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Barbara McClintock

American geneticist (1902–1992)

The daughter of a physician, McClintock was born in Hartford, Connecticut, and educated at Cornell's College of Agriculture, where she received her PhD in 1927 for work in botany. She remained at Cornell until 1936 supported by various grants from the National Research Council and the Guggenheim Foundation. But there was no future at Cornell for her as, until 1947, only the department of home economics appointed women professors. Fortunately a new genetics department was being set up in the University of Missouri by Craig Stadler, who knew and admired her work, and she was offered a post as assistant professor there, although it was made clear to her that any further advancement would be unlikely. She left in 1941, and in 1944 was elected to the National Academy of Sciences, becoming only the third woman to be so honored. McClintock then joined the Carnegie Institute's Cold Spring Harbor Laboratory, New York, where she remained until her death.

By the 1920s Morgan and other geneticists, working mainly with the Drosophila fruit fly, had established that gene action was connected with chromosomes and thereby established the new discipline of cytogenetics. Drosophila chromosomes, however, before the discovery of the giant salivary chromosomes by T. Painter in 1931, were too small to reveal much detail. McClintock chose to work with a variety of maize that possessed much more visible chromosomes. Further, the development of new staining techniques allowed McClintock to identify, distinguish, and number the ten maize chromosomes.

Morgan and his group had also demonstrated the existence of ‘linkage groups’ in Drosophila – groups of genes, such as those for white eyes and maleness, linked together because the genes themselves were sited near each other on a chromosome. In a series of papers published between 1929 and 1931, McClintock established similar linkage groups in maize. Because maize chromosomes were more visible under the microscope than those of Drosophila, McClintock was able to identify the chromosomal changes responsible for a change in phenotype and thus confirmed Morgan's work.

McClintock's own Nobel Prize for physiology or medicine, awarded in 1983, was for later work done on the so-called ‘jumping genes’. In the 1940s at Cold Spring Harbor, McClintock planted her maize and began to track a family of mutant genes responsible for changes in pigmentation. She was struck by the fact that mutation rates were variable. After several years' careful breeding, McClintock proposed that in addition to the normal genes responsible for pigmentation there were two other genes involved, which she called ‘controlling elements’.

One controlling element was found fairly close to the pigmentation gene and operated as a switch, activating and turning off the gene. The second element appeared to be located further away on the same chromosome and was a ‘rate gene’, controlling the rate at which the pigment gene was switched on and off. She further discovered that the controlling elements could move along the chromosome to a different site and could even move to different chromosomes where they would control different genes. McClintock gave a full description of the process of transposition, as it became known, in her 1951 paper, Chromosome Organization and Genic Expression. McClintock's work was largely ignored until 1960 when controlling elements were identified in bacteria by Monod and Jacob.

 
 
Genetics Encyclopedia: Barbara McClintock

Geneticist 1902-1992

Barbara McClintock was one of the most important geneticists of the twentieth century and among the most controversial women in the history of science. She made several fundamental contributions to our understanding of chromosome structure, put forward a bold and incorrect theory of gene regulation, and, late in her career, developed a profound understanding of the interactions among genes, organisms, and environments. She was born on June 16, 1902, in Hartford, Connecticut, the third of four children and the youngest daughter. She grew up in Brooklyn, New York, and in 1919 she enrolled in the agricultural college of Cornell University, where she received all her post-secondary education. She took a bachelor's degree in 1923, a master's in 1925, and a Ph.D., under the direction of the cytologist Lester Sharp, in 1927.

McClintock gravitated toward the cytology and genetics of maize, or Indian corn, and by 1929 she was a rising star in her field. Not quite single-handedly, she made possible the "golden age of maize genetics," from 1929 to 1935. During those years, McClintock published a string of superb papers identifying novel cytological phenomena and linking them to genetic events. Working with Harriet Creighton, she confirmed that chromosomes physically exchange pieces during the genetic phenomenon known as "crossing over." She was supported by a series of prestigious fellowships, from the National Research Council, the Guggenheim Foundation, and others, that took her from Cornell to the California Institute of Technology, and to Berlin and back. In 1935 she took a faculty position at the University of Missouri in Columbia. She was not happy there, however, and resigned in 1939, despite the apparent imminence of a promotion with tenure.

In 1941 she took a summer position at Cold Spring Harbor on New York's Long Island, at the Carnegie Institution of Washington's Department of Genetics. It was an ideal position for her, with no teaching or administrative duties. Within a year the post became permanent, and she remained there until her death. On arrival, she continued work that she had begun while at Missouri, investigating a phenomenon called the breakage-fusion-bridge (BFB) cycle. This is a repeating pattern of chromosome breakage she had discovered among strains of maize plants grown from X-rayed pollen. In 1944, during an experiment designed to use the BFB cycle to create new mutations, she discovered numerous "mutable" genes: genes that turned on and off spontaneously during development. In the cells of some of these new mutants lay her most important discovery, chromosome segments that move from place to place on the chromosome. That same year, the National Academy of Sciences honored a woman for only the third time in its eighty-year history when it elected McClintock a member.

During the rest of the 1940s McClintock developed a novel theory of how genes could control the development and differentiation of organisms.

The key to the theory was a new type of genetic element, not a gene but a gene-controller, that first appeared in her 1944 BFB experiment. These "controlling elements," she argued, inhibited or modulated the effects of the genes near them. She proposed that through coordinated movement from gene to gene (transposition) controlling elements executed the genetic program of development, much as the hammers on a player piano execute the program encoded on a piano roll.

Transposition in maize was confirmed immediately and repeatedly by other researchers. Few scientists, however, could accept her notion that the movements were coordinated. After about 1954, McClintock did little more with transposition, but she continued to work on genetic control for the rest of her long career. Her systems grew increasingly complex, and her thinking led her to comparisons between embryology and evolution.

