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| Barbara McClintock, linocut print, 11" x 14" by Ele Willoughby, 2026 |
I first read about Barbara McClintock years ago in 'Nobel Prize Women in Science' by Sharon Bertsch McGrayne. McClintock was one of the women portrayed who did actually win the award, rather than the several who should arguably have been included in various wins. Much more recently, I read 'The Exceptions: Nancy Hopkins, MIT and the Fight for Women in Science', by Kate Zernike. When I was an undergraduate and then graduate student in physics at University of Toronto, and served on the Gender Issues Committee, biologist Nancy Hopkins famously showed systemic discrimination against women faculty at MIT with hard data, gathered systematically, if surreptitiously, by literally measuring lab floor space. Her victory was all our victory, because she used the tools of science to convince scientists. Further, she and collaborators showed academics worldwide a strong, convincing, simple and effective strategy for showing discrimination was occurring. For instance, in 2002, the University of Toronto settled a class action suit from retired female faculty, acknowledging gender barriers and pay discrimination. The book is well-written, but I found it a bit hard to read. Barriers and discrimination Hopkins experienced in the 60s onwards, felt all too familiar for my experiences decades later. Further, as a physics student, one of a grand total of 2 women specialists as a undergrad, and as an obvious minority as a grad student, with a grand total of 0 then 1 female faculty, it was pretty undeniable that something was wrong and needed fixing. It took Hopkins, wary of feminism like many of her generation, a very long time to recognize she was facing discrimination and harassment. I found it rather hard to read about naive young Nancy who took so long to question things.
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| Roaslind Franklin, linocut, 11" x 14", by Ele Willoughby 2020 |
One of Hopkins' role models, somewhat hidden in the background was Barbara McClintock, who tried resolutely to recruit her. Young Nancy was a bit wary of McClintock whom she viewed as a bit of a square peg, maybe sidelined from the cutting edge of genetics unlike her undergraduate mentor Nobel laureate James Watson. She was both right and quite wrong, as it turned out. Outsider McClintock would go on to win the 1983 Nobel Prize in Physiology or Medicine. It was funny to read about Watson's role as champion and mentor in Nancy's life, knowing him more as a bit of a villain, not so much in Rosalind Franklin's life (as he is often portrayed with imprecision and hindsight), but as a villain in the story of what was and was not remembered about Franklin posthumously until more recently. Also, he has become increasingly notorious for ugly racist and sexist biases about people. Nancy was important to a lot of us women in science in the early 2000s, well beyond biology, for the way she spearheaded the survey of lab space offered women versus men faculty at MIT and gathered hard data of systemic biases. When Nancy was the first person to read Jim Watson's 'Selfish Gene', it did not occur to her that maybe the portrayal of Franklin as uptight harridan might not be the whole story or to wonder why this was the first time she had heard of her. When Watson told Nancy that McClintock was "difficult" she believed him. When McClintock showed Nancy a letter in which she had been described as the best person in the field and that it was a shame that she (McClintock) couldn't be hired because she was a woman, Nancy assumed the problem and McClintock's science both were a thing of the past. But what did come through to me in reading this book was how, all the while, Barbara McClintock was doing her own thing, quietly revolutionary on her own terms. Barbara was there supporting fellow women in science and understood the dynamics of gender in the scientific workplace before Nancy.
Barbara McClintock (1902-1992) was a cytologist and geneticist who studied maize genetics from graduate school through retirement. She had been named Eleanor when she was born the third of four children to British immigrant in Connecticut, Thomas Henry McClintock, a homeopathic physician, and his upper-middle-class, Mayflower-descendent Boston wife, housewife, artist and poet, Sara Handy McClintock. When she was young, her parents decided that Eleanor was too delicate and feminine and did not suit their solitary, active, independent child, so they renamed her Barbara. To help save money while her father established his medical practice, and relive her mother of caring for both a toddler Barbara and her new infant younger brother, Barbara was sent to live with an aunt and uncle in Brooklyn from the age of three until she began school. She was close with her father but had a difficult relationship with her mother from a young age.
From her uncle she learned to repair machinery and love nature. Her father raised her as a boy, giving her boxing gloves at age four. When a neighbour who disapproved of the athletic child who didn't play like a girl and tried to teach her "womanly" things, her mother told her to mind her own business. She played sports with the boys, but felt they merely tolerated her as a girl. When Barbara found a teacher "emotionally ugly" her parents let her stay home from school. She grew up with a sense of freedom.
