Barbara McClintock - Education and Research at Cornell

Education and Research At Cornell

McClintock began her studies at Cornell's College of Agriculture in 1919. She studied botany, receiving a BSc in 1923. Her interest in genetics had been sparked when she took her first course in that field in 1921. The course was based on a similar one offered at Harvard University, and was taught by C. B. Hutchison, a plant breeder and geneticist. Hutchison was impressed by McClintock's interest, and telephoned to invite her to participate in the graduate genetics course at Cornell in 1922. McClintock pointed to Hutchison's invitation as the reason she continued in genetics: "Obviously, this telephone call cast the die for my future. I remained with genetics thereafter." Although it has been reported that women could not major in genetics at Cornell, and therefore her MA and PhD— earned in 1925 and 1927, respectively—were officially awarded in botany, recent research has revealed that women did earn graduate degrees in Cornell's Plant Breeding Department during the time that McClintock was a student at Cornell.

During her graduate studies and postgraduate appointment as a botany instructor, McClintock was instrumental in assembling a group that studied the new field of cytogenetics in maize. This group brought together plant breeders and cytologists, and included, Charles R. Burnham, Marcus Rhoades, George Beadle (who became a Nobel laureate in 1958 for showing that genes control metabolism), and Harriet Creighton. Rollins Adams Emerson, head of the Plant Breeding Department, supported these efforts, although he was not a cytologist himself. McClintock's cytogenetic research focused on developing ways to visualize and characterize maize chromosomes. This particular part of her work influenced a generation of students, as it was included in most textbooks. She also developed a technique using carmine staining to visualize maize chromosomes, and showed for the first time the morphology of the 10 maize chromosomes. By studying the morphology of the chromosomes, McClintock was able to link, to a specific chromosome, groups of traits that were inherited together. Marcus Rhoades noted that McClintock's 1929 Genetics paper on the characterization of triploid maize chromosomes triggered scientific interest in maize cytogenetics, and attributed to his female colleague 10 of the 17 significant advances in the field that were made by Cornell scientists between 1929 and 1935.

In 1930, McClintock was the first person to describe the cross-shaped interaction of homologous chromosomes during meiosis. During 1931, McClintock and a graduate student, Harriet Creighton, proved the link between chromosomal crossover during meiosis and the recombination of genetic traits. They observed how the recombination of chromosomes and the resulting phenotype formed the inheritance of a new trait. Until this point, it had only been hypothesized that genetic recombination could occur during meiosis, although it had been shown genetically. McClintock published the first genetic map for maize in 1931, showing the order of three genes on maize chromosome 9. This information provided necessary data for the crossing over study she published with Creighton. In 1938, she produced a cytogenetic analysis of the centromere, describing the organization and function of the centromere.

McClintock's breakthrough publications, and support from her colleagues, led to her being awarded several postdoctoral fellowships from the National Research Council. This funding allowed her to continue to study genetics at Cornell, the University of Missouri, and the California Institute of Technology, where she worked with E. G. Anderson. During the summers of 1931 and 1932, she worked at Missouri with geneticist Lewis Stadler, who introduced her to the use of X-rays as a mutagen. (Exposure to X-rays can increase the rate of mutation above the natural background level, making it a powerful research tool for genetics.) Through her work with X-ray-mutagenized maize, she identified ring chromosomes, which form when the ends of a single chromosome fuse together after radiation damage. From this evidence, McClintock hypothesized that there must be a structure on the chromosome tip that would normally ensure stability. She showed that the loss of ring-chromosomes at meiosis caused variegation in maize foliage in generations subsequent to irradiation resulting from chromosomal deletion. During this period, she demonstrated the presence of what she called the nucleolar organizers on a region on maize chromosome 6, which is required for the assembly of the nucleolus.

McClintock received a fellowship from the Guggenheim Foundation that made possible six months of training in Germany during 1933 and 1934. She had planned to work with Curt Stern, who had demonstrated crossing-over in Drosophila just weeks after McClintock and Creighton had done so; however, in the meantime, Stern emigrated to the United States. Instead, she worked in Germany with geneticist Richard B. Goldschmidt. She left Germany early, amid mounting political tension in Europe, and returned to Cornell, remaining there until 1936, when she accepted an Assistant Professorship offered to her by Lewis Stadler in the Department of Botany at the University of Missouri–Columbia.