150 NUCLEOTIDE SEQUENCE IN DEOXYRIBONUCLEIC ACIDS 



deoxyribonucleic acid that enables it to impose itself upon, and 

 to paralyze, as it were, the bacterial host. 



c. What is meant by ''replacement"? 



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

 true — certainly cannot hold for the deoxyribonucleic 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 pyrimidine, not normally occurring in 

 nature, is, when offered to the cell, incorporated into a deoxyribo- 

 or ribonucleic acid "in the place" of a particular natural con- 

 stituent of the nucleic acid. Such statements are, of course, 

 derived from the existence of the pairing principles^, 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 30 with 

 respect to cytosine and 5-methylcytosine and in Table 32 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? (1) It means that Y is incorporated into the 

 deoxyribonucleic acid chain and that the cell, hence, must pos- 

 sess, or acquire, the enzymic apparatus necessary for the produc- 

 tion and utihzation of the derivative of Y that is the direct 

 precursor of the polynucleotide chain. (2) If X and Y share the 

 same functional group {e.g., 6-keto or 6-amino) through which 

 alone the selection of the members of the newly forming chain 

 takes place — as foretold by a mechanism postulating the replica- 

 tion of the complementary halves of a double strand^^ — it follows 

 that every X molecule along the chain has an equal chance of 

 being "replaced" by a Y molecule. 



d. Selection of a natural analogue 



When we deal with a natural analogue, such as the 5-methyl 

 derivative of cytosine, it is, of course, meaningless to speak of the 

 replacement of one by the other. Such a statement acquires 