Her family moved to Brooklyn in 1908, where Barbara completed high school in 1919, and discovered her love of science. She wanted to continue to Cornell's College of Agriculture but her mother did not want to allow it. She worried Barbara would become unmarriageable, a not uncommon attitude at the time. Mrs. McClintock had convinced Barbara's older sister to reject a scholarship to Vassar. Her father returned from France where he was serving in the army medical corps just in time to intercede on Barbara's behalf, and she managed to register just before the deadline.
Barbara thrived at college, participating in student government and joining a sorority (though she later broke her sorority pledge). Her friend circle were avant-guard, and most of her women friends were Jewish at a time when there was a large social gap between Jews and Gentiles at Cornell. She studied Yiddish, and when she found her friends were not welcome in sororities she rejected her own bid. She took up jazz music, playing tenor banjo in a group until the late hours began to interfere with her work. A 1921 field course taught by C.B. Hutchinson first peaked her interest in genetics, a brand new field of study. Hutchinson had been impressed and telephoned to invite her take the new graduate course in genetics the next year. She would later say, "Obviously, this telephone call cast the die for my future. I remained with genetics thereafter." She got her undergraduate degree in 1923, her MSc in 1925 and PhD in 1927, all officially in botany, though her graduate work focused on the cytogenetics of maize. Though just a petite younger woman grad student in pants and a man's shirt working with men in the fields and the lab, she was instrumental in assembling a group of plant breeders and cytologists working on maize cytogenetics, including her champion Marcus Rhoades, future Nobel laureate George Beadle and Harriet Creighton, and this group was supported by department head Rollins A. Emerson (who had rediscovered the laws of inheritance established by Mendel). The group remained close friends and allies through their careers. She worked as research assistant to botanists Lowell Fitz Randolph and Lester W. Sharp. She focused on ways to visualize and characterize maize chromosomes (packages of DNA carrying its genetic material). She was the first to show the 10 maize chromosomes using a staining method she developed using carmine looking at cells from the microspore rather than the root tip, effectively scooping her own supervisor who had been working for years on more effective way to image the maize chromosomes. She studied the morphology of chromosomes and was actually able to link chromosome groups to inherited traits.
Every visit home, her mother tried to convince her not to go back, afraid she would become a professor rather than a housewife. Barbara decided she was too independent for emotional relationships, that she was a "dominant person" who would make a man miserable. She broke up with her beau and remained single for the rest of her life. Zernike writes that she "nurtured a vision of herself as gender-free - feeling at home neither as a girl nor a boy - and once complained of her body as 'a nuisance'."*
She was the first to describe the cross-shaped interaction of the set of maternal and paternal chromosomes that pair up (called homologous chromosomes) during meiosis (cell division of germ cells). Together with Harriet Creighton in the 1930s, studying meiosis in corn cells and examining the gene positions on the chromosomes, she discovered that new combinations of nucleotides that make up DNA in offspring were related to the event of crossing. This proved inter-chromosomal genetic recombination, previously only a hypothesis. They published in 1931, only a few months before Curt Stern published the same observation for fruit flies. This landmark paper made her reputation. They went on to observe how the recombination was linked with new offspring traits. McClintock's chromosomal map was consistent with the linkage map published by her supervisor Hutchinson in 1921, and made a basis of her work with Creighton. They also showed that crossing-over occurs in sister chromatids (which occur when chromosomes are copied by DNA and tied together by a centromere).
Her success and the importance of her publications lead to receiving several National Research Council post-doctoral fellowships and she continued her research at Cornell, the University of Missouri and the California Institute of Technology, driving her model A Ford across the country. She was the first woman postdoc at Caltech, which required the board of trustees' approval. After a visit to the faculty club on her first day, unlike every other visiting scholar, she was never welcomed again. Warren Weaver of the Rockefeller Foundation deemed her "more boy than girl." It was very hard to find a permanent position during the Great Depression. In Missouri in 1931 and 1932, Stadler introduced her using x-rays as a mutagen; greatly increasing the rate of mutation above background levels was a very useful tool in genetic studies. She found ring chromosomes (where the ends of a single radiation-damaged chromosome fuse), which were first reported by Mikhail Sergeevich Navashin, in x-ray-mutagenized maize. She correctly inferred that normally, undamaged chromosomes must have a structure on the chromosome tip to ensure stability, eventually identified as telomeres. She showed that the loss of ring-chromosomes at meiosis caused variegation in subsequent maize generations due to chromosomal deletion. On chromosome 6, she demonstrated the presence of the nucleolus organizer region needed for the assembly of the nucleolus (the largest structure in the cell nucleus). She found that cells can be damaged during non-homologous recombination (the repair of double-strand breaks in DNA) and published this in 1933.