During the 1970s transposition was discovered in bacteria, and its biochemistry was explained in terms of DNA sequences and enzymatic action. Soon transposition was found to be nearly universal in the living world and was linked to medical fields such as cancer, virology, and immunology. McClintock experienced a rare scientific renaissance. Her theories of genetic control, never widely accepted and by this time rejected outright, were forgotten, and she was reborn as the discoverer of transposition. She won, unshared, the 1983 Nobel Prize in physiology or medicine "for her discovery of transposable genetic elements."

Since then, the experiments of other researchers have provided at least qualified support for even some of her wilder ideas, such as her conception of the genome as a "sensitive organ of the cell"; and the idea that any organism has the genetic instructions to make any other. Some of these findings had appeared by the time she died, on September 2, 1992, but it has been the various genome projects, human and otherwise, that have lent the strongest support to McClintock's dynamic, interactive vision of nature. Had she lived to be one hundred, she might well have been considered for a second Nobel, this time for her insights into the workings of the genome.

Bibliography

Comfort, Nathaniel C. The Tangled Field: Barbara McClintock's Search for the Patterns of Genetic Control. Cambridge, MA: Harvard University Press, 2001.

———. "Two Genes, No Enzyme: A Second Look at Barbara McClintock and the 1951 Cold Spring Harbor Symposium." Genetics 140, no. 4 (1995): 1161-1166.

Keller, Evelyn Fox. A Feeling for the Organism: The Life and Work of Barbara McClintock, 10th Anniversary Edition. New York: W. H. Freeman, 1993.

McClintock, Barbara. The Discovery and Characterization of Transposable Elements: The Collected Papers of Barbara McClintock. New York: Garland, 1987.

—Nathaniel Comfort

 
Biography: Barbara McClintock

Geneticist Barbara McClintock (1902-1992) received the Nobel Prize in Physiology for her discovery that genes could move from place to place on a chromosome.

Barbara McClintock was born in Hartford, Connecticut, on June 16, 1902. She had two older sisters and gained a brother when she was two. Her father, Thomas Henry McClintock, was a physician. Upon the birth of their son, the McClintocks sent Barbara off to live with relatives in the country, where she lived on and off until she was of school age. It was here that she developed the deep love of nature that lasted her lifetime. In 1908, the family moved to the Flatbush section of Brooklyn where her father had taken a job with Standard Oil. McClintock rejoined the family and attended the local school. Her love of nature, however, persisted.

After graduating from Erasmus High School in 1918, she took a job rather than go on to college, in part because of lack of parental support. She did so well at private studies, however, that the following year she was allowed to enter Cornell University as a biology major in the College of Agriculture. During her freshman and sophomore years, she had a normal college life, including dating and playing tenor banjo in a jazz band. She was elected president of the freshman class and was asked to join a sorority. Upon discovering that the sorority would not accept Jews, McClintock refused the invitation. She never hesitated to snub the social conventions of her time. Upon receiving her B.A. in 1923 she pressed on to take her M.A. in 1925 and her Ph.D. in 1927, studying cytology. She was appointed an instructor in Cornell's botany department.

The faculty at Cornell's agricultural school during those years was pioneering the development of hybrid corn, and McClintock soon discovered a way to identify individual chromosomes of maize. Between 1929 and 1931 she published, with others, nine papers describing her work. Then, in August 1931, the National Academy of Sciences published a paper on the subject, done jointly with Harriet Creighton, which has been described as "the cornerstone of experimental genetics."

Despite the world wide recognition for her work and temporary teaching positions as well as grants from such major foundations as the Guggenheim Fellowship and the Rockefeller Foundation, Cornell University refused her a tenured faculty position. She accepted one from 1939 to 1941 at the University of Missouri but it turned out badly. It was clear that while she might have gotten a regular appointment at a women's college, other doors were closed to her because of her gender. In 1941 her friend fellow geneticist Marcus Rhoades obtained an invitation for her to spend the summer at the Cold Spring Harbor Laboratory, run by the Carnegie Foundation of Washington on Long Island. The laboratory was a self contained facility that had its own summer houses for researchers. She was offered a one-year position December 1, 1941 and she remained there for the rest of her career well into the mid-1980s. During her first decade at the laboratory she won many honors, including presidency of the Genetics Society of America and election to the National Academy of Sciences, only the third woman to be admitted to that body.

It was during the decade of the 1940s that she began the work which was later to result in the Nobel Prize. Essentially, it was her discovery that genes "jumped" from place to place in a chromosome, what she called transposable genetic elements. Since accepted opinion had it that genes were static, rather like beads on a string, her theory was generally received with either hostility or a lack of understanding. Soon after she presented these findings at a symposium in 1951, she stopped publishing her work, so disappointed was she at its reception. Furthermore, the discovery of the double helix structure of DNA in 1953 turned many geneticists away from the "old-fashioned" technique of McClintock (careful experiment, observation, and recording) to the more mechanistic models of James Watson, Francis Crick, and their associates. Partly because of her solitary nature, but also partly because she wanted to stay in close touch with her experiments, McClintock chose to work alone rather than as part of a large research team. As a result, she was in physical and intellectual control of all aspects of her work. As one colleague put it, "she has a feeling for the organism."

The rediscovery of McClintock's work began in the mid-1960s with the study of aspects of bacteria and became unavoidable in the 1980s with the growth of genetic engineering. In 1981 she was awarded the prestigious Wolfe Prize in Medicine for her work, as well as the Lasker Award. The MacArthur Foundation appointed her its first Prize Fellow Laureate; then in 1983 she received the Nobel Prize for Physiology or Medicine.

A deeply private person, McClintock continued to pursue her work alone and with the same holistic perspective she used throughout her career. Although the basics of her experimental work were not only accepted but honored, some of her larger hypotheses were yet to find an audience.

