Meselson, Matthew Stanley (1930- )

US molecular biologist who, with Franklin Stahl, confirmed that replication of the genetic material DNA is semiconservative (that is, the daughter cells each receive one strand of DNA from the original parent cell and one newly replicated strand). Meselson was born in Denver, Colorado, and studied physical chemistry at the California Institute of Technology. He remained at Caltech, rising to professor of biology, until 1976, when he moved to Harvard. In 1963 he became a consultant to the US Arms Control and Disarmament Agency. Meselson and Stahl used the bacterium Escherichia coli for their work on DNA. They grew the bacteria in a culture medium containing nitrogen-15 (a heavy isotope of nitrogen), and then transferred the bacteria to a medium containing nitrogen-14 (the normal nitrogen isotope). Later, when they extracted the DNA, they obtained three different types: one containing only nitrogen-14, one containing only nitrogen-15, and a hybrid containing both nitrogen isotopes. On heating, the hybrid separated into two halves, one from the parental DNA and one that had been newly synthesized. These findings demonstrated that the double helix of DNA splits into two strands when the DNA replicates, with each of the single strands acting as a template for the synthesis of a complementary strand. Meselson has also investigated the molecular biology of nucleic acids, the mechanisms of DNA recombination and repair, and the processes of gene control and evolution.