She earned a Guggenheim Foundation Fellowship which allowed her to work in Germany with Richard B. Goldschmidt director of the Kaiser Wilhelm Institute for six months, but she left due to mounting political tensions there. Emerson hired her as an assistant in his Department of Plant Breeding at Cornell for three years and her work there lead to an offer of an assistant professorship from Lewis Stadler at the Department of Botany at the University of Missouri in 1936. There she continued her work on the effect of x-rays on maize cytogenetics. She found breakage and fusion in irradiated maize chromosomes and found spontaneous breakage in endosperm cells from some plants. She made a key cytogenetic discovery of a breakage-rejoining-bridge cycle. During mitosis (cell splitting) the broken chromatids were rejoined after chromosome replication. In anaphase (when replicated chromosomes are split), the broken chromosomes formed a bridge which was broken when the chromatids moved toward cell poles. These broken ends rejoined in the interphase (just prior to) the next mitosis, causing massive mutations which she could observe as variegation of the endosperm. In 1938 she published a cytogenetic analysis on the centromere, describing its organization, function and ability to divide. While her research was going well, and she was gaining recognition (she was elected vice-president of the Genetics Society of America in 1939) she was not happy at Missouri. She felt her "maverick" ways did not fit with what was expected of a "lady scientist." Whether it was wearing pants, allowing her students to work in the field after hours or simply climbing the fence when she forgot her key, she did not fit. When another woman named Barbara McClintock announced her engagement in the newspaper, she was hauled into her boss' office and told she would be fired if she married. She was excluded from faculty meetings, not told of advertised professorships elsewhere and convinced she would never be get tenure, so she decided to leave. She learned that if Stadler, who had hired her, moved to the University of California, her job would be in jeopardy. She took a leave of absence in 1941 and accepted a visiting professor at Columbia, where Rhoades was now teaching. Rhoades also offered to share his research field at Cold Spring Harbor Laboratory, a research institution funded by the Carnegie Institution of Washington on Long Island. She loved working the lab and said, "I was just so interested in what I was doing I could hardly wait to get up in the morning and get at it." In December, acting director Drosophila expert Milislav Demerec of the Carnegie Institution of Washington's Department of Genetics at Cold Spring Harbor offered her a temporary position there and then, when made Director he offered her a permanent position. She wavered at first but accepted and became a permanent member of staff in 1942. She had found the right fit, somewhere everyone wore jeans, teaching was not required and there were no restrictions on research.
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| Esther Lederberg, linocut print, 11" x 14" by Ele Willoughby, 2026. Esther Lederberg also worked at Cold Spring Harbor and published on N. crassa in the early 40s. |
There she used the breakage-rejoining-bridge cycle to map new genes. In 1944 she was elected to the National Academy of Sciences, only the third woman to be elected. George Beadle invited her to visit Stanford and work on the bread mold Neurospora crassa. He had used N. crassa to demonstrate the one gene one enzyme relationship and within two months McClintock had described its number of chromosomes (or karyotype) and its entire life cycle. Beadle said, "Barbara, in two months at Stanford, did more to clean up the cytology of Neurospora than all other cytological geneticist had done in all previous time on all forms of mold." In 1945 she was elected president of the Genetics Society of America. All of this she achieved before her Nobel award winning work.
Summer grad students at Cold Spring Harbor would play baseball in the evening, next to Barbara's cornfields. One of them, James Watson, said the ball went all too often into her fields and she would "get pretty mad... like your mother". Little did he know at the time that her cornfields would tell a much more complex story of the operation of genes than could be found in the structure of DNA alone. McClintock could be prickly and Watson also called her "the Katharine Hepburn of science."
She began her systematic study of mosaicism in maize in the summer of 1944 investigating the mechanisms of colour patterns and unstable inheritance. She had noted that colour patterns were too unstable over the generations to be explained by simple mutations. She discovered two dominant genetic loci she called Dissociation (Ds) because it could cause a dissociation (or break) and Activator (Ac) which was like a sort of switch. She found that corn colours, dominant or recessive were turned on or off by Ds. Strikingly, Ds could change position, or transpose, within the chromosomes, disrupting the colour gene. In a few cells it can jump again. The jumping is random, so multicoloured corn is common. She found that Ac terminated the transposition of Ds; the amount of jumping around done by Ds is determined by the numbers of copies of Ac in the cell. Ds had other varying effects on neighbouring genes, like making normally stable mutations unstable, if Ac was present. Then in 1948 she made the unexpected observation that both Ds and Ac could change position, or transpose, on the chromosome. Hence they are now known as transposons.