McClintock spent the remainder of her life studying transposition at Cold Spring Harbor. She died on September 2, 1992 shortly after her friends had celebrated her ninetieth birthday. In her obituary, Gerald R. Finks notes that her "burning curiosity, enthusiasm and uncompromising honesty serve as a constant reminder of what drew us all to science in the first place." In 1996 Cold Spring Harbor's DNA Learning Center held an exhibit in her honor featuring a replica of her original 1942 laboratory.

Further Reading

A good short sketch of McClintock's life and work may be found in Science, 222 (October 28, 1983). A full-length biography is Evelyn Fox Keller, A Feeling for the Organism: The Life and Work of Barbara McClintock (1983). Also see Long Island Business News, October 21, 1996.

 
Britannica Concise Encyclopedia: Barbara McClintock

(born June 16, 1902, Hartford, Conn., U.S. — died Sept. 2, 1992, Huntington, N.Y.) U.S. geneticist. She received her doctorate from Cornell University. In the 1940s and '50s, her experiments with variations in the coloration of kernels of corn revealed that genetic information is not stationary. She isolated two control elements in genetic material and found not only that they moved but that the change in position affected the behaviour of neighbouring genes, and she suggested that these elements were responsible for the diversity in cells during an organism's development. Her pioneering research, whose importance was not recognized for many years, eventually resulted in her being awarded a 1983 Nobel Prize.

For more information on Barbara McClintock, visit Britannica.com.

 
Columbia Encyclopedia: McClintock, Barbara,
1902–92, American geneticist. She discovered that certain genetic material, “transposable elements,” shifted its location in the chromosomes from generation to generation. At first ignored, her research was later recognized as a major contribution to DNA research. In 1983 she was awarded the Nobel Prize in Physiology or Medicine.
 
Wikipedia: Barbara McClintock
This article is about the American scientist. For the American illustrator, see Barbara McClintock (illustrator).
Barbara McClintock
Barbara_McClintock.png
Born June 16 1902(1902--)
Hartford, Connecticut, USA
Died September 2 1992 (aged 90)
Huntington, New York, USA
Nationality Flag of the United States United States
Field Genetics
Notable prizes Nobel_Prize.png Nobel Prize in Physiology or Medicine (1983)

Barbara McClintock (June 16, 1902 – September 2, 1992) was a pioneering American scientist and one of the world's most distinguished cytogeneticists. McClintock received her Ph.D. in botany from Cornell University in 1927, where she was a leader in the development of maize cytogenetics. The field remained the focus of her research for the rest of her career. From the late 1920s, McClintock studied chromosomes and how they change during reproduction in maize. Her work was groundbreaking: she developed the technique for visualizing maize chromosomes and used microscopic analysis to demonstrate many fundamental genetic ideas, including genetic recombination by chromosomal crossover during meiosis—a mechanism by which chromosomes exchange genetic information. She produced the first genetic map for maize, linking regions of the chromosome with physical traits, and she demonstrated the role of the telomere and centromere, regions of the chromosome that are important in the conservation of genetic information. She was recognized amongst the best in the field, awarded prestigious fellowships and elected a member of the National Academy of Sciences in 1944.

Biography

During the 1940s and 1950s, McClintock discovered transposition and used it to show how genes are responsible for turning physical characteristics on or off. She developed theories to explain the repression or expression of genetic information from one generation of maize plants to the next. Encountering skepticism of her research and its implications, she stopped publishing her data in 1953. Later, she made an extensive study of the cytogenetics and ethnobotany of maize races from South America. McClintock's research became better understood in the 1960s and 1970s, as researchers demonstrated the mechanisms of genetic change and genetic regulation that she had demonstrated in her maize research in the 1940s and 1950s. Awards and recognition of her contributions to the field followed, including the Nobel Prize in Physiology or Medicine awarded to her in 1983 for the discovery of genetic transposition; she is the first and only woman to receive an unshared Nobel Prize in that category.

Early life

Barbara McClintock was born in Hartford, Connecticut, the third of four children of physician Thomas Henry McClintock and Sara Handy McClintock. She was independent from a very young age, a trait McClintock described as her "capacity to be alone." From about the age of three until the time she started school, McClintock lived with an aunt and uncle in Massachusetts in order to reduce the financial burden on her parents while her father established his medical practice. The McClintocks moved to semi-rural Brooklyn, New York in 1908. She was described as a solitary and independent child, and a tomboy. She was close to her father, but had a difficult relationship with her mother.[1]

McClintock completed her secondary education at Erasmus Hall High School in Brooklyn. She discovered science at high school, and wanted to attend Cornell University to continue her studies. Her mother resisted the idea of higher education for her daughters, believing that it would make them unmarriageable, and the family also had financial problems. Barbara was almost prevented from starting college, but her father intervened, and she entered Cornell in 1919.

Education and research at Cornell

McClintock began her studies at Cornell's College of Agriculture in 1919. She studied botany, receiving a BSc in 1923. Her interest in genetics had been sparked when she took her first course in that field in 1921. The course was based on a similar one offered at Harvard University, and was taught by C. B. Hutchison, a plant breeder and geneticist.[2] Hutchinson was impressed by McClintock's interest, and telephoned to invite her to participate in the graduate genetics course at Cornell in 1922. McClintock pointed to 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."[3] Although it has been reported that women could not major in genetics at Cornell, and therefore her MA and PhD — earned in 1925 and 1927, respectively — were officially awarded in botany, recent research has revealed that women did earn graduate degrees in Cornell's Plant Breeding Department during the time that McClintock was a student at Cornell.[4]

