CHROMOSOME DUPLICATION AND GENETIC RECOMBINATION 191 



is paired with a complementary new strand composed of N ' ' nucleotides 

 from the medium. Thus all the replicated DNA is hybrid. At the next 

 replication, all the new strands are N^^, but so are half of the old 

 strands. At subsequent replications, each hybrid molecule gives rise to 

 one hybrid and one N'^ molecule. The original N^"^ strands maintain 

 their integrity and have been followed experimentally for four replications. 



The principal findings established by this experiment are; (1) DNA 

 consists of bipartite molecules, each containing two subunits; (2) after 

 each replication, the progeny DNA molecules are again bipartite, consist- 

 ing of one old and one new strand; and (3) each DNA subunit retains its 

 structural integrity for at least four replications and perhaps indefi- 

 nitely. DNA replication, then, consists of the doubling of duplex mole- 

 cules to produce two duplexes, each consisting of one old and one new 

 monoplex strand. 



What is the relation of the organized DNA of the intact cell to the 

 DNA molecules which, when extracted from E. coli, sediment in the 

 ultracentrifuge? E. coli contains about 6 x 10^ molecular weight of DNA 

 per bacterial nucleus; the extracted DNA is quite homogeneous in size, 

 with a molecular weight of about 7 to 8 x 10*^ as determined by the 

 cesium chloride method. On this basis, the extracted DNA consists pre- 

 sumably of about 1000 pieces. The genetic evidence based upon map- 

 ping of mutants indicates the presence of only one linkage group or 

 chromosome in E. coli, and therefore it seems likely that within the cell 

 the DNA is organized into a single structure and that it fragments upon 

 extraction. 



Chromosome duplication 



DNA replication has also been studied in a number of plants, by 

 methods in which the unit of observation was not the DNA molecule 



FIGURE 7.4. Ultraviolet absorption photographs showing DNA bands resulting from 

 density-gradient centrifugation of lysates of bacteria sampled at various times after 

 addition of excess N substrates to a grov/ing cultures of N '^-labeled cells. Regions 

 of equal density occupy the same horizontal position on each photograph. The genera- 

 tion times were estimated from measurements of bacterial growth. The densitometer 

 tracings shown in (b) are proportional to the DNA concentrations. The degree of 

 labeling of a species of DNA corresponds to the relative position of its band between 

 the bands of fully lab>eled and unlabeled DNA shown in the lowermost frame, which 

 serves as a density reference. A test of the conclusion that the DNA in the band of 

 intermediate density is just half-labeled is provided by the frame showing the mixture 

 of generations and 1 .9. The peak of intermediate density is centered at 50 ± 2 per 

 cent of the distance between the N'^ and N'' peaks (from Meselson and Stahl, 1958, 

 Proc. Natl. Acad. Sci., Wash., 44:671). 



