biosynthesis of nucleic acids 387 



4. The Synthesis of Virus Nucleic Acids 



With certain bacteriophages the maintenance of the physical integrity of 

 the virus particle after lysis of the host cell permits the isolation of uniform 

 and reproducible samples of newly synthesized nucleoprotein, and many of 

 the known polynucleotide precursors have been applied in studies of virus 

 reproduction, which is discussed in detail in Chapter 26. Virus multiplica- 

 tion obviously involves a distortion of the normal metabolic characteristics 

 of the host, and alterations of enzyme^^^'^'^ and metabolic patterns*''''""^''^ 

 of infected bacteria or tissues have been demonstrated. Even though the 

 metabolic machinery of the host is reoriented in virus replication, the pro- 

 cesses are probably not so basically altered but that studies of the origin 

 of bacteriophage represent an important facet of the general picture of 

 nucleic acid biosynthesis. 



The synthesis of the viral DNA appears (Chapter 26) to involve the 

 assembly of relatively small units which may be obtained from suitable 

 available sources (medium, host, or infecting particles) or by synthesis 

 de novo. These units certainly include intact purines, pyrimidines, and poly- 

 nucleotide phosphorus, and circumstantial evidence also suggests that 

 mono- or possibly oligonucleotides may be involved. There can be extensive 

 utilization of moieties from the DNA of the host, but if there is any transfer 

 of specific nucleic acid fragments of considerable size, from either the infect- 

 ing particles or the host, it has been masked by the nonspecific transfer 

 which occurs. 



5. The Question of the Character of the "Active" 

 Derivatives of the Purines and Pyrimidines 



The evidence from the intact animal, from microorganisms, and from 

 viral replication thus suggests a participation of phosphorylated derivatives 

 in the assembly of polynucleotides. In addition the presence in the cyto- 

 plasm of egg cells of large amounts of deoxyribosyl derivatives^^^-^"' (pre- 

 sumably of purines^"^ and thymine^"^) also suggests that these are stores of 



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