child who learned to read early, and his hero was his maternal grandfather, Albert Franck, a four-star general in the French army.
From the age of seven he attended the Lycée Carnot in Paris, which he later described in his autobiography as a cage. Around 1934, as the far right grew louder in France, he was harassed by right-wing students there. He was gifted in physics and mathematics, but the thought of two more years of cramming for the École Polytechnique horrified him. After watching a surgical operation he chose medicine instead, intending to become a surgeon.
War with the Free French
On June 2, 1940, his mother died. Within weeks the German army had overrun France. Jacob, then a second-year medical student, later said that her death and the occupation marked the end of his childhood. In June 1940 he left for England and joined General de Gaulle’s Free French Forces in London, one of the early volunteers.
He was sent to Africa as a medical officer and saw action in Fezzan, Libya, Tripolitania and Tunisia, where he was wounded. Posted to the Second Armoured Division, he was severely wounded in Normandy in August 1944 by a German air attack and spent seven months in hospital. For his service he received the Cross of the Liberation, France’s highest decoration of the war, together with the Legion of Honour and the Croix de Guerre. His injuries ended his hope of becoming a surgeon.
Finding his way to biology
After the war Jacob finished medical school and in 1947 submitted a thesis on the antibiotic tyrothricin. That year he married the concert pianist Lise Bloch. Unable to operate, he tried other work, including producing antibiotics at the Cabanel Centre, a job he later called futile. He felt drawn to research but discouraged by how little he knew.
In 1950 he asked André Lwoff to take him into his laboratory at the Pasteur Institute in Paris, and was accepted; he took a science degree the following year. There he studied bacteria and the viruses that infect them, called bacteriophages. With Élie Wollman, from 1954, he worked out how bacteria pass genes to one another in a kind of mating, and how a virus can lie hidden in a bacterium’s chromosome. By interrupting bacterial mating at timed intervals, they showed that genes are transferred in a fixed order and that the bacterial chromosome is a closed circle. He earned his science doctorate at the Sorbonne in 1954 and became head of the Department of Cell Genetics at the Pasteur Institute in 1960.
His earliest work at the Pasteur Institute was on lysogenic bacteria, which carry a dormant virus, a prophage, inside them. Jacob showed that such bacteria are immune to fresh infection by the same virus, because something in the cell keeps the prophage’s genes silent. That puzzle of how genes are kept quiet would lead directly to his greatest discovery. He and Wollman summarised their work on bacterial mating and hidden viruses in the book Sexuality and the Genetics of Bacteria, which became a standard text for a generation of geneticists.
How genes are switched on and off
In 1958 Jacob noticed a striking parallel between his work on hidden viruses and the studies of Jacques Monod, down the corridor, on how the bacterium E. coli digests milk sugar. Scientists knew that when E. coli finds itself in lactose instead of glucose, it starts making the enzymes it needs to use the new sugar, and stops when the sugar runs out. But no one knew how the cell decided which genes to use.
Jacob and Monod worked out the answer. The genes for digesting lactose sit together in a group they called an operon. Next to them is a stretch of DNA that acts as a switch. A separate regulator gene makes a repressor protein that sits on the switch and blocks the genes from being read. When lactose is present, a form of it binds to the repressor, the repressor lets go and the genes are read. In a 1961 paper they distinguished structural genes, which carry the recipe for a protein, from regulatory genes, which decide when that recipe is used. It was the first clear explanation of how a cell controls its genes, and they proposed that similar logic would govern genes in all living things.
The messenger
The operon model needed a go-between: a short-lived molecule that carried each gene’s instructions from the DNA to the ribosomes, the cell’s protein factories. Jacob and Monod called it messenger RNA. In June 1960 Jacob travelled to Caltech in California to test the idea with Sydney Brenner and Matthew Meselson. For weeks the experiment failed, until Brenner realised that the ribosomes needed more magnesium to hold together. In May 1961 their paper in Nature, together with a companion paper from François Gros and colleagues working with James Watson at Harvard, showed that unstable RNA molecules carry genetic information from genes to ribosomes.
