142 CHAHLKS A. THOMAS, .IK. 



A. rvToi.ocv or nACTERiA 



The cxccllcut i-c\-ir\v l)y KcllciilKiLici' ( 19()()l has coNcicd this subject 

 in (li'lail, and only sonic niajoi- coiichisions will he considcicd here. He 

 concludes that in bacteria the I)X.\ is ()r<4anize(| into well-defined areas 

 calieil iHicleaf x-acuoles which are x'aiiahlc in shape ami do not appear 

 to have a nicinhi'anc. These vacuoles aic lilled with a fibrillar material 

 about 20-60 A in di.ainetci-; the dense chromosomes which are sometimes 

 seen are thought to be a coagulation artifact. As yet, electron micro- 

 graphs are not able to reveal anything about the number of DNA mole- 

 cules in the fibiils, or anything about the linkages between DNA 

 molecules, although some recent work in this area appears promising 

 (Kleinschmidt, Oehatia, and Zahn, 1960). 



1^. OROWTH AND TRAN.SFER EXPERIMENTS 



Following the experiments of Meselson and Stahl (1958a,b) which 

 demonstrated that the rei)lication of E. roli DNA took place by the 

 formation of a structural unit which was one-half old and one-half 

 newly synthesized material, subsequent work has been aimed at (/) dis- 

 cerning the largest structure which will replicate by this semi-conserva- 

 tive fashion, and (3) identifying precisely the structure of the DNA 

 molecule which contains equal amounts of old and new material. The 

 first of these topics has been investigated by Forro and Wertheimer 

 (1960) by tritium autoradiography. His experiments have been consid- 

 ered in Chapter II. For the purposes of this discussion the salient point 

 is that by using young, small cells of E. coli 15T", the DNA seems to be 

 synthesized as though the bacteria contained only tii'o DNA-containing 

 structures which can survive many rounds of re|)lication. These findings 

 have been verified by Van Tubergen and Setlow (19611 using tritium 

 autoradiography. Exponinitially growing cultures of coli which probably 

 have two nuclear vacuoles were found to have foi<r structures which 

 were conserved during further replication. This would be just as expected 

 if each nuclear vacuole contained a single long DNA molecule, or a 

 molecule punctuated by a special non-DNA unit ("linkers") at various 

 points (Kellcnl)erger, 1960; Frecse, 1958). Thus, while the details of the 

 structure of such a unit are still unknown, one may consider the "nuclear 

 vacuole" as seen in electron micrograi)hs to consist of two DNA-con- 

 taining units which may (but rigorous proof is still lacking) be the two 

 polynucleotide chains of a very long DNA molecule. 



Further evidence on this point has come from the study of an E. coli 

 which simultaneously recpiires thymine, arginine, and uracil (Maal0e 

 and Hanawalt, 1961 ; llanawalt ct a/., 1961). The .starting point for tlie.se 



