IV. CELLULAR CONTROL OF DNA BIOSYNTHESIS 183 



The dependence of i)rinier activity on the physical state of DNA may 

 be inferred from in vitro studies of the primer requirements of cell-free 

 polymerizing systems. The availability of primer for DNA duplication 

 may involve several factors: (a) Is it necessary for DNA to exist in a 

 special form in order for it to be duplicated? (5) Is DNA synthesis 

 compatible with simultaneous RNA synthesis, and if not, to what extent 

 are' these two processes incompatible? (c) Is DNA structurally stabilized 

 within the cell and is this a necessary part of DNA replication? If so, 

 factors needed for this stabilization may control its replication, (d) Is 

 the replication of DNA physically ordered, with respect to the macro- 

 molecule itself, with a beginning, a middle, and an end? If so, a control 

 mechanism may involve the appropriate conditions for initiation of 

 synthesis at one end of the molecule, maintenance of synthesis through- 

 out the molecule, and termination of synthesis once the end of the mole- 

 cule is reached. The onset and cessation of synthesis may also involve 

 the activation of information contained within the molecule itself. 



Little experimental evidence is available to answer these questions. 

 Thus, the following discussion will be largely speculative. 



A. IS IT NECESSARY FOR DNA TO EXIST IN A SPECIAL FORM 

 IN ORDER FOR IT TO BE DUPLICATED? 



In vitro studies of the deoxynucleotide triphosphate polymerase have 

 shown that enzymes isolated from a variety of sources require primer in 

 order to act (see Chapter I). The ability of DNA to function as a primer 

 in these reactions varies. In some systems native DNA appears to 

 function efficiently as a primer, whereas in others heated or denatured 

 DNA is required. 



It appears difficult, therefore, to reach a general decision from such 

 experiments as to whether the jiolymerizing reaction utilizes a special 

 form of DNA as primer. Certainly, in some systems, denatured DNA 

 ai:)pcars to be required. 



It has been shown that denaturation of DNA by heat involves strand 

 separation (Meselson and Stahl, 1958). Similar separation occurs as a 

 result of DNA duplication in that one-half of the parent DNA molecule 

 is found together with an equal amount of new material in a "hybrid," 

 semi-conserved, daughter molecule (Meselson and Stahl, 1958; Sueoka, 

 1960; Taylor et al., 1957; Forro and Wertheimer, 1960). Despite these 

 findings, the presence during the DNA cycle of a distinct molecule or 

 species, which could serve as primer DNA, has seldom been observed. 

 In one experiment (Cavalieri and Rosenberg, 1961c), it was possible to 

 isolate a special species of DNA shortly before DNA replication in a 

 culture of E. coli T in which discontinuous DNA synthesis had been 



