Discovery of controlling elements

The discovery of transposable elements by Barbara McClintock is a remarkable story of careful study and insightful analysis in genetics. Mclintock 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 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 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. The controlling elements regulate the expression of other genes. The families of controlling elements are now recognized as members of the class of transposable elements that move through DNA intermediates. However, McClintock’s proposal that the controlling elements were mobile was not widely accepted for a very long time. Despite her extensive observations published in the 1930’s through the 1950’s, the interpretation that genetic elements could move was perhaps too novel. Indeed, the notion that transposable elements are active in a wide range of species as not widely accepted until the 1980’s, and new evidence continues to mount that transposable elements are more common than previously thought.



''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.''

McClintock’s seminal observations relied on two complementary approaches to understanding chromosome structure and function. One was cytological, using microscopy to examine the structure of chromosomes in corn, and the other used genetics to follow the fates of the chromosomes. A full exploration of the discovery of transposable elements is the subject of excellent books.

In essence, McClintock showed that certain crosses between maize cultivars (or strains) resulted in large numbers of mutable loci, i.e. the frequency of change at those loci is much higher than observed in other crosses. Her studies of the cultivars with mutable loci revealed a genetic element termed “Dissociation”, or Ds. Chromosome breaks occurred at the Ds locus; these could be seen cytologically, using a microscope to examine chromosome spreads from individual germ cells (sporocytes). The frequency and timing of these breaks is controlled by another locus, called “Activator” or Ac. In following crosses of the progeny, the position of Ds-mediated breaks changed, arguing that the Ds element had moved, or transposed. That was the basic argument for transposition.

''' Barbara McClintock Biographical Information Childhood'''

Barbara McClintock was born in Hartford, Connecticut, on June 16 in 1902 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. Early youth 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 on the theory 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. 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 the only one of its type offered to undergraduates in the United States at the time, and was taught by C. B. Hutchison, a plant breeder and geneticist. 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." 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. 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 Rollins Emerson, Charles R. Burnham, Marcus Rhoades, and George Beadle (who became a Nobel laureate in 1958 for showing that genes control metabolism).

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 that maize had 10 chromosomes. 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. They observed how the recombination of chromosomes and the resulting phenotype formed the inheritance of a new trait. Until this point, it had only been hypothesized that genetic recombination could occur during meiosis. McClintock published the first genetic map for maize in 1931, showing the order of three genes on maize chromosome 9. In 1932, 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 Thomas Hunt Morgan. 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, which she called the telomere. 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 crossover 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. 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. 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 was excluded from faculty meetings, and was not made aware of positions available at other institutions. 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." She was also aware that her position had been especially created for her by Stadler and may have depended on his presence. 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 In the summer of 1941 she took a leave of absence from Missouri to visit Columbia University, where her 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, and she joined the staff of the Carnegie Institution of Washington's Department of Genetics Cold Spring Harbor Laboratory. 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.

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, 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.As we said before, 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".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. 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. 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 Nature paper "Genetic regulatory mechanisms in the synthesis of proteins", McClintock wrote an article for American Naturalist comparing the lac operon and her work on controlling elements in maize.[ McClintock's contribution to biology is still not widely acknowledged as amounting to the discovery of genetic regulation. 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, other scientists Ac and Ds were cloned 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. 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. McClintock was the first woman to win an unshared Nobel Prize in that category and the third woman to win an unshared Nobel Prize in science. McClintock also received numerous honorary degrees, including those from Georgetown University, Harvard University, University of Cambridge, and Yale University. She was a member of various societies, including the American Academy of Arts and Sciences, the American Association for the Advancement of Science, the American Institute of Biological Sciences, the American Philosophical Society, the American Society of Naturalists, the Botanical Society of America, the Genetics Society of America (by which she was elected Vice President in 1939 and the first woman President in 1945), the National Academy of Sciences (she was the third woman ever elected), the National Women's Hall of Fame, the New York Academy of Sciences, and the Royal Society of London. Barbara McClintock died on 2 September 1992 at age ninety

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 for her research in the "evolution of genetic information and the control of its expression.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.