Summary
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1.
An experiment usingScilla sibirica endosperm with a synchronized mitosis for X-ray breakage studies is described.
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2.
So-called point error aberrations arise through treatment of prophase. They lead through intercalary and terminal union of chromatids to bridge formation at anaphase.
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3.
The bridges break mechanically at anaphase to produce fragments. Breakage, however, is seldom at the point of union.
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4.
After a dose of 54 r and temperature maintained at 18° C point errors appear in anaphases from 2–6 1/4 hours following irradiation. They increase in frequency with time.
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5.
Point errors are assumed to arise through breakage and reunion of sub-chromatids to give structures resembling meiotic chiasmata.
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6.
Associations at non-homologous loci arise through coiled and overlapping prophase chromatids. Thus, loop structures occur through intrachromatid union; whilst interchromatid union, between sisters or non-sisters, gives configurations with unequal arms distal to the point of union.
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7.
There is evidence of interference by the centromere, point errors therefore rarely occur in its vicinity.
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8.
The interpretation of sub-chromatid breakage in relation to chromosome structure and reproduction is briefly discussed.
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La Cour, L.F., Rutishauser, A. X-ray breakage experiments with endosperm I. Sub-chromatid breakage. Chromosoma 6, 696–709 (1953). https://doi.org/10.1007/BF01259956
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DOI: https://doi.org/10.1007/BF01259956