STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 195 



place during prophase prior to cell division. Thus, the twisted strips can be 

 separated from each other by winding the hehcal strips about an axis per- 

 pendicular to the axis of the heHx, as presented in Fig. 29. 



One consequence of such a mechanism is the suggestion that during separ- 

 ation of daughter chromosomes newly synthesized DNA goes to one chromo- 

 some and the original to the other. The absence of mixing, indeed, has been 

 demonstrated by the experiments of Meselson and Staid. However, an 

 astonishing set of observations has revealed that in the division process all 

 of the newly synthesized DNA of a chromosome enters one chromosome and 

 is separated from the parental DNA in another. Such a result has been 

 obtained by Taylor et al. (1957), who labeled the DNA of bean root by growth 

 in tritium-labeled thymidine and followed the fate of the chromosomes by 

 autoradiography after subsequent growth in unlabeled media. Thus, it is 

 clear that chromosome structure involves functional association of the 

 separate nucleoproteins in such a way as possibly to extend the Bloch 

 molecular mechanism to the replication and division of the entire 

 organelle. 



Kacser (1956) has suggested a mechanism for the rephcation of nucleo- 

 protein in cliromosomes, in which parental protein is combined with new 

 nucleic acid and parental DNA is combined with new protein. A double- 

 labeling experiment, performed as described by Taylor, should be able to 

 test this hypothesis. 



One possible role of the basic protein in chromosomes has been considered 

 to be that of neutralizmg the charge of the polyanion nucleic acid. Anderson 

 has discussed this question in great detail (Anderson, 1956a,b), suggesting 

 that the division mechanism arose initially as a consequence of the cycHc 

 changes in the rate of formation of nucleic acids and proteins. He has pro- 

 posed that the fmidamental mechanism of cell division is a consequence of the 

 cyclic changes m the ratio of charges on cellular colloids and has shown that 

 much of the data on division is compatible with this view. Of mterest from 

 this point of view are the well-known effects on division and nuclear structure 

 of a variety of polycations, such as protamine, spermidine, etc., and poly- 

 anions, such as heparin. Of great importance in this connection is the de- 

 veloping concept in virology of the synthesis of independent pools of viral 

 polymers, as in the proposal of the independent duplication of phage DNA 

 by Hershey and Melechen (1957) and of tobacco mosaic virus E,NA and pro- 

 tein. In the latter case, a mechanism of interdigitating polymerized protein 

 and polymeric RNA has clearly been developed; however, comparable 

 mechanisms of terminal organization are not yet known in other systems. 

 The normal roles of basic compounds, such as putrescine, spermine, etc., in 

 cell division and virus multiplication might well be explored from this point 

 of view. 



