graphic studies that labeled RNA precursors appear very rapidly in the 

 nucleolus (Figure 4-26) and prior to their appearance in cytoplasmic 

 RNA. While this suggests that the nucleolus is a site of RNA synthesis, 

 recent studies suggest that such synthesis may be initiated at the surface 

 of the chromosomes with subsequently rapid transfer to either the nucle- 

 olus or the cytoplasm (Goldstein and Micou, 1959; Taylor and Woods, 

 1959). 



The fate of nucleolar RNA is somewhat obscure as is its relationship 

 to that of the cytoplasmic RNA. Some studies, however, have indicated 

 that nuclear, and possibly nucleolar, RNA is transported to the cyto- 

 plasm presumably across the nuclear boundary (Goldstein and Plaut, 

 1955; Ficq, 1955; Zalokar, 1959). This idea is at least in accord with 

 the proposed mechanism of transfer of genetic information via an RNA 

 intermediate from the chromosome to cytoplasmic sites of action. Recent 

 labeling and enucleation studies of Prescott (1960) indicate that the 

 cytoplasm may be completely dependent on the nucleus for RNA 

 synthesis. 



Proteins 



The nucleus, like the cytoplasm, shows capacity for synthesis of 

 protein. The kinds of proteins produced as well as the sites of production 

 are somewhat controversial topics. Some workers, notably Brachet 

 (1957), have provided what seems to be sound evidence for involvement 

 of the nucleolus, but other work on other kinds of cells indicates that 

 there is little incorporation of labeled protein precursors into this nuclear 

 body (Carneiro and Leblond, 1959). Even Brachet's studies, as he 

 himself has pointed out, may be interpreted as indirect rather than direct 

 involvement of the nucleolus in nuclear protein synthesis. Carneiro and 

 Leblond (1959) have shown that labeled precursors of protein are 

 actively taken up in somatic cells of adult mice only by the chromosomes 

 and cytoplasm and that there is extensive and rapid turnover in both 

 sites. This suggests, but scarcely proves, that the chromosomes them- 

 selves represent the major sites of protein synthesis in the nucleus. 



It has been known since the days of Kossel and Meischer that the 

 cell nucleus contains basic protein complexed with nucleic acid. Modern 

 chemistry tends to classify basic proteins into two groups: (a) the 

 histories, and (b) the protamines. The distinction between these two is 

 not always clear-cut. In general, the former are more complex and have 

 a higher molecular weight than the latter, which are found only in sperm 

 nuclei. Both are rich in basic amino acids such as arginine and lysine 



STRUCTURE AND FUNCTION OF THE NUCLEUS / 109 



