Barbara McClintock Timeline

In 1930, McClintock was the first person to describe the cross-shaped interaction of homologous chromosomes during meiosis.

Image source: Meiosis

It is often reported that McClintock published the first genetic map for maize in 1931, showing the order of three genes on maize chromosome 9; however, her professor C. Hutchison had published the first genetic linkage maps for chromosome 9 in 1921 and 1922. Her map confirmed that gene arrangement was consistent with Hutchison's linkage map.

Image source: Genetic linkage

McClintock found the first ring chromosome in maize in 1931, though the first ring chromosome had been reported by Mikhail Sergeevich Navashin, whom she cited in her first study with Stadler.

Image source: Ring chromosome

During the summers of 1931 and 1932, McClintock worked at the University of Missouri with geneticist Lewis Stadler, who introduced her to the use of X-rays as a mutagen.

Image source: University of Missouri

McClintock received a fellowship from the Guggenheim Foundation that made possible six months of training in Germany during 1933 and 1934. Evidence shows she returned to work at Cornell after spending only five months in Germany.

In 1933, McClintock established that cells can be damaged when nonhomologous recombination occurs.

Image source: Homologous recombination

In 1936, McClintock accepted an Assistant Professorship offered to her by Lewis Stadler in the Department of Botany at the University of Missouri in Columbia.

In 1940, McClintock wrote to Charles Burnham, 'I have decided that I must look for another job.' She believed she would not gain tenure at Missouri, even though according to some accounts she knew she would be offered a promotion in spring 1942.

In early 1941, McClintock took a leave of absence from Missouri in hopes of finding a position elsewhere.

In December 1941, McClintock was offered a temporary research position by Milislav Demerec, the newly appointed acting director of the Carnegie Institution of Washington's Department of Genetics at Cold Spring Harbor Laboratory.

Image source: Cold Spring Harbor Laboratory

Although hesitant to make a long-term commitment, McClintock accepted the invitation and became a permanent member of the Cold Spring Harbor Laboratory staff in 1943.

In 1944, McClintock undertook a cytogenetic analysis of Neurospora crassa at the suggestion of George Beadle, who used the fungus to demonstrate the one gene–one enzyme relationship.

Image source: Neurospora crassa

In the summer of 1944 at Cold Spring Harbor Laboratory, McClintock began systematic studies on the mechanisms of the mosaic color patterns of maize seed and the unstable inheritance of this mosaicism.

Between 1948 and 1950, McClintock developed a theory by which these mobile elements regulated the genes by inhibiting or modulating their action.

Image source: Transposable element

In early 1948, McClintock made the surprising discovery that both Dissociation (Ds) and Activator (Ac) could transpose, or change position, on the chromosome—a landmark finding in genetics.

In 1950, McClintock reported her work on Ac/Ds and her ideas about gene regulation in a paper entitled 'The origin and behavior of mutable loci in maize,' published in Proceedings of the National Academy of Sciences.

Image source: Proceedings of the National Academy of Sciences of the United States of America

In summer 1951, McClintock presented her paper 'The origin and behavior of mutable loci in maize' at the annual symposium at Cold Spring Harbor Laboratory.

Based on reactions of other scientists to her work, McClintock felt she risked alienating the scientific mainstream, and from 1953 was forced to stop publishing accounts of her research on controlling elements.

McClintock published a paper in Genetics in 1953 where she presented all her statistical data, and undertook lecture tours to universities throughout the 1950s to speak about her work.

In 1957, McClintock received funding from the National Academy of Sciences to start research on indigenous strains of maize in Central America and South America.

The importance of McClintock's contributions was revealed in the 1960s, when the work of French geneticists François Jacob and Jacques Monod described the genetic regulation of the lac operon, a concept she had demonstrated with Ac/Ds in 1951.

Image source: Lac operon

Following Jacob and Monod's 1961 Journal of Molecular Biology paper 'Genetic regulatory mechanisms in the synthesis of proteins,' McClintock wrote an article for American Naturalist comparing the lac operon and her work on controlling elements in maize.

McClintock officially retired from her position at the Carnegie Institution in 1967 and was made a Distinguished Service Member of the Carnegie Institution of Washington.

Image source: Carnegie Institution for Science