RNA-Synthese in der Mitose I. Beziehungen zwischen dem Kondensationsgrad der Chromosomen und dem Einbau von 3H-Uridin

Abstract

Introduction. Depression of RNA synthesis is well documented, but there are only vage suggestions on the regulation mechanism involved. One of the reasons discussed is the condensation of chromatin which could repress the template activity of DNA. The aim of the present study was to look for possible relationships between RNA synthesis and the degree of condensation during mitosis. Materials and Methods. Asynchronous cultures of L-cells were incubated for 10 minutes with 1 μC/ml 3H-uridine and fixed. Mapped mitotic cells were grouped into 12 different stages covering early prophase through early interphase. Following counting of silver grains after autoradiography the cells were stained according to the Feulgen procedure. Two different parameters were used as a measure for the “degree of condensation”: 1. The mean optical density measured with the scanning cytophotometer UMSP I (C. Zeiss) and 2. The reciprocal surface area occupied by the Feulgen stained material. This latter parameter was elaborated by photographing the cells and planimetry of the enlarged images. In addition the optical path difference in the cytoplasm, condensed and uncondensed chromatin of pro- and interphase nuclei and in chromosomes of mid-mitosis cells was measured. A Zeiss Jamin-Lebedeff-type interference microscope was used for this purpose. The relative frequency, i. e. the duration of the different mitotic stages was also counted. Results. The number of silver grains during mid-mitosis (late prophase to late telo-phase) was roughly ten percent of that of early prophase or early interphase (fig. 1). These changes in RNA synthetic activity occur within less than 15 minutes. Optical path differences do not considerably change in the cytoplasm (fig. 2) and in the heteropyknotic part of the chromatin, while in mid-mitosis the isopyknotic chromatin acquires a density comparable to that of the heteropyknotic chromatin (fig. 3). During metaphase reciprocal area and mean optical density rise to values two to three times that of interphase and early prophase (fig. 4 and 5). There exists a significant correlation between the “degree of condensation” of chromatin and the number of silver grains, which is thought to reflect RNA synthesis. This correlation can be demonstrated during the condensation process in pro-phase (fig. 6) as well as during the decondensation of chromosomes in telophase and early interphase (fig. 7). Discussion. The observed correlation between the compactness of chromosomes and their ability to prime for RNA synthesis can be interpreted as a causal relationship. Depression of RNA synthesis may during mitosis merely be the result of chromosome condensation with no specific regulation mechanism involved. Under these aspects the mitotic chromosome is a model for the regulation of transcription processes by chromatin condensation.