McClintock, Barbara (1902–1992)

US geneticist who was awarded a Nobel Prize for Physiology or Medicine in 1983 for her discovery of mobile genetic elements, meaning she discovered ‘jumping’ genes (genes that can change their position on a chromosome from generation to generation). This would explain how originally identical cells take on the specialized functions of skin, muscle, bone, and nerve, and also how evolution could give rise to the multiplicity of species.

McClintock was born in Hartford, Connecticut, and studied botany at Cornell University, New York, obtaining her PhD in 1927. As an undergraduate there, and from 1941 at the Carnegie Institute, New York, she studied maize chromosomes. She observed that the patterns on twin sectors of maize seedlings were the inverse of one another, and that pigmentation of certain kernels did not correspond to their genetic makeup. Realizing that as a single cell divided into sister cells, one gained what the other had lost, she deduced that not all genes behave in the same way: some genes can switch others on and off, moving from one place to another on one chromosome, or even ‘jumping’ from one chromosome to another. These jumping genes acted as regulators and were later discovered in bacteria and in fruit flies.

McClintock's discovery that genes are not stable overturned one of the main tenets of heredity laid down by Gregor Mendel. It had enormous implications and explained, for example, how resistance to antibiotic drugs can be transmitted between entirely different bacterial types.

She utilized X-rays to induce chromosomal aberrations and rearrangements and examined the ways in which chromosomes repair such damage. This information helped other scientists understand the problems of radiation sickness after the explosion of the atom bomb at Hiroshima, Japan.