Biographical Memoirs Volume 68 (1995) / Chapter Skim
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Barbara McClintock
Barbara McClintock
Pages 211-236
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From page 211... ...
The chromosomal basis of heredity was aIreacly well established by the time McClintock began her graduate training in the Botany Department at Cornell University. McCTintock made her first significant contribution as a graduate student, cleveloping cytological techniques that allowed 211
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From page 212... ...
Among the progeny of plants that hacI received a broken chromosome from each parent, she observer! unstable mutations at an unexpecteclly high frequency, as well as a unique mutation that clefinecI a regular site of chromosome breakage.
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From page 213... ...
But in the late 1960s evidence began to accumulate that bacteriophages and bacteria contain mobile DNA sequences. During the following two clecacles, it became clear that transposable elements are not only ubiquitous but are extraordinarily abundant in the genomes of many organisms.
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She reported enjoying this time and attributed her later interest in cars to watching her uncle struggle with his vehicle's frequent malfunct~ons. McCTintock returned home to attend school, and in 1908 the family moved to Brooklyn, New York.
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But she knew, as well, that she had to follow her own inclinations whatever the consequences. At the time McClintock graduated from high school in HIS, the family situation was difficult.
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From page 216... ...
But while Cornell had a group of outstanding geneticists, genetics was taught in the plant breeding department, which clicl not take female graduate students. So McClintock registerec!
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From page 217... ...
In the course of her triploid studies, she had discovered that the metaphase or late prophase chromosomes in the first microspore mitosis were far better for cytological discrimination than were root tip chromosomes in paraffin sections. In a few weeks' time she had prepared an idiogram of the maize chromosomes, which she published in Science.
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From page 218... ...
The work, authored by Creighton and McClintock, was published in the Proceedings of the National Academy of Sciences in 1931 and was perhaps McClintock's first seminal contribution to the science of genetics, many more of which were to follow. Among the most important of her discoveries during the next few years, sometimes made alone, sometimes together with others, were that sister chromatics also exhibit genetic and cytological crossing-over, that genes can be physically localized on the chromosomes, that nonhomologous chromosome pairing has genetic consequences, that the formation of ring-shaped chromosomes accounts for certain types of phenotypic variegation, that the centromere is divisible, that broken chromosomes can undergo repeated cycles of fusion and breakage, and that a particular chromosomal site, the nucleolus organizer region (NOR)
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From page 219... ...
Describing the effect of McClintock's NOR publication, cell biologist Joseph Gall has written: Out of the hundreds of papers we have each read, a half dozen or so stick in our minds because of their beautiful logic, their clarification of an otherwise obscure set of data, or simply their technical elegance.... For me, one of Barbara McClintock's early cytogenetic papers falls in this categoryher analysis of the nucleolus of maize published in 1934 in the Zeitschr~ft fur Zellforschung and Mikroskopische Anatomie under the title, "The relation of a particular chromosomal element to the development of the nucleoli in Zea ways." In 1933 McCTintock received a Guggenheim Fellowship to go to Germany.
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She learner! that the ends of newly broken chromosomes tend to fuse with each other, creating dicentric chromosomes that break again when a cell divicles and chromosomes are distributed to the daughter cells.
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From page 221... ...
She was a staff member of the Carnegie Institution of Washington's Genetics Department until ~ 967, whereupon she became distinguished service member of the Carnegie Institution, remaining at CoIcl Spring Harbor until her death in 1992. Carnegie gave her the freedom to do her work unfettered by teaching and other academic duties.
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From page 222... ...
the presidency of the Genetics Society, respectively, McCTintock reportecl in the Yearbook of the Carnegie Institution of Washington on her analysis of progeny grown from self-pollinatecl plants obtained by crossing parents, each of which bore a broken chromosome 9. She cletectec3 many mutations among these progeny, including the expected terminal deficiencies, some internal deficiencies of various sizes, and some "provocative" mutants that showed variegation from the recessive to the dominant phenotype.
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From page 223... ...
By the time she prepared her paper for the Cold Spring Harbor Symposium of 1951, McCTintock hack isolatect unstable alleles of at least four different genes. Some were caused by the insertion of the Ds element and so requirecl the presence of Ac for instability.
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From page 224... ...
by transposable elements. More fascinating yet, McCTintock found that the Spm element couIcI become heritably inactivated by a genetic mechanism that differs strikingly from conventional mutation by its reversibility.
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Her experiments were very complex en cl cliff~cult to communicate even to the quickest of mincis. Mel Green recounts that shortly after the 1951 CoIcl Spring Harbor Symposium, he and several other geneticists queried Sturtevant, arguably one of the century's leacling geneticists, about what McClintock tract said.
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From page 226... ...
But transposition remained McCTintock's central passion. By the time of her formal retirement, she tract accumulated a rich store of knowlecige about the genetic behavior of two markedly different transposable element families.
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From page 227... ...
Mobile genetic elements were no longer abstract concepts. Although the study of maize transposable elements had been an active and productive fielcl of research since Emerson's original studies on variegation at the P locus long before McClintock explicates!
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From page 228... ...
in molecular biological investigations. T had been asked to give a seminar at the CoIct Spring Harbor Laboratory on my postdoctoral work in Don Brown's laboratory at the Carnegie Institution of Washington's Department of Embryology in Baltimore.
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She continued to live at Cold Spring Harbor, spending her last years in a
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Even when ~ visited CoIcI Spring Harbor in 1991 to give a course lecture on the molecular genetics of the maize transposable elements, McClintock sat through the entire session, which lasted late into the evening. Her questions were penetrating en c!
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From page 231... ...
in the celebration of her ninetieth birthday at the home of Jim Watson, not far from her moclest apartment on the laboratory grour~cis. She knew nothing of the book but recognized her friends even Harriet Creighton, her first "unofficial" graduate student, had macle the trek to Coicl Spring Harbor.
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From page 232... ...
Botstein; Cold Spring Harbor: Cold Spring Harbor Press, 1992~. The Bateson quotation appears in E
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19:191-237. 1934 The relation of a particular chromosomal element to the development of the nucleoli in Zea mays.
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Cold Spring Harbor Symp. Quant.
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From page 235... ...
BARBARA McCLINTOCK 1965 235 The control of gene action in maize. BrooLhaven Symp.
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