84 ERWIN CHARGAFF 



unlikely to exhibit uniqueness of quality and will, therefore, not be sufficient 

 to define a sequence unambiguously. Uniqueness may, however, be 

 expected, at least occasionally, when very large clusters can be included 

 in the survey. We have made a beginning with regard to clusters of 

 pyrimidine nucleotides [46]. When a deoxyribonucleic acid is cleaved by 

 acid under moderate conditions (o-i M H0SO4, 30 min., 100°) and the 

 fragments are fractionated on DEAE-cellulose by means of a lithium 

 chloride gradient, the phosphorylated pyrimidine oligonucleotides can be 

 separated in order of increasing chain length. Further separation within 

 the different size groups then is based on two-dimensional chromato- 

 graphy on filter paper. More than 97" o of the nucleic acid can be accounted 

 for. Oligonucleotides of more than five pyrimidines in a row are rare, but 

 occasionally perceptible. Almost all possible combinations of pyrimidine 

 nucleotide units are encountered, among them di-, tri-, and tetrathymidylic 

 acids, all carrying an additional terminal phosphate group at 3'. We are at 

 present engaged in extending the sensitivity of the method by the use of 

 radioactive labels. 



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 deoxyribonucleic acid that enables it to 

 impose itself upon, and to paralyze, as it were, the bacterial host. 



WHAT IS MEANT BY "REPLACEMENT"? 



The platitude that "nobody is irreplaceable" — if it ever holds true — 

 certainly cannot hold for the deoxvribonucleic acids, if they carry the 

 biological functions ascribed to them currently. Nevertheless, one often 

 finds the statement in the literature that an analogue of a purine or pyrimi- 

 dine, not normally occurring in nature, is, when offered to the cell, in- 

 corporated into a deoxyribo- or ribonucleic acid "in the place" of a 

 particular natural constituent of the nucleic acid. Such statements are, of 

 course, derived from the existence of the complementariness principles [i] ; 

 i.e. they are based on the fact that certain nucleic acid constituents are 

 pleromers, to use the nomenclature suggested above. Good examples of 

 such pleromerism can be seen in Table III with respect to cytosine and 5- 

 methylcytosine and in Table V with respect to thymine and 5-bromouracil ; 

 a more comprehensive selection of such instances will be given at the end 

 of this paper. 



What does the assertion that X can be " replaced " in part by Y actually 

 signify.? (i) It means that Y is incorporated into the deoxyribonucleic 

 acid chain and that the cell, hence, must possess, or acquire, the enzymic 



