Cell Division. II. A Theoretical Approach to Chromosomal Movements and the Division of the Cell

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Abstract
Movements observed during cell division may best be explained on the basis of the physicochemical properties of gels and the formation of contractile fibrils of colloidal dimensions. The possible mechanisms by which contractile systems may be formed in cells involves protein as the chief constituent although other substances such as nucleic acids and acid polysaccharides may be involved. After a review of the methods used in the formation of artificial protein fibers, the author suggests that although attempts have been made to apply similar principles to fibril formation in cells, it does not appear likely that the sol-gel changes in the cell are an expression of the formation of contractile fibrils formed from interlinked unrolled polypeptide chains. Rather, the formation of thread-like particles by the end-to-end aggregation of globular molecules without gross denaturation has been demonstrated. The principle of linear aggregation of colloids has found numerous applications in biological systems. The cell is postulated to be composed of a series of colloidal aggregates which, though held together by a wide spectrum of forces, are nevertheless sufficiently labile to be considered in equilibrium with either smaller complexes or free colloidal particles in solution. The implications of this concept for cell division are discussed. Soluble complexes are believed to be of central importance in the economy of the cell because (1) the activities of soluble enzymes may be regulated by the degree to which they are aggregated with other enzymes to form complexes; (2) soluble complexes may serve to solubilize and transport molecules of limited solubility, and (3) the soluble complexes, if in the form of linear aggregates, will have a considerable effect on the physical properties of the cytoplasm. The soluble complexes are also believed to be intermediates in the formation of many cellular structures. The cytoplasm is thought to be a labile system in a state of dynamic equilibrium. Structural complexes include such a variety of intermolecular linkages that it is difficult to decide what constitutes a nucleus or a mitochondrion. Various cell components can, it is true, be isolated and studied biochemically but this fact does not defeat the postulated concept of labile systems since (1) equilibria in colloid systems are often established very slowly, and (2) the equilibrium may be shifted very far in one direction, for example, in the direction of structure. This equilibrium, by its very existence, accounts for the finite turnover rates of all cellular proteins examined so far, even those of nondividing cells. The formation of the asters, the production of the spindle, and the breakdown of the nuclear envelope are discussed on the basis of the molecular changes which could account for the chromosomal movements. The reconstruction of the resting cell and the charge on the sytoplasmic colloids are discussed in relation to the postulated theory. More than 200 references are given.