somes at the subsequent metaphase will contain a labeled chromatid 

 (Hughes, 1958). Studies using tritiated thymidine as a means of follow- 

 ing DNA metabolism during chromosome replication indicate that this 

 label is built into the DNA of the chromosome as a part of a physical 

 entity that remains intact during succeeding replications (Hughes, 

 1958). The interphase chromosome before replication would be com- 

 posed of two such entities which are probably complementary to each 

 other. After replication of each entity to give a chromosome with four 

 entities, the whole chromosome then divides so that by metaphase, each 

 chromatid or daughter chromosome regularly receives an "original" and 

 a "new" entity. Whether or not the two physical entities of the interphase 



Figure 4-25. Diagrammatic Representation of the Structural Organiza- 

 tion and Mode of Replication of the Chromosome as Revealed by Labeling 

 with Tritiated Thymidine. The nonlabeled DNA subunits are shown as 

 solid lines, while the labeled subunits appear as dashed lines. The dots rep- 

 resent grains as seen in the autoradiographs. (From Taylor, J. H., Woods, 

 P. S., and Hughes, W. L., 1957. "The Organization and Duplication of Chro- 

 mosomes as Revealed by Autoradiographic Studies Using Tritium-labeled 

 Thymidine," Proc. Natl. Acad. Sci.. 43, Fig. 3, p. 125.) 



chromosome associated with the uptake of tritiated thymidine represent 

 the chromonemata is not known. 



While the evidence is fairly good that the major role of DNA is ge- 

 netic, studies of protein synthesis in isolated nuclei have suggested another 

 possible function, namely, as a cofactor in the nuclear aerobic synthesis 

 of ATP. Allfrey and Mirsky (1959) have shown that when a large 

 fraction of the DNA is removed from isolated nuclei there is a loss of 

 capacity for ATP synthesis, amino acid incorporation, and for RNA 

 synthesis. This loss can be reversed by addition of DNA from almost 

 any source, RNA, polyadenylic acid, and a number of nonnucleotides 

 such as heparin and chondroitin sulfate. These results have been ex- 

 plained on the basis that DNA facilitates ATP synthesis which, in turn, 

 is necessary for production of RNA and protein in the nucleus. 



STRUCTURE AND FUNCTION OF THE NUCLEUS / 107 