By carefully controlling crosses, and observing the colouration patterns in generations of crops and careful microscopic analysis, she was able to describe the relationship between Ds and Ac. She saw that Ac controls how Ds jumps from chromosomes 9, and that when it does the chromosome breaks. Ds which suppresses the aleurone protein colour gene prior to transposition, and releases it when it jumps, resulting in pigment synthesis in cells. The Ds jumps are random, and may happen in some cells and not in others, so colour mosaicism is common in maize. The size of coloured spots depends on the stage of development at dissociation. She called Ds and Ac "controlling units" or "controlling elements" to distinguish them from genes and hypothesized that this gene regulation could explain how multicellular life with a single genome can produce cells of different functions. Thus the genome was not simply a static set of instructions - dogma at the time. In 1950, she published "The origin and behaviour of mutable loci in maize" in the Proceedings of the National Academy of Sciences and presented a paper of the same name the next year at the annual Cold Spring Harbor symposium. These described this research, the instability due to Ac, or both Ds and Ac in four genes and how those genes can unpredictably revert to the wild phenotype. She also explained there were "families" of transposons which did not interact with each other. She described her colleagues' response to this work as "puzzlement, even hostility," and said, "They thought I was crazy, absolutely mad," but she persisted with her research and published a subsequent paper in Genetics in 1953 about her statistical data and lectured on the work through the 50s. She found another element she called Suppressor-mutator (Spm) for which some versions could transpose on their own and some could not but which, when present, fully rather than partially suppressed mutant genes. McClintock worried she would alienate her colleagues if she continued to publish on controlling elements, so after 1953, she stopped. She felt the need to wait until the field was ready for conceptual change. When she presented her observation that maize did not follow Mendelian distributions where colours would be dictated strictly by copies of dominant or recessive genes, geneticist Sewal Wright suggested she (like other women) simply did not understand the underlying mathematics. During a visit, McClintock threw Joshua Lederberg and colleagues out of her lab for arrogance. He later declared, "By God, that woman is either crazy or a genius." According to McGrayne, a leading molecular biologist described her as "just an old bag who'd been hanging around Cold Spring Harbor for years." McClintock had no patience for arrogance. Barbara did not feel the need to keep defending her work. She said, "If you know you are on the right track, if you have this inner knowledge, then nobody can turn you off... no matter what they say." While McClintock thought her colleagues deemed her mad, she was respected. It might be more accurate to say her colleagues struggled to understand her work. Geneticist Alfred H. Sturtevant said in 1951, "I didn't understand one word she said, but if she says it is so, it must be so!"
She got a grant in 1957 from the National Academy of Sciences to research Central and South American indigenous maize strains so she could investigate the evolution of maize through chromosomal changes. In South America, she could work on a larger scale. In 1959 she was elected a fellow of the American Academy of Arts and Sciences. Through extensive work in the 60s and 70s looking at chromosomal, morphological and evolutionary characteristics of maize strains she and her colleagues published The Chromosomal Constitution of Races of Maize, an influential publication for cytogenetics, paleobotany, ethnobotany and evolutionary biology. She had officially retired in 1967, becoming a Distinguished Service Member of the Carnegie Institution of Washington, which allowed her to continue working with grad students as an emeritus scientist at Cold Spring Harbor. She also received the Kimber Genetics Awards.
In 1961, French geneticists François Jacob and Jacques Monod described genetic regulation of the lac operon and McClintock wrote an article for American Naturalist comparing this to her work on controlling elements in maize. She had effectively discovered genetic regulation though biology was slow to recognize this. When her colleagues finally saw transposons in bacteria, yeast and bacteriophages in the late 60s and early 70s, they had the tools to investigate the molecular basis for transposition, and her discoveries began to receive the credit they deserved. Today, mutant plants are generated using the Ac/Ds mechanism, to characterize gene function. We now know that transposons make up the majority, in fact, 85% of the maize genome and a significant portion of our own. McClintock did not get everything right but she had made a huge leap forward, years before anyone else.
