CHAPTER III 



Nucleic Acids and Protein Synthesis 



A. THE POSITION OF DNA IN PROTEIN 

 SYNTHESIS 



For reasons which have been considered in Chapter I of the present 

 book, it is almost certain that DNA ultimately controls the primary struc- 

 ture of the individual proteins. A question then arises — are the amino 

 acids arranged in the correct sequence under the immediate action of the 

 gene, is specific DNA the protein-forming system? 



At the present time it is convenient to examine separately the data 

 obtained with higher organisms and with bacteria. 



1. Higher Organisms 



In animal or plant tissue, asking whether DNA is directly and 'person- 

 ally' involved in assembling the proteins amounts to asking whether all 

 proteins are made in the chromatin. From what we have seen in the pre- 

 ceding chapters, the answer to this question is definitely negative. Complete 

 proteins endowed with their normal biochemical properties or enzymic 

 activity can be made in the absence of the nucleus, e.g. in isolated pieces of 

 cytoplasm or in isolated mitochondria, which contain no DNA. Most 

 cytoplasmic proteins are made in the cytoplasm, and DNA is not a consti- 

 tuent of the system which is immediately involved in their making. 



The synthesis of nuclear proteins must be examined more closely. 

 Fractionation of isolated nuclei after incorporation of amino acids indi- 

 cated that the rate of labelling differs considerably among the various 

 nuclear proteins. Histone is almost completely inert, whereas two protein 

 fractions which are found in close association with DNA and with RNA 

 respectively incorporate amino acids very rapidly. 



In vitro, the incorporation is inhibited and finally suppressed in the 

 presence of deoxyribonuclease, which destroys DNA. The degree of 

 impairment becomes greater as more and more of the DNA is depolymer- 

 ized and removed from the nucleus. However, the amino acid incorporating 

 ability is not irreversibly destroyed by treatment with deoxyribonuclease, 

 for it can be largely restored by the addition of a DNA supplement (AUfrey 

 et al, 1955 ; Mirsky et. al, 1956). 



These very striking results at first were taken as direct evidence that 



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