Biographical Memoirs Volume 87 (2005) / Chapter Skim
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Raymond Elliott Zirkle
Raymond Elliott Zirkle
Pages 374-390
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From page 375... ...
In later studies he and his coworkers developed microbeam technology, which they used to dissect the functions of particular cell structures for chromosome movements during mitosis. Ray Zirkle was born in Springfield, Illinois, and spent his early years on a farm in northern Oklahoma.
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From page 376... ...
Zirkle's thesis research was the forerunner of experiments and ideas that occupied him for the rest of his scientific career. In this research he irradiated spores of the fern Pteris longiflora with alpha particles emitted from a polonium source and studied the effects on their subsequent germination.
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From page 377... ...
During this time he investigated the quantitative relationship between ionization per unit path of alpha particles and their biological effectiveness, which was not clear-cut from the data obtained up to that time. By placing fern spore nuclei either near the beginning of the path (where the ionization density was low)
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From page 378... ...
In an incisive theoretical analysis he used existing knowledge of the specific ionization properties of different types of radiation to interpret experimental results with alpha particles, neutrons, and X rays. At one end of the spectrum are X or gamma rays, which set electrons in motion with the lowest specific ionization (ions per unit path traversed)
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From page 379... ...
William Doyle, a faculty colleague of Ray's, recalled to me the many pleasant Saturday afternoons when he and his wife visited the Zirkles and enjoyed games of bridge with them. The era of nuclear energy was spawned by the first chain reaction, produced by Enrico Fermi and colleagues under the West Stands of the athletic stadium at the University of Chicago.
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From page 380... ...
As knowledge of the chemical composition of biological material began to accumulate, he attempted to relate the chemical effects caused by the absorption of radiant energy to the ultimate biological effects. He fully appreciated that an understanding of the multitude of diverse radiobiological effects -- such as gene mutations, chromosome breaks, increased membrane permeability, inhibition of cell division, induction of neoplasms, and lethality of cells and organisms -- would require a detailed knowledge of the intervening chemical modifications.
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From page 381... ...
They interpreted these results as being consistent with a theory in which cell division in haploid cells can be inhibited by inactivating any one of multiple chromosomal sites with a single ionizing particle, whereas in diploids it is necessary to inactivate both members of an allelic pair of corresponding sites. Although the observed variations of relative biological effectiveness with linear energy transfer were not consistent with simple target theory, they could be explained in terms of chemical intermediates that diffuse from their places of origin in the ionization tracks to the sensitive chromosomal sites.
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From page 382... ...
For biological material Zirkle and Bloom concentrated on actively dividing mitotic cells in cultures derived from newt heart. These large, relatively flat cells were excellent specimens for this type of experiment because of their favorable dimensions and their ability to proliferate at ambient temperature.
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From page 383... ...
On the fourth floor, directly above the microbeam apparatus, was the temperamental Van de Graaff generator, its collimated proton beam protruding through a hole in the floor. After I passed the requisite qualifying examinations in physics and physical chemistry, I joined the Zirkle-Bloom laboratory as a neophytic graduate student of Ray Zirkle.
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From page 384... ...
The ease with which this instrument could be used for perturbing the mitotic process by chromosome ablation or spindle destruction led to a large number of experiments, many with interesting, informative outcomes. Localization of the irradiated regions of chromosomes could readily be verified because the intense ultraviolet energy absorbed by the chromosomes caused a dramatic decrease in refractive index at the irradiated site and a concomitant loss of DNA from the site, a phenomenon that was termed "paling." Zirkle and his coworkers selectively irradiated isolated centrophilic chromosomes that had not yet become aligned on the metaphase plate either in the kinetochore region or in a distal part of the chromosome.
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From page 385... ...
These experiments clearly demonstrated a distinction between the molecular mechanisms responsible for moving chromatids to the spindle poles during normal anaphase and those responsible for moving the poles apart prior to cytokinesis. Zirkle and Bloom amassed a huge collection of 16-mm movie films that documented the orderly progression through mitosis of normal cells and the abnormalities that occur in microbeam-irradiated cells.
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From page 386... ...
His experiments with alpha-particle-irradiated fern spores -- which showed especially high sensitivity of nuclei compared with cytoplasm and demonstrated the importance of ionization density-were seminal discoveries, made prior to an understanding of the molecular basis of gene expression. Similarly his microbeam experiments provided the first evidence for the importance of kinetochores in mitotic checkpoint control.
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From page 387... ...
; American Society of Naturalists; American Society of Plant Physiologists; American Roentgen Ray Society; National Academy of Sciences; and the American Philosophical Society. Member of the following committees: Biophysical Sciences Training Committee, National Institutes of Health (1958 1962)
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From page 388... ...
2:251-274. 1935 Biological effectiveness of alpha particles as a function of ion concentration produced in their paths.
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From page 389... ...
Effects of ploidy and linear energy transfer on radiobiological survival curves.
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From page 390... ...
Action spectra for mitotic spindle destruction and anaphase delay following irradiation of the cytoplasm with an ultraviolet microbeam. Photochem.
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