French biochemist who, with François Jacob (Jacob, François), did much to elucidate how genes regulate cell metabolism by directing the biosynthesis of enzymes. The pair shared, along with André Lwoff (Lwoff, André), the Nobel Prize for Physiology or Medicine in 1965.

In 1961 Jacob and Monod proposed the existence of a messenger ribonucleic acid (mRNA), a substance whose base sequence is complementary to that of deoxyribonucleic acid (DNA) in the cell. They postulated that the messenger carries the “information” encoded in the base sequence to ribosomes, the sites of protein synthesis; here the base sequence of the messenger RNA is translated into the amino acid sequence of a proteinaceous enzyme (biological catalyst).

In advancing the concept of gene complexes that they called operons (operon), Jacob and Monod postulated the existence of a class of genes that regulate the function of other genes by affecting the synthesis of messenger RNA. For this work, which has been proved generally correct for bacteria, the two men were awarded a Nobel Prize.

Monod's book-length essay Le Hasard et la nécessité (1970; Chance and Necessity) argued that the origin of life and the process of evolution are the result of chance. Monod joined the staff of the Pasteur Institute in Paris in 1945 and became its director in 1971.