262 NUCLEIC ACIDS AND GROWTH 3 



and in the best preparations of RNA (for instance RNA isolated from crystalline 

 tobacco mosaic virus), the native molecule is extremely elongated, almost like a 

 thread; the average molecular weight of the thymus DNA fibre is very large 

 (around 6,000,000) 



The chemical and physical data which have just been summarized, as well as 



Fig. I . Watson and Crick's structure for DNA. 



crystallographic evidence, have recently led Watson and Crick (1953a) to the idea 

 that the fundamental DNA unit contains two helically intertwined chains. As 

 seen in Fig. i, these chains are united by hydrogen bonds between pairs of bases in a 

 specific way : adenine with thymine and guanine with cytosine. The sequence of 

 the bases on one chain can be irregular: but whenever a certain nucleotide occurs 

 on one chain, its partner on the other chain is specifically determined. 



Such a structure is of considerable interest to biologists, because, as pointed out 

 by Watson and Crick (1953b) themselves, this "complementary" structure of 

 DNA readily suggests a self-duplicating mechanism: if the two chains unwind 

 and separate, each can serve as a template for the formation of its complement. 

 Later we will return to this important point when we discuss the biological role 

 of DNA; meanwhile let us note that, while the DNA structural model proposed by 

 Watson and Crick (1953a) fits in very well with a number of experimental facts, 

 it is still hypothetical. 



(c) Cytochemical techniques for detection 



Nucleic acids have aroused much interest among biologists because they are 

 unusually easy to detect in ordinary histological sections by cytochemical proce- 

 dures. These procedures can be based on the properties of any of the three main 

 constituents of nucleic acids, i.e. H3PO4, sugar and bases. 



A remarkable property of deoxyribose, which is not shared by ribose, is the 

 ability to give the SchifiF aldehyde reaction with fuchsin bleached with SOj. This 

 property is the basis of the extremely valuable Feulgen reaction for DNA (Feulgen 

 and Rossenbeck, 1924): the tissue sections are subjected to mild acid hydrolysis 

 in order to remove the DNA purines; the rest of the DNA molecule, which stays 

 at its initial site, stains strongly violet when the hydrolyzed section is dipped into 

 the Schiff reagent (Fig. 2, a). 



The presence of phosphoric acid in the nucleic acid molecule is of course responsi- 

 ble for their acidic character, and therefore their affinity for basic dyes. In the case 

 of tissue sections, the nucleic acids are thus basophilic and are easily stained with 