During her graduate studies and her postgraduate appointment as a 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, Charles R. Burnham, Marcus Rhoades, and George Beadle (who became a Nobel laureate in 1958 for showing that genes control metabolism), and Harriet Creighton.[5] Rollins Adams Emerson, head of the Plant Breeding Department supported these efforts, although he was not a cytologist himself.[6] McClintock's cytogenetic research focused on developing ways to visualize and characterize maize chromosomes. This particular part of her work influenced a generation of students, as it was included in most textbooks. She also developed a technique using carmine staining to visualize maize chromosomes, and showed for the first time the morphology of the 10 maize chromosomes.[7] By studying the morphology of the chromosomes, McClintock was able to link to a specific chromosome groups of traits that were inherited together. Marcus Rhoades noted that McClintock's 1929 Genetics paper on the characterization of triploid maize chromosomes triggered scientific interest in maize cytogenetics, and attributed to his female colleague 10 of the 17 significant advances in the field that were made by Cornell scientists between 1929 and 1935.[8]

In 1930, McClintock was the first person to describe the cross-shaped interaction of homologous chromosomes during meiosis. During 1931, McClintock and a graduate student, Harriet Creighton, proved the link between chromosomal crossover during meiosis and the recombination of genetic traits.[9] They observed how the recombination of chromosomes and the resulting phenotype formed the inheritance of a new trait.[10] Until this point, it had only been hypothesized that genetic recombination could occur during meiosis, although it had been shown genetically. McClintock published the first genetic map for maize in 1931, showing the order of three genes on maize chromosome 9.[11] This information provided necessary data for the crossing over study she published with Creighton.[12] In 1938, she produced a cytogenetic analysis of the centromere, describing the organization and function of the centromere.

McClintock's breakthrough publications, and support from her colleagues, led to her being awarded several postdoctoral fellowships from the National Research Council. This funding allowed her to continue to study genetics at Cornell, the University of Missouri - Columbia, and the California Institute of Technology, where she worked with E. G. Anderson.[13] During the summers of 1931 and 1932, she worked with geneticist Lewis Stadler at Missouri, who introduced her to the use of X-rays as a mutagen. (Exposure to X-rays can increase the rate of mutation above the natural background level, making it a powerful research tool for genetics.) Through her work with X-ray-mutagenized maize, she identified ring chromosomes, which form when the ends of a single chromosome fuse together after radiation damage. From this evidence, McClintock hypothesized that there must be a structure on the chromosome tip that would normally ensure stability. She showed that the loss of ring-chromosomes at meiosis caused variegation in maize foliage in generations subsequent to irradiation resulting from chromosomal deletion. During this period, she demonstrated the presence of what she called the nucleolar organizers on a region on maize chromosome 6, which is required for the assembly of the nucleolus during DNA replication.

McClintock received a fellowship from the Guggenheim Foundation that made possible six months of training in Germany during 1933 and 1934. She had planned to work with Curt Stern, who had demonstrated crossing-over in Drosophila just weeks after McClintock and Creighton had done so; however, in the meantime, Stern emigrated to the United States. Instead, she worked in Germany with geneticist Richard B. Goldschmidt. She left Germany early, amid mounting political tension in Europe, and returned to Cornell, remaining there until 1936, when she accepted an Assistant Professorship offered to her by Lewis Stadler in the Department of Botany at the University of Missouri - Columbia.[14]

University of Missouri - Columbia

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.[15] 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.[1] 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."[16] Initially, McClintock's position had been especially created for her by Stadler and may have depended on his presence.[17] 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.[18] Recent evidence reveals that McClintock more likely decided to leave Missouri because she had lost trust in her employer and in the University administration.[19] 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.

Cold Spring Harbor

After her year-long temporary appointment, McClintock accepted a full-time research position at Cold Spring Harbor. Here, she was highly productive and continued her work with the breakage-fusion-bridge cycle, using it to substitute for X-rays as a tool for mapping new genes. In 1944, in recognition of her prominence in the field of genetics during this period, McClintock was elected to the National Academy of Sciences — only the third woman to be so elected. In 1945, she became the first woman president of the Genetics Society of America. In 1944 she undertook a cytogenetic analysis of Neurospora crassa at the suggestion of George Beadle, who had used the fungus to demonstrate the one gene–one enzyme relationship. He invited her to Stanford to undertake the study. She successfully described the number of chromosomes, or karyotype, of N. crassa and described the entire life cycle of the species. N. crassa has since become a model species for classical genetic analysis.[20]

Discovery of controlling elements

The relationship of Ac/Ds in the control of the elements and mosaic color of maize. The seed in 10 is colorless, there is no Ac element present and Ds inhibits the synthesis of colored pigments called anthocyanins. In 11 to 13, one copy of Ac is present. Ds can move and some anthocyanin is produced, creating a mosaic pattern. In the kernel in panel 14 there are two Ac elements and in 15 there are three.
Enlarge
The relationship of Ac/Ds in the control of the elements and mosaic color of maize. The seed in 10 is colorless, there is no Ac element present and Ds inhibits the synthesis of colored pigments called anthocyanins. In 11 to 13, one copy of Ac is present. Ds can move and some anthocyanin is produced, creating a mosaic pattern. In the kernel in panel 14 there are two Ac elements and in 15 there are three.

In the summer of 1944 at Cold Spring Harbor, McClintock began systematic studies on the mechanisms of the mosaic color patterns of maize seed and the unstable inheritance of this mosaicism. She identified two new dominant and interacting genetic loci that she named Dissociator (Ds) and Activator (Ac). She found that the Dissociator did not just dissociate or cause the chromosome to break, it also had a variety of effects on neighboring genes when the Activator was also present. In early 1948, she made the surprising discovery that both Dissociator and Activator could transpose, or change position, on the chromosome.

She observed the effects of the transposition of Ac and Ds by the changing patterns of coloration in maize kernels over generations of controlled crosses, and described the relationship between the two loci through intricate microscopic analysis. She concluded that Ac controls the transposition of the Ds from chromosome 9, and that the movement of Ds is accompanied by the breakage of the chromosome. When Ds moves, the aleurone-color gene is released from the suppressing effect of the Ds and transformed into the active form, which initiates the pigment synthesis in cells. The transposition of Ds in different cells is random, it may move in some but not others, which causes color mosaicism. The size of the colored spot on the seed is determined by stage of the seed development during dissociation. McClintock also found that the transposition of Ds and the is determined by the number of Ac copies in the cell.

