SUMMARIZING REMARKS 149 



size groups then is based on two-dimensional chromatography on 

 filter paper. 



In this manner, essentially the entire pyrimidine phosphorus 

 of a nucleic acid can be accounted for (see the examples in Table 

 33). With the amounts usually employed for hydrolysis (20 mg 

 of nucleic acid) the isolation of sequences longer than nine is 

 difficult; but there is no doubt that with more material and larger 

 columns the separation of even longer units will be possible, 

 especially if by the use of radioactive labels the sensitivity of the 

 detection of fractions could be extended. What will become, 

 however, increasingly harder or even impossible, is the separation 

 of very large clusters into individual components, e.g., the dis- 

 crimination between Tispie and T14CP16, etc., let alone the 

 preparation of the many sequential isomers of this type. 



One gains the impression that almost all possible combinations 

 of pyrimidine nucleotide units are encountered. But there is one 

 important, and possibly quite significant, exception. There seems 

 to exist a certain bias in the arrangement of homologous pyrimi- 

 dine clusters: many nucleic acids appear to contain much longer 

 all-thymine than all-cytosine units. This apparent trend may be 

 exemplified by the instance of the deoxyribonucleic acid of 

 E. coli which contains nearly equimolar quantities of thymine 

 and cytosine (compare Table 32). One should have expected 

 similar proportions of the stretches of varying length comprising 

 only one or the other of these pyrimidines. Actually, no cytosine 

 units longer than C3 have been observed in the hydrolysates, 

 whereas T4 and T5 units have invariably been found, together 

 with the various mixed fragments such as T3C, T2C2, TC3, T4C, 

 T3C2 and T2C3. In general, cytosine is found in a relatively 

 greater abundance among the smaller units; the converse is true 

 of thymine. 



These and related methods, employing specific enzymes, will 

 doubtless have to be refined considerably, before more definitive 

 answers to the sequence problem and its biological meaning 

 become possible. It would, for instance, be of the greatest interest 

 to determine the nature of the lethal sequence of a parasitic 



References p. 159 



