IV. CELLULAR CONTROL OF DNA BIOSYNTHESIS 185 



Diiponol to obtain their DNA. The temperature at which lysis is carried 

 out and the m^racellular environment of the DNA at the time of lysis, 

 appear to be important in determining the quantity of "denatured DNA" 

 obtained. The evidence indicated that the intracellular environment is 

 detemiining the temperature at which denaturation of the cellular DNA 

 takes place. During the DNA cycle the amount of denatured material 

 isolated varies. Further studies in this and other systems should facili- 

 tate the investigation of control mechanisms in which primer require- 

 ments are involved. 



All of the evidence available indicates that in the process of replica- 

 tion the DNA molecule must change its state (splitting the molecule in 

 half). However, it is not certain that this does not occur as an automatic 

 result of the replication process instead of as a first step. From the 

 experiments cited, it would appear that the latter may be true. Such 

 a step may then represent an important controlling factor in the biosyn- 

 thesis of DNA. 



There is no evidence for the existence of a biological system capable 

 of converting native into primer DNA. It is tempting to assign such a 

 role to DNase and an increase in activity of this enzyme has been 

 shown shortly before DNA synthesis occurs in regenerating liver (Brody 

 and Balis, 1959). In vitro studies have shown that a brief treatment with 

 this enzyme results in strand separation measured as a large increase 

 in the reactivity of the nucleoside bases with formalin fSarkar and 

 Dounce, 1961; Sarkar, 1961). Moreover, DNA treated in this way 

 becomes active when tested in a biological system requiring denatured 

 DNA as primer (Sarkar and Dounce, 1961; Dounce et al., 1961; Mont- 

 savinos and Canellakis, 1961). Unfortunately, it appears that during 

 such treatment the molecule suffers several random scissions, a result 

 which appears to be incompatible with the lack of observed breakage in 

 the replication of the chromosome.^ It is, of course, possible that specific 

 DNase action (Laskowski, 1961) might cause non-random scissions com- 

 patible with the structural integrity of DNA. 



A more attractive possibility is suggested by the observations on 

 the conditions under which the "denatured DNA" is obtained from 

 cells. Changes in the intracellular environment of DNA may lower the 

 temperature of denaturation into the physiological range. When this 

 occurs, a portion of the DNA molecule would suddenly denature and 

 synthesis would be initiated. Such a hypothesis could easily explain the 

 timing of DNA synthesis and would account for the obsen^ed effects of 

 temperature upon the DNA cycle (see Section V.C below). 



In vitro studies would indicate that such an idea is feasible. Hama- 



^ DNA, in which scissions have occurred as a result of X-radiation in vivo, 

 undergoes degradation upon replication (Kas and DracuHc, 1961). 



