Mitotic Asymmetry: Differential Behaviour of Sister Nuclei

Abstract

<p>Caffeine, a trimethylxanthine, is a potent inhibitor of cytokinesis in-plant cells; it induces the formation of binucleate cells. The use of binucleate cells is particularly revealing since they make it possible to compare the behaviour of sister nuclei in a binucleate cell with the behaviour of nuclei in sister cells, i.e. to compare the behaviour of sister nuclei in one cytpplasmic environment or in two separate environments. We have used this technique to study the asymmetry of mitosis and the consequences of this asymmetry. With binucleate cells we have shown that a number of differences exhibited by the nuclei of sister cells, i.e. differences in nuclear size, protein content and RNA content, are also exhibited by the sister nuclei of binucleate cells. The fact that these differences occurred in binucleate cells indicates that differences between sister nuclei are inherent and arose as a result of the mitosis from which the nuclei were formed.</p> <p>The subsequent behaviour of the sister nuclei is also affected by the asymmetrical mitoses. Sister nuclei of binucleate cells showed a differential growth pattern and when supplied with ³H-uridine a differential ability to synthesize RNA. An analysis of nuclei of sister cells revealed an identical pattern of behaviour. This suggests that the asymmetry of division not only produces sister nuclei of different size and macromolecular content but also results in functional differences between the two sister nuclei.</p> <p>It is proposed that a large degree of heterogeneity in cell size, nuclear size, macromolecular contents of cells and nuclei, and in cell cycle duration is the result of asymmetrical mitoses. The asymmetrical mitoses are responsible for generating, at every mitosis, physical differences between sister cells and these physical differences are functionally related to the differential behaviour of sister nuclei and sister cells.</p>