Summary
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1.
In the loops of primary spermatocyte nuclei actinomycin causes characteristic and reproducible disintegrations of the structural elements composing the loops. The alterations become visible for the first time about 6 hours after injection of actinomycin and reach a maximum after about 24–30 hours.
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2.
The structure of the nucleolus is also changed by actinomycin. It looses its cortical “ribosomal” layer and detaches from the nuclear membrane to which it is normally attached.
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3.
All structural damage of the loops and the nucleolus caused by actinomycin is reversible. The regeneration of the loops starts approximately 40 hours after injection and is completed after about 100–120 hours.
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4.
In males with two Y chromosomes which form loops of different morphology because of genetical differences the actinomycin effects similar alterations as in normal males. During the regeneration each Y chromosome autonomously determines its specific type of loops.
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5.
The results are in good agreement with our hypothesis that the loops are modifications of the chromosome structure at sites of active genes.
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Herrn Prof. Dr. H. Bauer zum 60. Geburtstag.
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Meyer, G.F., Hess, O. Struktur-Differenzierungen im Y-Chromosom von Drosophila hydei und ihre Beziehungen zu Gen-Aktivitäten. Chromosoma 16, 249–270 (1965). https://doi.org/10.1007/BF00325994
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DOI: https://doi.org/10.1007/BF00325994