In 1963 Jacob and Brenner proposed the replicon model, explaining how a bacterium controls the copying of its chromosome. In 1965 Jacob, Monod and Lwoff shared the Nobel Prize in Physiology or Medicine for discoveries concerning the genetic control of enzyme and virus synthesis.
Why it matters
Every cell in a human body carries the same genes, yet a liver cell, a nerve cell and a white blood cell are utterly different. The reason is gene regulation: each cell switches on a different set of genes. Jacob and Monod gave biology its first working model of that process, and gene regulation has since become one of the largest fields in molecular biology. It explains how an embryo develops, how cells respond to hormones and drugs, and how cancers arise when growth genes are switched on at the wrong time.
The lac operon itself became a workhorse of biotechnology, widely used as an on-off switch to make bacteria produce useful proteins on demand. And messenger RNA, the molecule Jacob helped discover, became the basis of a new kind of medicine. As the Pasteur Institute noted in 2021, the unstable messenger RNA identified in 1961 is the foundation of the COVID-19 vaccines given to people around the world, once later researchers found ways to stabilise it and deliver it into cells.
The ideas Jacob and Monod introduced together, messenger RNA, regulatory genes, the operon and allosteric proteins whose shape changes when a small molecule binds, became part of the basic vocabulary of biology. They guide how researchers look for the genetic switches that go wrong in inherited disease and cancer, and how drug developers design medicines that turn particular genes or proteins on or off.
Jewish identity
Jacob grew up in an assimilated French Jewish family. In his autobiography he described his father as a conformist in religion, while his mother and the other relatives most important to him were secular Jews. He had a bar mitzvah, and soon afterwards, he said, he became an atheist. His grandfather’s career as a general had shown that a Jew could rise to the top of French society, but his schooldays brought him face to face with the antisemitic right of the 1930s.
His decision in 1940 to cross to England, rather than live under German occupation and the Vichy regime that soon passed anti-Jewish laws, placed him among the early volunteers of the Free French. The Jewish Virtual Library and other references count him among Jewish Nobel laureates.
Writer and academician
Jacob held a chair in cell genetics created for him at the Collège de France from 1964 to 1991, and chaired the board of the Pasteur Institute from 1982 to 1988. In later decades he turned to the early development of the mouse embryo, using mouse tumour cells as a tool to study the genetic circuits that guide how cells specialise.
He was also an elegant writer. His history of heredity, La Logique du vivant (1970), traced how biologists came to understand living things. In The Possible and the Actual and in a celebrated 1977 essay, Evolution and Tinkering, he argued that evolution works like a tinkerer, reusing old parts for new purposes, rather than like an engineer working from a plan. His autobiography La Statue intérieure (1987) was published in English as The Statue Within. He received the first Lewis Thomas Prize for writing about science and in 1996 was elected to seat 38 of the Académie française.
Family and legacy
Jacob and Lise Bloch had four children: Pierre, who became a philosopher, Laurent, Odile and Henri. Lise died in 1983, and in 1999 he married Geneviève Barrier. François Jacob died in Paris on April 19, 2013, aged 92.
Few scientists have also been war heroes, and fewer still have changed how humanity understands life itself. The young medical student who left France in 1940 to fight for its freedom returned to help reveal how genes are controlled and how their messages are carried. Those ideas now underpin modern genetics, developmental biology, biotechnology and the mRNA vaccines that helped the world through a pandemic.
Sources & further reading
François Jacob, Biographical, NobelPrize.org · François Jacob (1920–2013), The Embryo Project Encyclopedia, Arizona State University · Discovery of messenger RNA in 1961, Institut Pasteur · Francois Jacob, Jewish Virtual Library · François Jacob, Wikipedia · Researched from the sources above and checked against them. Spotted an error? Tell us.
A life of tikkun olam. This is part of tikkun olam, the Jewish idea of repairing the world, and every one of us can add to it.