BARBARA McCLINTOCK: LIFE & WORK*
by Lee B. Kass, Visiting Professor, Cornell University (*modified from Kass 2007b)
Barbara McClintock (1902-1992), one of the foremost women scientists in 20th century America, is most noted for her pioneering research on transposable elements in maize. For this work she was awarded the Nobel Prize in Physiology or Medicine in 1983. She was the third woman to receive an unshared Nobel Prize in the sciences.
Born in Hartford, Connecticut, on June 16, 1902, Barbara McClintock was raised in Brooklyn, New York. After graduating from Erasmus Hall High School, she entered Cornell University at age 17, and in 1923 earned a B.S. in agriculture, concentrating in plant breeding and botany. She received both her master's (1925) and doctoral degrees (1927) from Cornell's College of Agriculture. She majored in cytology with Lester W. Sharp in the Department of Botany, and minored in genetics and zoology with Allan C. Fraser and Hugh D. Reed in the Departments of Plant Breeding and Zoology, respectively. As a graduate student, McClintock was a research and teaching assistant in the Department of Botany, Cornell University, College of Agriculture. During these years, Sharp referred both botany and plant breeding graduate students and post-doctoral researchers to her. Most notable were George Beadle (Ph.D. 1930), who learned cytology from McClintock, and went on to head the biology division at Caltech and win a Nobel Prize (1958); and L. J. Stadler (NRC Fellow 1926), later elected to the National Academy of Sciences.
McClintock's career as one of the most prominent geneticists of the 20th century was launched while she was at Cornell. Upon receiving her doctorate, McClintock was made an Instructor. At that time, this appointment was the first step leading to tenure at colleges and universities like Cornell. Jobs in academia were scarce during the Depression, and jobs for women were limited. While employed at Cornell, Instructor McClintock continued to mentor and collaborate with graduate students. She befriended graduate student Marcus Rhoades (Ph.D. 1932), who also rose to preeminence in genetics and was McClintock's lifetime supporter. From 1927 to 1931, she taught undergraduate and graduate courses in Cornell's Department of Botany.
In early 1929, McClintock published her Ph.D. dissertation in Genetics, the foremost journal in the field. Within two years, she had published six other articles in major journals, all of which made important contributions to the newly emerging field of plant cytogenetics, and furthered the world's knowledge about the location of genes on chromosomes. McClintock collaborated with students on the most notable of these investigations.
Instructor McClintock gave graduate students Henry Hill and Harriet Creighton two important projects for their thesis research, and co-authored these pioneering contributions with them. The first was a method to connect chromosomes with linkage groups in corn (McClintock & Hill 1931) and the second was the cytological proof for crossing over (McClintock 1931, Creighton & McClintock 1931). Creighton and McClintock's significant study gave further confirmation to T. H. Morgan's chromosome theory of inheritance, for which he won a Nobel Prize in 1933. These collaborative projects were based on important work that McClintock had pioneered: identification of corn's ten chromosomes at mitosis (and later at meiotic pachytene stage), confirmation of Belling's translocation hypothesis, and the sequence of the genes in Chromosome 9. Creighton (Ph.D. 1933) became head of Botany at Wellesley College and President of the Botanical Society of America in 1956.
From 1931 through 1934, sponsored by two National Research Council Fellowships, and a prestigious Guggenheim Fellowship, McClintock traveled to a series of important research institutions across the U.S., Germany, and back to Cornell, where she worked in the Department of Plant Breeding as an assistant to R. A. Emerson, head of the department. There, she conducted research, funded by the Rockefeller Foundation, which would provide insights to an understanding of variegation, and would eventually lead to her Nobel award-winning investigations.
In 1936, McClintock accepted an appointment as Assistant Professor of Botany at the University of Missouri to join L. J. Stadler's genetics research group. Upon learning that the research unit might be eliminated, and preferring research over teaching, McClintock requested a leave of absence from Missouri in 1941 to seek employment elsewhere. In 1943, she accepted a position as a permanent staff member of the Carnegie Institution of Washington's Department of Genetics at Cold Spring Harbor on Long Island, New York. It was there, continuing work she began at Cornell and at Missouri, that she discovered "mobile genetic elements" in corn, for which she was awarded the Nobel Prize in Medicine or Physiology in 1983. She remained at Cold Spring Harbor for the duration of her career, accepting only short term appointments at national and international institutions elsewhere.
McClintock achieved considerable recognition within her lifetime. In 1944, prior to her most celebrated work, she was elected to the National Academy of Sciences, the third woman so honored. McClintock also became the first woman elected Vice President (1939) and President (1945) of the Genetics Society of America. By 1947, she received the Achievement Award from the American Association of University Women.
But it is for McClintock's work with maize, beginning in the mid-1940s, her meticulous observations of the dynamism of the genome, her communications of her theory of genetic transposition-the idea that genes could spontaneously change their position on a chromosome-that reinforced her reputation as a pioneering geneticist, which was widely acknowledged in later years. In 1957, the Botanical Society of America recognized her achievements with their esteemed Merit Award, and Cornell appointed her one of their first A.D. White Professor's-at-Large in 1965 (renewed in 1971).
McClintock also won a number of prizes during her later career. A few months before she formally retired in 1967, she received the Kimber Genetics Award from the National Academy of Sciences. In that year, the Carnegie Institution of Washington appointed her a Distinguished Service Member, one of their highest honors, which made it possible to continue working at Cold Spring Harbor Laboratory. During the 1970s she received the National Medal of Science (1970), the Lewis S. Rosensteil Award (1978), and the Louis and Bert Freedman Foundation Award (1978). A few years before receiving the Nobel Prize, she was honored with many awards; more notable were the Thomas Hunt Morgan Medal, the Wolf Foundation Prize in Medicine, a shared Albert Lasker Basic Medical Research Award, and the first Prize Fellow Laureate of the MacArthur Foundation.
McClintock's life as a scientist was not always easy. Full appreciation of the implications of her work on mobile genetic elements, which challenged generally held beliefs that the chromosome had a stable structure, was not possible until molecular biologists found similar phenomena in bacteria and other organisms. Her work continues to influence and inspire the field of transposons and genomic dynamics in plants.
As one of the early women scientist in this country, McClintock earned timely recognition for her pioneering achievements, gaining a star in American Men of Science by 1944. Yet, as an aspiring young geneticist, she experienced injustice because of her gender. Determined to succeed in her chosen field, and respected and helped by devoted colleagues, McClintock eventually found a position at an institution at Cold Spring Harbor that gave her the freedom to pursue her love of science and which, she said, "fit her personality rather well."
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Photo above: Cornell University 1929: (from right to left) Barbara McClintock with Professor R.A. Emerson and his graduate students Marcus Rhoades and George Beadle (kneeling), and Postdoctoral Fellow Charles Burhnam (used with permission of W. B. Provine and M. Sorrells).
Barbara McClintock and Lewis Stadler at the University of Missouri in the early days of maize genetics.