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protein from this material. Finally, recent work, reviewed in Chapter 23 

 dealing with the biosynthesis of purines, has shown that the purine ring is 

 preferentially sunthesized from simple precursors such as carbon dioxide, 

 formate, and glycine; it does not arise directly from arginine and histidine, 

 as was believed when Caspersson worked out his theory in 1940. 



It should also be recalled that in those days very little was known about 

 the existence of various cytoplasmic fractions, all of them containing PN A : 

 it is therefore not surprising that no reference is to be found, in Caspersson's. 

 theory, to microsomes, mitochondria, or cell sap PNA. 



The relationship existing between these various types of particles is the 

 first problem to be discussed here: we shall then consider the origin of 



Fig. 1. Diagrammatic view of the cytoplasmic protein-forming system. Nl: nucle- 

 olus; Chr: nucleolus-associated chromatin; M.n.: nuclear membrane; A.n.cyt.: cyto- 

 plasmic PNA; Prot. cyt.: cytoplasmic proteins. The arrows indicate the migration 

 of proteins from the heterochromatin, direct or via the nucleolus, to the nuclear mem- 

 brane (after Caspersson'"*). 



