Barbara McClintock

For the illustrator named Barbara McClintock see Barbara McClintock (illustrator).

Education and research at Cornell

McClintock commenced study at the College of Agriculture at Cornell University in 1919. She studied botany, earning her B.Sc. in 1923. During her undergraduate studies her interest in genetics was sparked when she took her first course in genetics in 1921. The course was the only course of its type offered to undergraduates in America at the time and was convened by C. B. Hutchison, a plant breeder and geneticist. Hutchinson took notice of McClintock's interest and telephoned her to invite her to participate in the graduate genetics course at Cornell in 1922. McClintock credits Hutchinson's invitation as the reason she continued in genetics, "Obviously, this telephone call cast the die for my future. I remained with genetics thereafter." {{ref|McClintockAutobiography}}

Related Topics:
Botany - 1923 - C. B. Hutchison

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Women could not major in genetics at Cornell, and as she continued with her graduate studies, receiving an M.A. in 1925 and a Ph.D. 1927, her degrees were technically awarded in botany. During her graduate studies, and her post graduate appointment as an 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 Rollins Emerson, George Beadle who became a Nobel laureate in 1958 for showing that genes control metabolism, Charles Burnham, and Marcus Rhoades. McClintocks cytogenetic research focused on developing ways to visualize and characterize maize chromosomes. McClintock developed a technique using carmine staining to visualize maize chromosomes, and showed for the first time that maize has 10 chromosomes. By studying the banding patterns of the chromosomes McClintock was able to link groups of traits that were inherited together to a specific chromosome. Marcus Rhoades notes that McClintock's 1929 Genetics paper on the characterization of the chromosomes of triploid maize triggered scientific interest in maize cytogenetics.{{ref|MRhoades}}

Related Topics:
Cytogenetics - Maize - Rollins Emerson - George Beadle - Charles Burnham - Marcus Rhoades - Carmine - Link - Genetics - Triploid

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In 1930 McClintock was the first to describe the cytological pattern of cross-shaped synapsis of homologous chromosomes during meiosis in heterozygotes for reciprocal translocation. During 1931, McClintock and graduate student Harriet Creighton proved the link between the crossing over of chromosomes during meiosis and recombination of genetic traits. They observed the recombination of chromosomes and the resulting phenotype form the inheritance of a new trait. Until this point it was only hypothesized that genetic recombination could occur during meiosis. McClintock also published the first genetic map for maize in 1931, showing the order of three genes on maize chromosome 9.

Related Topics:
1931 - Harriet Creighton - Crossing over - Chromosomes - Meiosis - Genetic recombination

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In 1932 she produced a cytogentic analysis of the centromere, describing the organization and function of the centromere.

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Following her influential publications McClintock was awarded several postdoctoral fellowships from the National Research Council which allowed her to continue to study genetics at Cornell, and at University of Missouri - Columbia, and the California Institute of Technology where she worked with Thomas Hunt Morgan. During the summers of 1931 and 1932 she worked with geneticist Lewis Stadler at Missouri, and he introduced her to the use of X-ray as a mutagen. Through her work with X-ray mutagenised maize she identified ring-chromosomes. Ring-chromosomes were formed when the ends of a single chromosome fused together after radiation damage. From this McClintock hypothesized that there must be a structure on the chromosome tip that would normally ensure stability, which she called the telomere. She showed that the loss of ring-chromosomes at meiosis caused variegation in maize foliage in generations subsequent to irritation, resulting from chromosomal deletion. During this period she also demonstrated the presence a region on maize chromosome 6 that she called the nucleolar organizers which assembles the nucleolus during DNA replication.

Related Topics:
National Research Council - University of Missouri - Columbia - California Institute of Technology - Thomas Hunt Morgan - Lewis Stadler - X-ray - Mutagen - Ring-chromosome - Telomere - Variegation - Nucleolar organizer - Nucleolus

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McClintock also received a fellowship from the Guggenheim Foundation which allowed her six months of training in Germany in during 1933 and 1934, She had planned to work with Curt Stern who had demonstrated crossover in Drosophila weeks after McClintock and Creighton, however he had recently emigrated to the United States. So during her time in Germany she worked with geneticist Richard B. Goldschmidt. She left Germany early amidst mounting political tension in Europe. She returned to Cornell where she remained until 1936, when she accepted an Assistant Professorship offered to her by Lewis Stadler at the University of Missouri - Columbia in the Department of Botany.

Related Topics:
Guggenheim Foundation - Curt Stern - Drosophila - Richard B. Goldschmidt - 1936

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