190 KARL (!. LARK 



fert'utiation is to ])lay a coiilrolliii'i; role in DNA hiosyntlicsis then the 

 availability of thi.s information must be under some on-off type of 

 control, similar to the oonti-ol of operator <j;enes by oytoplasmie repressors 

 (see Jacob and Monod, 19611. However, a siniilai' t>|)c of cytoplasmic 

 repressor would not appear to be involved, since conditions leading to 

 the accumulation of RNA, DNA, or i)roteiii within tlic cell do not affect 

 the timing of DNA synthesis (Maruyania and Lark. I9()li. whicli is 

 also independent of tlu^ synthesis or lack of synthesis of DNA itself 

 (Lark, 1960). 



Guttes and Guttes (1961) have suggested that the nucleolus may 

 play the role of an "oiierator" since its synthesis is initiated simul- 

 taneously with that of DNA. 



V. Other Factors Affecting DNA Biosynthesis and the DNA Cycle 



A. THE LOCALIZATION OF DNA SYNTHESI.S 



The in vivo synthesis of DNA depends upon its environment within 

 the cell. Thus, fragments of DNA (resulting from radiation damage) 

 which are left in the cytoplasm as micronuclei can no longer serve as 

 foci for DNA synthesis although the parent nucleus still does (Das and 

 Alfert, 1961b). In ciliates different nuclei such as the macronucleus and 

 the micromicleus have been found to follow different patterns of DNA 

 synthesis, the micronucleus entering its period of DNA synthesis when 

 the macronucleus has finished and when the cell is beginning to divide 

 (McDonald, 1962; Prescott ef al, 1962). 



These data would indicate that the nucleus represents a unique 

 environment for DNA .synthesis; and it has been shown that specific 

 DNA precursors such as thymidine, when taken into the cell from the 

 environment, will rapidly apjiear in their free form, or as nucleotides 

 (thymidine mono-, di-, or triphosiihate), in the nucleus, but are not 

 found in the cytoplasm (Crathorn and Shooter, 1960). Howe\-ci'. good 

 evidence has been accumulated demonstrating that the deoxyribonucleo- 

 tide polymerase is found in the cytoplasm, but 7wt in the nucleus of 

 mammalian cells (Smellie of a/.. 1959; Smellie and Eason, 1961; Prescott 

 et al., 1962a). In l)ringing together all of these components refjuired 

 for the synthesis of DNA, a controlling factor may be the permeability 

 of the nuclear membrane. 



A further factor may be the association within th(> nucleus of enzyme, 

 substrate, and primer. Billen 1 1962b) has recently demonstrated that in 

 E. coli the polymerase is complexed witli the DNA. It is not known 

 whether the formation of this complex is a controlling factor in DNA 



