54 R. MARKHAM 



and thymidine. Where ambiguity about the sugar may exist the deoxy- 

 riboside is specified as in deoxyadenosine, the term "adenosine" being taken 

 to imply riboadenosine. 



2. Nucleotides 



The phosphoric esters of the nucleosides are called nucleotides. It will be 

 seen from the structure of ribose and deoxyribose that several nucleotides 

 can exist. Ribose in the furanose form has two secondary alcoholic groups 

 (2 and 3), and one primary alcoholic group (5), and so can form three mono- 

 ester phosphates. In addition two diester phosphates can be formed, namely, 

 the 2 : 3 (cyclic) phosphate and the 3 : 5 (cyclic) phosphate. The latter is 

 purely of academic interest at the present time, as far as ribonucleic acid is 

 concerned. 



Deoxyribose nucleosides have only one primary and one secondary 

 alcoholic group, and so only form two monoester phosphates, namely, the 

 nucleoside 5'-phosphate and the nucleoside 3'-phosphate. They can also 

 form 3' : 5' (cyclic) phosphates. 



All of these compounds except two may be obtained from nucleic acids 

 during their degradation (the nucleoside 3' : 5'-phosphates have not been 

 encomitered so far). 



Nucleotides are usually named after their bases, for example, adenylic 

 acid, deoxycytidylic acid, and so on. This nomenclature is ambiguous because 

 of the possibility of several nucleotides existing, so that if the nucleotide 

 has to be specified more precisely, the nucleoside and its substituent are 

 both specified, as in deoxyguanosine 3'-phosphate. 



B. The Ribonvdeic Acids 



The nucleic acids which have been found in all those plant viruses which 

 have been analyzed so far are ribonucleic acids. The fact that the sugar is 

 ribose has been shown several times, usually by paper chromatography, which 

 eifectively separates all the pentoses. No unusual bases have been found, 

 so that the only nucleotides encoimtered have been adenylic acid, guanylic 

 acid, cytidylic acid, and uridylic acid. These nucleotides may be liberated 

 by enzymatic or chemical hydrolysis, and then may be analyzed by ion 

 exchange chromatography, or more rapidly on paper. Then the relative 

 quantities of the four compounds may be estimated by means of their 

 ultraviolet absorption (the molar extinction coefficients are of the order of 

 10*). Alternatively, the nucleic acids may be hydrolyzed to give adenine, 

 guanine, cytidylic acid, and uridylic acid (as well as ribose and phosphate, 

 which are not estimated) which may be separated readily on paper chromato- 

 grams. Such analyses may be made on much less than one milligram of 



