Barbara McClintock

Barbara McClintock

American geneticist, Nobel Prize in Physiology or Medicine, 1983
Date of Birth: 16.06.1902
Country: USA

Biography of Barbara McClintock

Barbara McClintock, an American geneticist, was born in Hartford, Connecticut. She was the youngest of three daughters of Thomas Henry McClintock, a doctor, and his wife Sarah Handy. As a child, Barbara spent a lot of time in rural areas of Massachusetts with her paternal aunt and uncle, enjoying the outdoors. When she was 8 years old, her family moved to Flatbush, Brooklyn, where her father worked as a doctor for the Standard Oil Company. Barbara was interested in figure skating and other sports, often playing games with the boys in her neighborhood. She also developed a habit of reading and contemplating in solitude.

In 1918, Barbara graduated from a Brooklyn high school. Despite resistance from her parents, who did not want her to continue her education but eventually gave in, Barbara was enrolled at Cornell University in Ithaca, New York in 1919 to study biology in the College of Agriculture. She was elected president of the freshman women's club at the university. Initially, she actively participated in social activities and even played the banjo in a jazz orchestra. However, upon learning that her Jewish friends were not invited to a women's organization, she decided not to join. Instead, she attended lectures on genetics and impressed her professors, who offered her the opportunity to work with graduate students, despite being younger than them. In 1923, Barbara received her bachelor's degree and continued her studies specializing in cytology, genetics, and zoology at the botanical department.

Barbara's favorite objects of genetic research were the fruit fly (Drosophila melanogaster) and corn (Zea mays). The popularity of the fruit fly among scientists was due to its short life cycle, high fertility, and distinct physical characteristics. Corn, on the other hand, had a longer life cycle, but its colorful grains and leaves made it suitable for genetic research. While still a junior at the university, Barbara worked with the staff of the College of Agriculture who were making the first steps in the field of genetics. Professor R.A. Emerson was studying corn genetics and hybrid lines. Barbara worked with him and established promising professional contacts with two senior students, George W. Beadle and Marcus Rhoades, who later became renowned geneticists.

In 1924, while working on her thesis, Barbara developed a method to study individual corn chromosomes under a microscope. This led her to the idea of studying the correlation between chromosomes and phenotypic traits. In the same year, she was appointed as an assistant in the botanical department and obtained her master's degree in 1925. Two years later, she earned her Ph.D., writing a dissertation on her developed method. From 1927 to 1931, she worked as a lecturer in the botanical department. During these years, Barbara continued her study of corn chromosome morphology and their correlation with phenotypic traits in adult plants. Together with Harriet Creighton, Barbara discovered that corn chromosomes exchange genetic material and information during chromosome crossovers in early meiosis stages. Meiosis is the process of cell division that produces gametes, or sex cells, with half the number of chromosomes as somatic, or body, cells. During fertilization, the number of chromosomes doubles.

Between 1929 and 1931, Barbara published 9 papers in biological and genetic journals. In 1931, she presented her work on the correlation between cytological and genetic crossovers in Zea mays at Cornell University. Thomas Hunt Morgan, a prominent geneticist, was impressed by her research. He offered her the opportunity to publish her materials in the prestigious journal "Proceedings of the National Academy of Sciences." Her article, titled "A Correlation of Cytological and Genetical Crossing Over in Zea mays," was published in the journal in August 1931. In the same year, Barbara received a grant from the National Research Council, allowing her to pursue further research on corn genetics for the next two years. She also became a research associate at Morgan's department at the California Institute of Technology, working at Cornell University and the University of Missouri in Columbia, studying the correlation between X-ray-induced genetic mutations in corn chromosomes and their phenotypic manifestations in plants. Barbara identified that ring chromosomes were associated with the appearance of variegated colors in corn kernels. She also discovered nucleolar chromosomes involved in ribosome biosynthesis, which are centers for cellular protein synthesis. A grant from the Guggenheim Foundation allowed her to spend 1933 at the Kaiser Wilhelm Institute in Berlin. However, due to the rise of Nazism, she returned to Cornell the following year. Until 1936, she worked as a research associate in Emerson's department and then became an assistant professor of botany at the University of Missouri. Realizing her limited chances for further advancement, Barbara left the university in 1941 and spent the summer working in the biological laboratory of her old friend Marcus Rhoades in Cold Spring Harbor, New York. In the fall, she accepted a position as a staff member at the Carnegie Institution of Washington in Cold Spring Harbor, where she continued her research on corn genetics.