Between 1948 and 1950, she developed a theory by which these mobile elements regulated the genes by inhibiting or modulating their action. She referred to Dissociator and Activator as "controlling units"—later, as "controlling elements"—to distinguish them from genes. She hypothesized that gene regulation could explain how complex multicellular organisms made of cells with identical genomes have cells of different function. McClintock's discovery challenged the concept of the genome as a static set of instructions passed between generations. In 1950, she 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 the journal Proceedings of the National Academy of Sciences. In summer 1951, when she reported on her work on gene mutability in maize at the annual symposium at Cold Spring Harbor, the paper she presented was called "Chromosome organization and genic expression".[21]

Her work on controlling elements and gene regulation was conceptually difficult and was not immediately understood or accepted by her contemporaries; she described the reception of her research as "puzzlement, even hostility".[22] Nevertheless, McClintock continued to develop her ideas on controlling elements. She 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.[23] She continued to investigate the problem and identified a new element that she called Suppressor-mutator (Spm), which, although similar to Ac/Ds displays more complex behavior. Based on the reactions of other scientists to her work, McClintock felt she risked alienating the scientific mainstream, and from 1953 stopped publishing accounts of her research on controlling elements.[24]

The origins of maize

McClintock's microscope and ears of corn on exhibition at the National Museum of Natural History.
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McClintock's microscope and ears of corn on exhibition at the National Museum of Natural History.

In 1957, McClintock received funding from the National Science Foundation, and the Rockefeller Foundation sponsored her to start research on maize in South America, an area that is rich in varieties of this species. She was interested in studying the evolution of maize, and being in South America would allow her to work on a larger scale. McClintock explored the chromosomal, morphological, and evolutionary characteristics of various races of maize. From 1962, she supervised four scientists working on South American maize at the North Carolina State University in Raleigh. Two of these Rockefeller fellows, Almeiro Blumenschein and T. Angel Kato, continued their research on South American races of maize well into the 1970s. In 1981, Blumenschein, Kato, and McClintock published Chromosome constitution of races of maize, which is considered a landmark study of maize that has contributed significantly to the fields of evolutionary botany, ethnobotany, and paleobotany.

Rediscovery of McClintock's controlling elements

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. This honor allowed her to continue working with graduate students and colleagues in the Cold Spring Laboratory as scientist emerita. In reference to her decision 20 years earlier no longer to publish detailed accounts of her work on controlling elements, she wrote in 1973:

Over the years I have found that it is difficult if not impossible to bring to consciousness of another person the nature of his tacit assumptions when, by some special experiences, I have been made aware of them. This became painfully evident to me in my attempts during the 1950s to convince geneticists that the action of genes had to be and was controlled. It is now equally painful to recognize the fixity of assumptions that many persons hold on the nature of controlling elements in maize and the manners of their operation. One must await the right time for conceptual change.[25]

The importance of McClintock's contributions only came to light in the 1960s, when the work of French geneticists Francois Jacob and Jacques Monod described the genetic regulation of the lac operon, a concept she had demonstrated with Ac/Ds in 1951. Following Jacob and Monod's paper 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.[26] McClintock's contribution to biology is still not widely acknowledged as amounting to the discovery of genetic regulation.[24]

McClintock was widely credited for discovering transposition following the discovery of the process in bacteria and yeast in the late 1960s and early 1970s. During this period, molecular biology had developed significant new technology, and scientists were able to show the molecular basis for transposition. In the 1970s, Ac and Ds were cloned by other scientists and were shown to be Class II transposons. Ac is a complete transposon that can produce a functional transposase, which is required for the element to move within the genome. Ds has a mutation in its transposase gene, which means that it cannot move without another source of transposase. Thus, as McClintock observed, Ds cannot move in the absence of Ac. Spm has also been characterized as a transposon. Subsequent research has shown that transposons typically do not move unless the cell is placed under stress, such as by irradiation or the breakage, fusion, and bridge cycle, and thus their activation during stress can serve as a source of genetic variation for evolution. McClintock understood the role of transposons in evolution and genome change well before other researchers grasped the concept. Nowadays, Ac/Ds is used as a tool in plant biology to generate mutant plants used for the characterization of gene function.

Honors and recognition

McClintock was awarded the National Medal of Science by Richard Nixon in 1971. Cold Spring Harbor named a building in her honor in 1973. In 1981 she became the first recipient of the MacArthur Foundation Grant, and was awarded the Albert Lasker Award for Basic Medical Research, the Wolf Prize in Medicine and the Thomas Hunt Morgan Medal by the Genetics Society of America. In 1982 she was awarded the Louisa Gross Horwitz Prize from Columbia University for her research in the "evolution of genetic information and the control of its expression." Most notably, she received the Nobel Prize for Physiology or Medicine in 1983, credited by the Nobel Foundation for discovering "mobile genetic elements", over thirty years after she initially described the phenomenon of controlling elements.

She was awarded 14 Honorary Doctor of Science degrees and an Honorary Doctor of Humane Letters. In 1986 she was inducted into the National Women's Hall of Fame. During her final years, McClintock led a more public life, especially after Evelyn Fox Keller's 1983 book A feeling for the organism brought McClintock's story to the public. She remained a regular presence in the Cold Spring Harbor community, and gave talks on mobile genetic elements and the history of genetics research for the benefit of junior scientists. An anthology of her 43 publications The discovery and characterization of transposable elements: the collected papers of Barbara McClintock was published in 1987. McClintock died near Cold Spring Harbor in Huntington, New York, on September 2, 1992 at the age of 90; she never married or had children.

