188 KARL G. LARK 



DNA conipU'xcs witli the concurrent release of the DNA molecule, 

 which if unstable may then untlei-<z;o change. 



C. IS DNA STRUCTURALLY STABILIZED WITHIN THE CELL AND IS THIS A 

 NECESSARY PART OF DNA REPLICATION? 



^luch evidence has accumulated to demonstrate that DNA exists 

 within the cell in the form of nucleoprotein (Bloch, 1958). We have 

 already discussed the evidence which exists for the role of protein in 

 controlling DNA synthesis. De (1961) has shown that histone is synthe- 

 sized sinmltaneously with DNA in the chromatin of interphase plant 

 cells. Additional protein and histone synthesis was demonstrated in the 

 nucleolus during Go, following DNA rei)lication (see also Woodard et al., 

 1961). In the studies cited above with Euplotes, it was noted that the 

 synthesis of DNA and of protein or histone (incorporation of thymidine 

 and of histidine) were extremely active in the same region of the synthe- 

 sizing band. This region of active synthesis, however, was extremely low 

 in its content of both DNA and protein. The region of the band immedi- 

 ately adjacent contained large amounts of both. It thus would appear 

 that the synthesis of both DNA and protein proceed simultaneously. An 

 interesting point in this connection is that this protein can apparently 

 be synthesized in the absence of RNA, since this region of the macro- 

 nucleus is devoid of this nucleic acid. Thus, this nuclear protein may be 

 synthesized by a special mechanism which may, as pointed out earlier, 

 be chloramphenicol-insensitivc (Nathans and Lipmann, 1961). The role 

 of this protein is unknown. However, it has been suggested that such 

 protein may serve to hold several DNA molecules together, linking them 

 into a chromatid (Dounce, 1959; Kellenberger, 1960). This might 

 account for the marked instability of such large chromatids which, al- 

 though they can be shown to exist within the cell, break down, upon 

 extraction, into smaller pieces (Burgi and Hershey, 1961). The molecular 

 weight of the latter suggests a frequent and uniform occurrence within 

 the chromatid of weak links. It has been suggested that the synthesis of 

 DNA may be controlled by the rate of synthesis of these "linkers" 

 (Kellenberger, 1960). 



D. IS THE REPLICATION OF DNA PHYSICALLY ORDERED, WITH RESPECT TO THE 

 MACROMOLECULE ITSELF, WITH A BEGINNING, A MIDDLE, AND AN END? 



No evidence exists to indicate whether the DNA molecule, the 

 chromatid, or portions of the latter are synthesized in a linear progres- 

 sion or not. The evidence for a preferentially timed synthesis of individ- 

 ual chromosomes has been discussed in a previous chapter (Chapter II). 

 Exjieriments on density labeled chromatin indicate that the proportion 



