Monolayers of chromatin structural units about 33.0 nm in width enclosed on both sides by extensions of the nuclear envelope, called sheets, and located either in the cytoplasm (c. .n. .c type), or within the nucleus (c. .n. .n type), are common in cultured cells of Burkitt's lymphoma. The sheets are absent from mitotic cells except at telophase where, unlike interphase, type c. .n. .n is more numerous than c. .n. .c. The degree of nuclear asymmetry is defined in terms of the increase in enclosing membranes over that required to enclose the same area in a circular configuration. The percentage number (Ps) of cells with nucleus-associated sheets averaged over all stages in the cell cycle, increases with cell viability and with nuclear asymmetry. However, during the cycle there is a marked diminution in Ps during the S-phase of DNA synthesis when nuclear asymmetry itself does not change. Hence, it is suggested, and data on other cell types support the hypothesis, that nuclear asymmetry is a necessary but not sufficient factor in causing sheets to form. Microtubules are present within the cytoplasm and their morphological arrangement suggests a role in determining nuclear asymmetry. Treatment with a microtubule depolymerizing agent, colcemid, does not alter either the existing nuclear asymmetry or Ps, but when cells are treated early in S-phase the reappearance of sheets in the G2 phase of the cell cycle is considerably delayed. The reappearance takes place when the microtubules are still depolymerized. It is suggested that synthesis of membrane in excess of what is needed to enclose a sphere results in nuclear asymmetry and associated membrane-enclosed monolayers, the resulting nuclear conformation, including the distribution of membrane between types c. .n. .n and c. .n. .c, depending on what is energetically favoured. No biochemical function has yet been assigned to sheets.

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