CHROMOMERES AND PROTEINS 



necessary for conversion into the desoxyribose form required for 

 the rapid multipHcation of nuclei which follows fertilization. 

 Ribose nucleotides are capable of some polymerization. Their 

 main work, however, seems to be less mechanical and more 

 physiological than that of their desoxyribose relatives. They are 

 concerned with protein reproduction. They are regularly found in 

 the smaller viruses, in the plastids of plants and in other small 

 protein bodies of the cytoplasm. The viruses and plastids are 

 characteristically self-propagating bodies like the genes of the 

 nucleus. Ribose nucleic acid, therefore, may well do for these 

 proteins in a small way what the desoxyribose does for the more 

 highly organized chromosome, namely, act as the agent of their 

 reproduction. 



What picture do we then get of the chemical relations of the 

 nucleus and the cell? The nucleus is a differentiated and highly 

 organized, mechanically stable and totally balanced system of 

 protein-producing units. The cytoplasm has no mechanical stability 

 to give it a permanent and total balance. Indeed, as we shall see, 

 differentiation between cells depends on changes in its balance. It is 

 not surprising, therefore, that the permanence of control in heredity 

 and development so largely comes from the nucleus. At the same 

 time the existence of self-propagating bodies in the cytoplasm 

 would indicate that the control of the nucleus was not total, im- 

 mediate and absolute. The cytoplasm must always feed the nucleus 

 with the materials from which it builds up its specialized structures 

 and with which it in turn feeds the cytoplasm. Each must feed 

 the right materials to the other. When, as we saw, the nucleus 

 of one species is put in the cytoplasm of another, breakdown often 

 results from the failure of this reciprocal process. In development, 

 therefore, we camiot argue for the greater importance of one or the 

 other. It is only the long-term relations concerned with the per- 

 manence of heredity that enable us to take their interactions to pieces 

 and show where precedence and authority lie. 



This picture opens our eyes to a number of new possibilities. 

 First, although there are many animals and plants in which the 

 differentiation of the heterochromatin is not visible, it seems likely 

 that the two kinds of genes are always present. The one kind gives 

 the simpler products and should have less specific, and individually 



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