University of Missouri
During her time at Missouri, McClintock expanded her research on the effect of X-rays on maize cytogenetics. McClintock observed the breakage and fusion of chromosomes in irradiated maize cells. She was also able to show that, in some plants, spontaneous chromosome breakage occurred in the cells of the endosperm. Over the course of mitosis, she observed that the ends of broken chromatids were rejoined after the chromosome replication. In the anaphase of mitosis, the broken chromosomes formed a chromatid bridge, which was broken when the chromatids moved towards the cell poles. The broken ends were rejoined in the interphase of the next mitosis, and the cycle was repeated, causing massive mutation, which she could detect as variegation in the endosperm. This cycle of breakage, fusion, and bridge, also described as the breakage–rejoining–bridge cycle, was a key cytogenetic discovery for several reasons. First it showed that the rejoining of chromosomes was not a random event, and secondly it demonstrated a source of large-scale mutation. For this reason, it remains an area of interest in cancer research today.
Although her research was progressing at Missouri, McClintock was not satisfied with her position at the University. She recalled being excluded from faculty meetings, and was not made aware of positions available at other institutions. In 1940 she wrote to Charles Burnham, "I have decided that I must look for another job. As far as I can make out, there is nothing more for me here. I am an assistant professor at $3,000 and I feel sure that that is the limit for me." Initially, McClintock's position had been especially created for her by Stadler and may have depended on his presence. McClintock believed she would not gain tenure at Missouri, although according to some accounts she knew she would be offered a promotion by Missouri in the spring of 1942. Recent evidence reveals that McClintock more likely decided to leave Missouri because she had lost trust in her employer and in the University administration. In early 1941 she was invited by the Director of the Department of Genetics at Cold Spring Harbor to spend her summer there. She took a leave of absence from Missouri in hopes of finding a position elsewhere. She also accepted a visiting Professorship at Columbia University, where her former Cornell colleague Marcus Rhoades was a professor. He offered to share his research field at Cold Spring Harbor on Long Island. In December 1941 she was offered a research position by Milislav Demerec, the newly appointed acting director, and she joined the staff of the Carnegie Institution of Washington's Department of Genetics Cold Spring Harbor Laboratory.
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