Legacy

Since her death, McClintock has been the subject of the biographical work by science historian Nathaniel C. Comfort, in The tangled field : Barbara McClintock's search for the patterns of genetic control. Comfort's biography contests some claims about McClintock, described as the "McClintock Myth", which he claims was perpetuated by the earlier biography by Keller. Keller's thesis was that McClintock was long ignored because she was a woman working in the sciences, whereas Comfort asserts that McClintock was well regarded by her professional peers, even in the early years of her career.[27] Although Comfort argues that McClintock was not a victim of sex discrimination, she has been widely written about in the context of women's studies, and most recent biographical works on women in science feature accounts of her experience. She is held up as a role model for girls in such works of children's literature as Edith Hope Fine's Barbara McClintock, Nobel Prize geneticist, Deborah Heiligman's Barbara McClintock: alone in her field and Mary Kittredge's Barbara McClintock. A recent biography for young adults by Naomi Pasachoff, Barbara McClintock, Genius of Genetics, provides a new perspective, based on the current literature.[28]

On May 4, 2005 the United States Postal Service issued the American Scientists commemorative postage stamp series, a set of four 37-cent self-adhesive stamps in several configurations. The scientists depicted were Barbara McClintock, John von Neumann, Josiah Willard Gibbs, and Richard Feynman. McClintock was also featured in a 1989 four-stamp issue from Sweden which illustrated the work of eight Nobel Prize-winning geneticists. A small building at Cornell University and a laboratory building at Cold Spring Habor Laboratory bear her name to this day.

Key publications

  • McClintock, Barbara (1929) A cytological and genetical study of triploid maize. Genetics 14:180–222
  • Creighton, Harriet B., and McClintock, Barbara (1931) A Correlation of Cytological and Genetical Crossing-Over in Zea Mays. Proceedings of the National Academy of Sciences 17:492–497
  • McClintock, Barbara (1931) The order of the genes C, Sh, and Wx in Zea Mays with reference to a cytologically known point in the chromosome. Proceedings of the National Academy of Sciences 17:485–91
  • McClintock, Barbara (1941) The stability of broken ends of chromosomes in Zea Mays, Genetics 26:234–82
  • McClintock, Barbara (1945) Neurospora: preliminary observations of the chromosomes of Neurospora crassa. American Journal of Botany. 32:671–78
  • McClintock, Barbara (1950) The origin and behavior of mutable loci in maize. Proceedings of the National Academy of Sciences. 36:344–55
  • McClintock, Barbara (1953) Induction of instability at selected loci in maize. Genetics 38:579–99
  • McClintock, Barbara (1961) Some parallels between gene control systems in maize and in bacteria. American Naturalist 95:265–77
  • McClintock, Barbara., Kato, T. A. & Blumenschein, A. (1981) Chromosome constitution of races of maize. Its significance in the interpretation of relationships between races and varieties in the Americas.. Colegio de Postgraduados, Chapingo, Mexico

