The repair time for chromosome breaks induced by X-irradiation of unstimulated (G0) and stimulated (G1) human lymphocytes has been determined by dose fractionation studies. In both types of cells repair time was approx. 4–5 h. Treatment with hydroxyurea, a DNA synthesis inhibitor, did not prevent or delay the rejoining of broken chromosomes, whereas treatment with cycloheximide, a potent protein synthesis inhibitor, did. Thus, the repair of radiation-induced chromosome breaks in human lymphocytes is similar to the repair observed with plant cells.

The repair of x-ray-induced chromosome aberrations in stimulated and unstimulated human lymphocytes.

http://link.springer.com/content/pdf/10.1007/978-1-4684-8479-3_2.pdf

The Structure and Replication of Eukaryotic Chromosomes

Studies on in vitro lymphocyte proliferation in cultures synchronized by the inhibition of DNA synthesis. I. Variability of S plus G2 periods of first generation cells.

The ultrastructural localization of deoxyribonucleic acid (DNA) synthesis was studied by electron-microscope autoradiography in human transforming lymphocytes, embryonic lung fibroblasts, epithelial cells and normoblasts. Euchromatin was found to be active in DNA synthesis in all cell types studied, whereas heterochromatin was inactive. However, DNA synthesis was also prominent in the regions where heterochromatin was thought to be decondensing to form euchromatin. Analysis of sequential changes in nuclear morphology of the transforming lymphocyte suggested that there is decondensation of heterochromatin during the S -phase until none is left. In nuclei with no heterochromatin a prominent localization of DNA synthesis was at the nuclear membrane. This sequence of complete decondensation of heterochromatin also seemed likely for fibroblasts and epithelial cells. Normoblasts however showed no stage where the nucleus was wholly euchromatic and it is suggested that in this cell decondensation of heterochromatin for replication is localized and transient.

Nuclear Morphology and the Ultra-Structural Localization of Deoxyribonucleic Acid Synthesis during Interphase

Publisher Summary This chapter discusses the variable condition of euchromatin and heterochromatin. Several terms were used to describe late-replicating chromosome material, replacing the customary term heterochromatin which was introduced by Heitz. The varying degree of condensation of meiotic chromosomes was called heteropycnosis. The chapter discusses the structure characterization of late-replicating chromosome material. According to the electron microscope studies euchromatin and heterochromatin both consist of 200-A fibrils but differ either in manner of spiralization or in condensation system. A varying content of nucleic acid and a different degree of spiralization are commonly held responsible for the different stainability. Heterochromatin in the interphase cell nucleus can fuse into one body (the collective chromocenter) or can be distributed in several nuclear regions. The Lyon hypothesis explained several puzzling phenomena and was of great help in the development of human genetics in spite of its deficiencies. Inactivation, however, represents only one aspect of the mode of action of late-replicating chromosome material. The cytological variability and the many clinical symptoms related to aberrations of late-replicating autosomes show clearly the great significance attached to the mode of action of this chromosome material.

Ludmila Ya. Levina

Papers

Autoradiographical study on the time of nuclear protein synthesis in human leucocyte blood culture.