World of Scientific Discovery on Barbara McClintock
Barbara McClintock was born on June 16, 1902, in Connecticut. The daughter of a physician, she grew up in the Flatbush section of Brooklyn, New York--a rural area during that time. During high school, McClintock's interests turned from sandlot baseball to science. Despite her parents' misgivings, she persuaded them to let her study at Cornell University, an agricultural college in Ithaca, New York. McClintock was fascinated with a genetics course she took in her junior year. However, in the plant breeding department where most of the genetics courses were taught, women were not permitted as graduate students. So McClintock signed up in the botany department and became assistant to a cytology professor who encouraged her interest in plant genetics.
After McClintock completed her graduate work at Cornell, she stayed on as an instructor and researcher, associating herself with Rollins Emerson's famous maize (corn) genetics group. During this time, she discovered the function of each chromosome in corn. Cultivating her own crops on university grounds, she spent summers recording changes in each plant, then returned to the lab after the harvest. By observing cell divisions at various stages of a corn plant's development, she was able to tell what each chromosome did in the process. Between 1929 and 1931, she published nine papers on the subject of genetics.
In 1931, McClintock assigned graduate student Harriet Creighton (1909- ) a problem in genetic crossover in corn. With McClintock's guidance, Creighton was able to show that genetic information is transferred after chromosomes cross over during the formation of sex cells. While on a visit to Cornell, Thomas Hunt Morgan, one of the pioneers in the study of genetics, urged McClintock and Creighton to publish their findings on genetic crossover in corn. They did, receiving favorable reviews for their work.
McClintock's major contribution to genetics came after she left Cornell, which at the time had no women above the level of professor outside the home economics department. It was difficult at first, because she had gained a reputation as being a difficult, independent loner. She worked briefly at the University of Missouri, but felt isolated from the administration and constrained by its bureaucratic rules (she often ignored building hours and would break into her lab on Sundays). In 1941, she found a permanent position at Cold Spring Harbor Laboratory on Long Island, a residential research site run by the Carnegie Institution of Washington, where she could conduct her experiments free of administrative interference.
Over the years, McClintock had developed an interest in the mutations caused by x-rays. She was so familiar with maize plants and their chromosomes that she instantly recognized anything unusual. One season, some of her corn seedlings had developed odd streaks and spots on the leaves. There appeared to be a pattern, but it was unlike any pattern made from the x-rays. Something in the plant itself seemed to be controlling the appearance of the patterns. She studied more closely and found that many color patches showed up in opposite pairs. For instance, if part of a leaf contained more green streaks than usual, a nearby portion would contain fewer than usual.
To McClintock, these were not signs of disorder, but of a larger and different system of order. She set out to make that difference understandable. For six years, she researched her unusual observations, and in 1951, she presented her theory of genetic transposition (also called genetic mobility or gene jumping). According to her theory, genes in a chromosome could actually move from one position to another within that same chromosome. McClintock saw transposition as a way that individual organisms regulate their development, and even their survival, in times of stress. McClintock's discovery, however, was met with derision. The majority of geneticists believed that genes were preprogrammed, and, like beads on a string, occupied a fixed place on chromosomes. Any changes, therefore, occurred only as a result of random accidents. When McClintock presented her findings at a symposium, her colleagues dismissed her as "an old bag who'd been hanging around Cold Spring Harbor for years."
By the 1970s, McClintock had expanded her theory, arguing that genes could not only change their positions on a chromosome, but that they could also serve different purposes in those different positions. Slowly, her ideas gained acceptance, as scientists began to witness transposition in other life forms. But it wasn't until 1983, some 30 years after her discovery, that McClintock received the Nobel Prize in physiology or medicine. She was also the recipient of the National Medal of Science (1970) and the first MacArthur Laureate Award (1981), a lifetime annual prize of $60,000.
When asked about the long delay in recognition for her discovery, McClintock observed, "If you know you're right, you don't care. You know that sooner or later, it will come out in the wash." McClintock died on September 2, 1992.
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