DEOXYPENTOSE NUCLEIC ACIDS 53 



d. Molecular structure 



Deoxypentose nucleic acids are polynucleotide chains that are 

 held together by phosphoric acid diester bridges between the 

 deoxypentose molecules. Our knowledge of the sugar found in 

 different preparations rests mostly on chromatographic evidence 

 which has up to now yielded no indication of the presence of 

 any but 2-deoxyribose^. The position of the phosphate bridges 

 has been studied in some detail^^ only in the deoxyribonucleic 

 acid of calf thymus in which it appears to be 3', 5'. No deter- 

 minations of end groups have as yet been possible nor has the 

 existence of a terminal phosphate group been demonstrated. 

 Strictly speaking, it cannot yet be asserted that the deoxypentose 

 nucleic acids occur as open and not as closed chains. It is, 

 however, probably too early to worry about the tail before we 

 can describe the animal. 



The excellent X-ray photographs obtained at King's Col- 

 \QgQ27, 28 ^jj^ ^jjg discovery of the general unity relationships men- 

 tioned before have formed the basis of an interesting structural 

 proposal^®' ^^. This model, which in the recent past has been 

 unveiled repeatedly and sometimes very amusingly*, assumes a 

 heUcal dyad in which two coiled strands are held together by a 

 specific pairing of the nitrogenous constituents showing the unity 

 relationships mentioned before. This conception has several at- 

 tractive features; it makes good use of some of the experimentally 

 established facts and is, on the whole, not incompatible with the 

 X-ray^2 and part of the chemical evidence. But between what is 

 plausible and what is true in the natural sciences there lies an 

 ocean which has seldom been crossed. 



The invention of models is an essentially sterile undertaking 

 unless two things are thereby accomplished: (a) a satisfying ex- 

 planation of all known and sometimes seemingly contradictory 



* "This pairing is likely to be so fundamental for biology that I can- 

 not help wondering whether some day an enthusiastic scientist will 

 christen his newborn twins Adenine and Thymine!" (Ref. 31). No doubt, 

 this too will happen; but is "enthusiastic" the correct epithet? 



References p. 60 



