11. REPLICATION OF DNA IN CHROMOSOMES SI 



merits on semi-conservative replication which indicate separation of the 

 two complementary chains of a double helix comes from a series of 

 papers by Cavalieri and Rosenberg (1961a,b,c). These authors main- 

 tain, primarily on the basis of the kinetics of molecular weight reduction 

 by enzymes, that DNA extracted from rapidly growing cells is four- 

 stranded. In particular their results indicate that the particles of DNA 

 isolated from E. coli such as those used by Meselson and Stahl (1958) 

 are composed of paired double helices held together by hyj^thetical 

 biunial bonds. According to their interpretation these are the bonds 

 broken by heating and the separation of hybrid DNA observed by 

 Meselson and Stahl involved not the unwinding of the polynucleotide 

 chains, but the separation of a heavy Watson-Crick double helix from 

 a double helix of lower density. Later Hall and Cavalieri (1961) pub- 

 lished electron micrographs of E. coli DNA which indicated that at least 

 some particles of DNA consisted of paired double helices. In addition, 

 the mass per unit length of the particles, calculated from measurements 

 in the electron micrograjihs and molecular weights determined by the 

 light-scattering technique, indicated that the particles contained between 

 3 and 4 polynucleotide chains. On the other hand, there are a number of 

 experiments which clearly indicate that agents known to break hydrogen 

 bonds cause the separation of the strands of DNA particles. Among 

 these are heat, low or high hydrogen ion concentration, and agents that 

 compete for hydrogen-bonding sites or destroy the water layer of 

 hydration, such as urea and formamide (Doty, 1960; Marmur and Doty, 

 1961; and Schildkraut et al., 1961). The separated strands will recom- 

 bine if the population of molecules is not too heterogeneous. In addition, 

 hybrid molecules can be produced by heating and slowly cooling DNA's 

 with different isotopic densities. 



Whatever may prove to be the significance of these conflicting inter- 

 pretations of experiments on strandedness of different DNA samples, the 

 Watson-Crick scheme for replication, in which specific pairing of com- 

 plementary bases is the principal role of the template, is likely to 

 dominate our thinking and the design of experiments in the foreseeable 

 future (see Chapters III, IV, and V). The semi-conservative mode of 

 replication is likewise not a trivial quirk of nature. A phenomenon so 

 universal would not be preserved in evolution so consistently unless it 

 played a fundamental role in reproduction. The only basis on which it 

 makes any sense is the separation of complementary units as a part of 

 the process of replication. One of the most puzzling jiroblems of chro- 

 mosome reproduction as stated earlier is the packaging and sorting of 

 the tremendously long chromosomal strands if the axial element is a 

 single DNA helix. A model that folds and forms temporary H linkers 