When the Carnegie Institute of Genetics closed in the early 70s, her corn field was removed to become a library parking lot. McClintock continued working and collaborating with colleagues Ben and Frances Burr at Brookhaven National Laboratory, where she replanted her seeds and with Nina Federoff at the Carnegie Institute of Embryology in Baltimore.
In 1970 she received the National Medal of Science. She was the first woman to do so. In 1973 Cold Spring Harbor named a building in her honour. The awards kept coming: the Louis and Bert Freedman Foundation Award (1978), the Lewis S. Rosensteil Award (1978), she received the first MacArthur Foundation Grant, the Albert Lasker Award for Basic Medical Research, the Wolf Prize in Medicine, the Thomas Hunt Morgan Medal from the Genetics Society of America, the Louisa Gross Horwitz Prize from Columbia University, and in 1983 the Nobel Prize for Physiology or Medicine for discovering "mobile genetic elements," 30 years after her discovery. She was the first woman to win the prize solo. When alerted of her win by a 6:00 phone call from the New York Times the morning it was announced, post-doc Grey Freyer found her collecting walnuts and congratulated her. "For what?" she asked. "You won the Nobel Prize!" he told her. "That's nice," she said. The administrative director got her to issue a press release, in which she wrote that it seemed unfair "to reward a person for having so much pleasure, over the years, asking the maize plant to solve specific problems and then watching its responses." When the reporters showed up, they found her again out collecting walnuts in dungarees and shirt. Colleague escorted her to the press conference but she was wearing a Groucho mask as a disguise. She told reporters "I've had a very, very, satisfying and interesting life."
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| Barbara McClintock in the Groucho mask |
That year physicist, feminist and historian of science Evelyn Fox Keller published 'A Feeling for the Organism', a biography which brought McClintock's story to the public. In 1986 she was inducted in the National Women's Hall of Fame. In 1987, The Discovery and Characterization of Transposable Elements: The Collected Papers of Barbara McClintock was published. In 1989 she was elected a Foreign Member of the Royal Society. In 1993 she received the Benjamin Franklin Medal for Distinguished Achievement in the Sciences of the American Philosophical Society. She received 14 honorary doctorates and an Honorary Doctor of Humane Letters. She remained active in the Cold Spring Harbor community, giving talks on mobile genetic elements and the history of genetics for junior scientists for the rest of her life, playing tennis, collecting black walnuts for baking for colleagues. She would place the walnuts in her driveway and roll her Honda Accord over them, to crack them open to make brownies! Known for her lengthy and thorough answers to student questions, she had brownies at the ready when they looked tuckered out. Jim Watson hosted a large party with many of her colleagues for her 90th birthday. She died of natural cause at 90 years of age in 1992. Her colleagues remembered her brilliance, intensity, scientific focus, devoted mentorship, quick wit and sense of fun. Steven Jay Gould wrote, "Her discovery of transposable elements in maize - so-called jumping genes - first presented in the early 1950s before her field had any language to express such a heterodox idea, was, in retrospect, the beginning of modern molecular genetics." Posthumously, in 2005 the USPS issued a Barbara McClintock stamp, the McClintock Prize was named in her honour in 2013, and in 2024 the plant Stellaria mcclintockiae was named in her honour.
References
Chomet, P., and R. Martienssen. Barbara McClintock's Final Years as Nobelist and Mentor: A Memoir. Cell, 170, pp. 1049-1054. September 7, 2017.
Barbara McClintock. NobelPrize.org, Nobel Prize Outreach 2026. January 25, 2026.
Barbara McClintock, Wikipedia, accessed June, 2026.
Barbara McClintock's World, Weed to Wonder, Cold Spring Harbor Laboratory, accessed June, 2026.
Kim, Taeah. The Life of Barbara McClintock and her Jumping Gene. AMI Webinar Online Salon, 2020.
Krueger, Brian. Barbara McClintock discovered a little thing called the transposable element in 1950. omicly.com, May 5, 2024.
Madrigal, Alexis. What was Barbara McClintock's "mysticism"? Oakland garden club. September 2, 2023.
McGrayne, Sharon Bertsch. Nobel Prize Women in Science. Birch Lane Press, New York. 1993.
Pearse, Yewande. Meet Barbara McClintock, who used corn to decipher 'jumping genes,' Massive Science, May 11, 2018.
Zernike, Kate. The Exceptions: Nancy Hopkins, MIT and the Fight for Women in Science. Scribner, New York, 2023.