References

  1. ^ a b Keller, Evelyn Fox (1983) A feeling for the organism. W. H. Freeman and Company, New York ISBN 0-7167-1433-7
  2. ^ Kass, L. B. and W. B. Provine. I997. Genetics in the roaring 20s: The influence of Cornell's professors and curriculum on Barbara McClintock's development as a cytogeneticist. American Journal of Botany Abstracts. 84 (6, Supplement): 123. Kass, L. B., 2000. Barbara McClintock, *Botanist, cytologist, geneticist. American Journal of Botany 87(6): 64. Available online: Symposium website, Symposium Botany in the Age of Mendel, Abstract #193.
  3. ^ McClintock, Barbara. A short biographical note: Barbara McClintock (1983) Nobel Foundation biography .pdf
  4. ^ Kass, Lee B. 2003. Records and recollections: A new look at Barbara McClintock, Nobel Prize-Winning geneticist. Genetics 164 (August): 1251-1260. Kass Lee, B. 2007b. Barbara McClintock (1902-1992), on Women Pioneers in Plant Biology, American Society of Plant Biologists website, Ann Hirsch editor. Published online, March 2007: American Society of Plant Biologists.
  5. ^ Kass, Lee B. 2005. Harriet Creighton: Proud botanist. Plant Science Bulletin. 51(4): 118-125. Available online, December 2005: Planet Science Bulletin. Kass Lee, B. 2007. Harriet B. Creighton (1909-2004), on Women Pioneers in Plant Biology, American Society of Plant Biologists website, edited by Ann Hirsch. Published online, February 2007: American Society of Plant Biologists.
  6. ^ Kass, Lee B. and Christophe Bonneuil. 2004. Mapping and seeing: Barbara McClintock and the linking of genetics and cytology in maize genetics, 1928-1935. Chapt 5, pp. 91-118, in Hans-Jörg Rheinberger and Jean-Paul Gaudilliere (eds.), Classical Genetic Research and its Legacy: The Mapping Cultures of 20th Century Genetics. London: Routledge. Kass, Lee B. Chris Bonneuil, and Ed Coe. 2005. Cornfests, cornfabs and cooperation: The origins and beginnings of the Maize Genetics Cooperation News Letter. Genetics 169 (April): 1787-1797. Available online, May 6, 2005: American Genetics Society.
  7. ^ Kass, Lee B. and Christophe Bonneuil. 2004. Mapping and seeing: Barbara McClintock and the linking of genetics and cytology in maize genetics, 1928-1935. Chapt 5, pp. 91-118, in Hans-Jörg Rheinberger and Jean-Paul Gaudilliere (eds.), Classical Genetic Research and its Legacy: The Mapping Cultures of 20th Century Genetics. London: Routledge
  8. ^ Rhoades, Marcus M. The golden age of corn genetics at Cornell as seen though the eyes of M. M. Rhoades undated .pdf NLM profile
  9. ^ Coe, Ed and Lee B. Kass. 2005a. Proof of physical exchange of genes on the chromosomes. Proceedings of the National Academy of Science 102 (No. 19, May): 6641-6656. Available online, May 2, 2005: PNAS.
  10. ^ Creighton, Harriet B., and McClintock, Barbara (1931) A Correlation of Cytological and Genetical Crossing-Over in Zea Mays. Proceedings of the National Academy of Sciences 17:492–497
  11. ^ McClintock, Barbara (1931) The order of the genes C, Sh, and Wx in Zea Mays with reference to a cytologically known point in the chromosome. Proceedings of the National Academy of Sciences 17:485–91
  12. ^ Coe, Ed and Lee B. Kass. 2005a. Proof of physical exchange of genes on the chromosomes. Proceedings of the National Academy of Science 102 (No. 19, May): 6641-6656.
  13. ^ Kass, Lee B. 2003. Records and recollections: A new look at Barbara McClintock, Nobel Prize-Winning geneticist. Genetics 164 (August): 1251-1260.
  14. ^ Kass, Lee B. 2005. Missouri compromise: tenure or freedom. New evidence clarifies why Barbara McClintock left Academe. Maize Genetics Cooperation Newsletter 79: 52-71.
  15. ^ McClintock, Barbara (1941) The stability of broken ends of chromosomes in Zea Mays, Genetics 26:234–82
  16. ^ McClintock, Barbara. Letter from Barbara McClintock to Charles R. Burnham (16 September 1940) .pdf
  17. ^ Kass, Lee B. 2003. Records and recollections: A new look at Barbara McClintock, Nobel Prize-Winning geneticist. Genetics 164 (August): 1251-1260. Kass, Lee B. 2005. Missouri compromise: tenure or freedom. New evidence clarifies why Barbara McClintock left Academe. Maize Genetics Cooperation Newsletter 79: 52-71
  18. ^ Comfort, Nathaniel C. (2002) Barbara McClintock's long postdoc years. Science 295:440
  19. ^ Kass, Lee B. 2003. Records and recollections: A new look at Barbara McClintock, Nobel Prize-Winning geneticist. Genetics 164 (August): 1251-1260. Kass, Lee B. 2005. Missouri compromise: tenure or freedom. New evidence clarifies why Barbara McClintock left Academe. Maize Genetics Cooperation Newsletter 79: 52-71
  20. ^ McClintock, Barbara (1945) Neurospora: preliminary observations of the chromosomes of Neurospora crassa. American Journal of Botany. 32:671–78
  21. ^ McClintock, Barbara (1950) The origin and behavior of mutable loci in maize. Proceedings of the National Academy of Sciences. 36:344–55
  22. ^ McClintock, Barbara. "Introduction" in The discovery and characterization of transposable elements: the collected papers of Barbara McClintock
  23. ^ McClintock, Barbara (1953) Induction of instability at selected loci in maize. Genetics 38:579–99
  24. ^ a b Comfort, Nathaniel, C.(1999) "The real point is control": The reception of Barbara McClintock's controlling elements. Journal of the History of Biology 32:133–6
  25. ^ McClintock, Barbara. Letter from Barbara McClintock to J. R. S. Fincham (1973) .pdf
  26. ^ McClintock, Barbara (1961) Some parallels between gene control systems in maize and in bacteria. American Naturalist 95:265–77
  27. ^ Comfort, Nathaniel C. (June 2001). The Tangled Field: Barbara McClintock's search for the patterns of genetic control. Cambridge, MA: Harvard University Press. ISBN 0-674-00456-6. 
  28. ^ Pasachoff, Naomi. 2006. Barbara McClintock, Genius of Genetics. Enslow Publishers, Inc.