The 1940s were particularly productive for Barbara. During the winter months, she analyzed the results of experiments conducted in the previous summer and planned experiments for the following year. In the summers, she grew corn on a plot in front of the laboratory. Early experiments led her to the idea of mobile genetic elements in corn chromosomes. In the winter of 1943/44, she planned a program of experiments hoping to confirm her theory. In the summer of 1944, Barbara noticed that twin plants had varying degrees of leaf coloration intensity, with some having strongly colored stripes and others having weakly colored stripes. She observed a similar phenomenon in the kernels of the ears, leading her to conclude that one of the twin plants possessed a specific genetic system that the other plant did not. This phenomenon is now called genetic transposition, and the genes involved are called transposons or jumping genes. The experimental results allowed Barbara to formulate a clear model of the genetic system, which included two transposing genes: Dissociator, named Ds gene, and Activator, named Ac gene. According to her observations, the genetic system worked as follows: if the Ds gene moved near a chromosomal region next to a structural gene (e.g., a gene controlling the pattern of alternating stripes on corn leaves), it suppressed the phenotypic expression of the structural gene, resulting in pale stripes on the leaves. However, this suppression by the structural gene only occurred if the Ac gene occupied positions near the two other genes. If the Ac gene transposed to a more distant region, the suppression by the Ds gene did not occur, and the stripes on the leaves were bright. According to Barbara's conclusions, one of the transposing genes was a suppressor gene, while the other lifted its suppressing effect.

Barbara presented her research on corn genetics and mobile genetic systems at a symposium in Cold Spring Harbor in 1950. Due to the fact that her hypothesis of mobile, transposing genes contradicted the prevailing genetic dogma of genes as stable components of chromosomes, her work was not taken seriously. Additionally, being a woman, she faced discrimination in the scientific community. Disappointed, Barbara temporarily stopped publishing the results of her experiments. From 1958 to 1960, she did not conduct any research and instead focused on training cytologists from South American countries through a program offered by the National Academy of Sciences. By the time she resumed her work on corn genetics and transposing genes, bacterial geneticists had discovered regulatory genes in bacteria, reminiscent of what Barbara had found in corn.

Barbara McClintock was awarded the Nobel Prize in Physiology or Medicine in 1983 for her discovery of transposable genetic elements. More than three decades had passed since the work that led to this recognition. In an interview with her biographer, Evelyn Fox Keller, Barbara expressed her approach to work, saying, "When you look at these things, they become a part of you. And you forget yourself. That's the main thing – that you forget yourself." Despite being referred to as a maverick by members of the Nobel committee, Barbara continued her research on corn genetics in her laboratory and experimental field in Cold Spring Harbor. She never married.

In addition to the Nobel Prize, Barbara received the Kimber Award in Genetics from the National Academy of Sciences (1967), the National Science Foundation's Medal for Scientific Achievement (1970), the Albert Lasker Award for Basic Medical Research (1970), the Wolf Prize in Medicine from the Wolf Foundation (1981), and the Louisa Gross Horwitz Prize from Columbia University (1982). In 1981, she received a MacArthur Fellowship. Barbara was a member of the National Academy of Sciences, the American Society of Naturalists, the American Philosophical Society, the Botanical Society of America, and the Genetics Society of America. She received honorary degrees from the University of Rochester, Smith College, the University of Missouri, Yale University, Williams College, and New York University.