Barbara McClintock

Barbara McClintock an American cytogeneticist (1902-1992) whose discovery of transposable genetic elements, or jumping genes, in maize transformed the understanding of the genome. Her work was long neglected, and she received the Nobel Prize in Physiology or Medicine in 1983, more than thirty years after her decisive findings.

Early Life and Education

Barbara McClintock was born in 1902 in Hartford, Connecticut, and grew up in Brooklyn, New York, the daughter of a physician and a musically gifted mother. She was an independent and solitary child who found pleasure in intellectual pursuits, and she entered Cornell University in 1919. There she discovered a love of science, took a course in genetics and decided to make it her career.

She received her bachelor's degree in 1923 and her master's in 1925, and in 1927 she was awarded a doctorate in botany at Cornell. She remained at the university as an instructor and research associate, working on the genetics of maize, and she became a central member of a group of young researchers who were transforming the study of chromosomes.

At a time when women had difficulty obtaining academic appointments, she supported herself through research fellowships and temporary positions for many years. She worked at Cornell, at the University of Missouri and finally at the Cold Spring Harbor Laboratory on Long Island, where she found the stability and freedom that her work required.

Cytogenetics and the Maize Chromosomes

McClintock became a master of the microscopic study of chromosomes. She developed techniques for staining and identifying the individual chromosomes of maize, an organism with only ten pairs, and she produced the first cytological map correlating the visible chromosomes with the genes they carried. Her careful preparations made maize one of the best-understood organisms in genetics.

In the late 1920s and 1930s she demonstrated the role of the chromosome in heredity and showed how the physical exchange of material between chromosomes during cell division produces new combinations of genes. Her work helped to establish that genes are arranged in a definite order along the chromosomes, and she clarified the behaviour of the nucleolar organiser region.

She observed that chromosomes could break and rejoin, and that the broken ends behaved in ways that demanded explanation. These observations led her to ask how the ends of chromosomes are protected from fusion, and eventually to the recognition that the chromosome is a dynamic structure capable of rearrangement. Her insight prepared the ground for everything that followed.

Transposable Elements and Controlling Elements

In the 1940s, working at Cold Spring Harbor, McClintock made the discovery for which she is famous. Studying the inheritance of pigmentation in maize kernels, she found that certain genetic elements could move from one location to another within the chromosomes, inserting themselves at new sites and altering the activity of neighbouring genes. She called them controlling elements.

Her experiments showed that these elements could switch genes on and off and that their movement explained a wide range of puzzling hereditary patterns. She argued that the genome is not a static collection of fixed instructions but a dynamic system in which genes are regulated and rearranged, and she proposed that controlling elements might have a role in development and evolution.

The results were so unexpected that they met with scepticism and, for the most part, silence. When she reported her findings in the early 1950s, few geneticists could connect them with the molecular understanding of the gene that was then emerging. Discouraged by the neglect, she stopped publishing detailed accounts of the work, though she continued her experiments for years.

Method, Intuition and the Culture of Maize

McClintock's method rested on an intimate knowledge of the organism she studied. She grew generation after generation of maize, examined the colours and patterns of its kernels, and prepared thousands of microscopic slides of its chromosomes. She insisted that the researcher must develop a feeling for the organism, and she often spoke of the patience and openness required to let the material reveal its own logic.

She was noted for the intensity of her concentration and for the intuitive leaps by which she connected observations that others could not integrate. Colleagues describe long solitary hours at the microscope and a willingness to trust hunches that she then tested with rigorous genetic crosses. Her ability to see the chromosome as a dynamic system came from this combination of intimacy and imagination.

She was generous with her knowledge and trained many students and visitors, but she was also reserved and worked best alone. She held a position at the Carnegie Institution's Department of Genetics at Cold Spring Harbor for practically the whole of her mature career, and it was there, in a small laboratory overlooking the harbour, that she did her greatest work.

Recognition and Legacy

For decades McClintock's discovery was largely ignored, and only with the development of molecular genetics in the 1960s and 1970s did the significance of transposable elements become clear. Similar elements were found in bacteria, in other plants and in animals, and their study became a major field. Her controlling elements were recognised as the first evidence that genes can move.

She received many honours, including election to the National Academy of Sciences and the presidency of the Genetics Society of America, and in 1983 she was awarded the Nobel Prize in Physiology or Medicine, the first woman to win that prize unshared. She was then eighty-one years old and had continued to work at Cold Spring Harbor until shortly before her death.

She died in 1992 on Long Island. Her work is now recognised as one of the foundations of modern genetics, and transposable elements are known to play a central part in genome structure, evolution and disease. Her perseverance in the face of indifference, and her conviction that the organism itself would teach the attentive observer, remain an example of scientific integrity.

See also

• Transposable element
• Maize
• Cold Spring Harbor Laboratory
• Cytogenetics