Further reading

  • Bogdanov, Yu. F. (2002) A life devoted to science. In "Commemoration of the 100th anniversary of the birth of Barbara McClintock". Russian Journal of Genetics 38:984–87 PMID 12430570
  • Coe, Ed and Lee B. Kass. 2005. Proof of physical exchange of genes on the chromosomes. Proceedings of the National Academy of Science 102 (No. 19, May): 6641-6656. Available online, May 2, 2005: PNAS abstract
  • Comfort, Nathaniel, C. (1999) "The real point is control": The reception of Barbara McClintock's controlling elements. Journal of the History of Biology 32:133–62 PMID 11623812
  • Comfort, Nathaniel C. (2001) The tangled field: Barbara McClintock's search for the patterns of genetic control Harvard University Press, Cambridge, MA. ISBN 0-674-00456-6
  • Fedoroff, Nina V. (1995). Barbara McClintock. Biographical Memoirs of the National Academy of Science. 68:211–36
  • Fedoroff, Nina V. 2002. The well mangled McClintock myth. Trends in Genetics 18 (7): 378-379.
  • Kass, L. B. 1999. Current list of Barbara McClintock's publications. Maize Genetics Cooperation Newsletter 73: 42-48. Available online, 1998: Maize Genetics Cooperation Newsletter
  • Kass, Lee B. 2000. McClintock, Barbara, American botanical geneticist, 1902-1992. Pp. 66-69, in Plant Sciences. edited by R. Robinson. Macmillan Science Library, USA.
  • Kass, L. B. 2002. The Tangled Field, by N. Comfort. Isis. 93 (4): 729-730.
  • Kass, Lee B. 2003. Records and recollections: A new look at Barbara McClintock, Nobel Prize-Winning geneticist. Genetics 164 (August): 1251-1260.
  • Kass, L. B., 2004. Identification of photographs for the Barbara McClintock papers on the National Library of Medicine website. Maize Genetics Cooperation Newsletter 78: 24-26, available online, 2003: Maize Genetics Cooperation Newsletter
  • Kass, Lee B. 2005. Harriet Creighton: Proud botanist. Plant Science Bulletin. 51(4): 118-125. Available online, December 2005: Botanical Society of America
  • Kass, Lee B. 2005. Missouri compromise: tenure or freedom. New evidence clarifies why Barbara McClintock left Academe. Maize Genetics Cooperation Newsletter 79: 52-71; article without footnotes or photographs; available, online April 2005: Maize Genetics Cooperation Newsletter
  • Kass Lee, B. 2007. Harriet B. Creighton (1909-2004), on Women Pioneers in Plant Biology, American Society of Plant Biologists website, edited by Ann Hirsch. Published online, February 2007: American Society of Plant Biologists
  • Kass Lee, B. 2007. Barbara McClintock (1902-1992), on Women Pioneers in Plant Biology, American Society of Plant Biologists website, Ann Hirsch editor. Published online, March 2007: American Society of Plant Biologists
  • Kass, Lee B. and Christophe Bonneuil. 2004. Mapping and seeing: Barbara McClintock and the linking of genetics and cytology in maize genetics, 1928-1935. Chapt 5, pp. 91-118, in Hans-Jörg Rheinberger and Jean-Paul Gaudilliere (eds.), Classical Genetic Research and its Legacy: The Mapping Cultures of 20th Century Genetics. London: Routledge.
  • Kass, L. B. and R. P. Murphy. 2003. Will the real Maize Genetics Garden please stand up? Maize Genetics Cooperation Newsletter. 77: 41-43. Available online, 2003: Maize Genetics Cooperation Newsletter
  • Kass, L. B and W. B. Provine. 1999 (&1998). Formerly restricted interview with Barbara McClintock, now available at Cornell University Archives. Maize Genetics Cooperation Newsletter. 73: 41. Available online, 1998: Maize Genetics Cooperation Newsletter
  • Kass, Lee B. Chris Bonneuil, and Ed Coe. 2005. Cornfests, cornfabs and cooperation: The origins and beginnings of the Maize Genetics Cooperation News Letter. Genetics 169 (April): 1787-1797. Available online, May 6, 2005: Genetics Society of America
  • Jones, R.N. 2005. McClintock's controlling elements: the full story. Cytogenetics Research 109:90–103 PMID 15753564
  • Keller, Evelyn Fox (1983) A feeling for the organism. W. H. Freeman and Company, New York ISBN 0-7167-1433-7
  • Lamberts, William J. (2000) McClintock, Barbara. American National Biography Online. Oxford University Press
  • McClintock, Barbara. (1987). The discovery and characterization of transposable elements: the collected papers of Barbara *McClintock, ed John A. Moore. Garland Publishing, Inc., ISBN 0-8240-1391-3.
  • Fedoroff, Nina V and Botstein, David (1993) The Dynamic Genome: Barbara McClintock's Ideas in the Century of Genetics. Cold Spring Harbor Laboratory Press, New York. ISBN 0-87969-396-7

Archives and research collections

External links


Wolf Prize in Medicine Laureates

George Snell / Jean Dausset / Jon J. van Rood (1978) • Roger Sperry / Arvid Carlsson / Oleh Hornykiewicz (1979) • César Milstein / Leo Sachs / James L. Gowans (1980) • Barbara McClintock / Stanley Norman Cohen (1981) • Jean-Pierre Changeux / Solomon H. Snyder / James W. Black (1982) • Donald F. Steiner (1984) • Osamu Hayaishi (1986) • Pedro Cuatrecasas / Meir Wilchek (1987) • Henri G. Hers / Elizabeth F. Neufeld (1988) • John Gurdon / Edward B. Lewis (1989) • Maclyn McCarty (1990) • Seymour Benzer (1991) • Judah Folkman (1992) • Michael Berridge / Yasutomi Nishizuka (1994) • Stanley B. Prusiner (1995) • Mary F. Lyon (1997) • Michael Sela / Ruth Arnon (1998) • Eric Kandel (1999) • Avram Hershko / Alexander Varshavsky (2001) • Ralph L. Brinster / Mario Capecchi / Oliver Smithies (2002) • Robert Weinberg / Roger Y. Tsien (2004) • Alexander Levitzki / Anthony R. Hunter / Anthony Pawson (2005)

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Nobel Prize in Physiology or Medicine Laureates

Baruch Blumberg / Daniel Gajdusek (1976) • Roger Guillemin / Andrew W. Schally / Rosalyn Yalow (1977) • Werner Arber / Daniel Nathans / Hamilton O. Smith (1978) • Allan Cormack / Godfrey Hounsfield (1979) • Baruj Benacerraf / Jean Dausset / George Snell (1980) • Roger Sperry / David H. Hubel / Torsten Wiesel (1981) • Sune Bergström / Bengt I. Samuelsson / John Vane (1982) • Barbara McClintock (1983) • Niels Jerne / Georges Köhler / César Milstein (1984) • Michael Brown / Joseph L. Goldstein (1985) • Stanley Cohen / Rita Levi-Montalcini (1986) • Susumu Tonegawa (1987) • James W. Black / Gertrude B. Elion / George H. Hitchings (1988) • J. Michael Bishop / Harold E. Varmus (1989) • Joseph Murray / E. Donnall Thomas (1990) • Erwin Neher / Bert Sakmann (1991) • Edmond Fischer / Edwin G. Krebs (1992) • Richard J. Roberts / Phillip Sharp (1993) • Alfred G. Gilman / Martin Rodbell (1994) • Edward B. Lewis / Christiane Nüsslein-Volhard / Eric F. Wieschaus (1995) • Peter Doherty / Rolf M. Zinkernagel (1996) • Stanley B. Prusiner (1997) • Robert F. Furchgott / Louis Ignarro / Ferid Murad (1998) • Günter Blobel (1999) • Arvid Carlsson / Paul Greengard / Eric Kandel (2000)

Complete roster | (1901-1925) | (1926-1950) | (1951-1975) | (1976-2000) | (2001-2025)


Persondata
NAME McClintock, Barbara
ALTERNATIVE NAMES
SHORT DESCRIPTION cytogeneticist
DATE OF BIRTH June 16 1902
PLACE OF BIRTH Hartford, Connecticut
DATE OF DEATH September 2 1992
PLACE OF DEATH Huntington, New York

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