110 



KERATIN AND KERATINIZ ATION 



the nucleic acid is probably combined with basic proteins to form a 

 nucleoprotein, probably also a helix with the polypeptide chain closely 

 linked to the DNA helix (Wilkins et al, 1959). 



Except in very general outline it must be admitted that nucleo-cyto- 

 plasmic relations remain very obscure. A current hypothesis is illustrated 



Fig. 49. Principle cytological features of a cell synthesizing protein for 

 secretion. N, nucleus; Nu, nucleolus; m, mitochondrion; R, reticulum 

 (basophilic); P, an RNA-containing particle; G, Golgi apparatus; S, 

 secretory granule. The arrows A and B symbolize the exchange of 

 control between cell and environment. 



in Fig. 49. Working copies in the form of RNA molecules are formed from 

 segments of the DNA molecules (master copies) of the chromosomes. The 

 gene is here pictured as a copiable segment of the DNA thread. These 

 RNA molecules fold up with protein into corpuscles and accumulate first 

 as a nucleolus (perhaps in this phase some nuclear synthesis occurs): they 

 then pass through the nuclear membrane either to join with a cytoplasmic 

 membrane to form part of the basophilic reticulum of a secretory cell 



