CHROMOSOME DUPLICATION AND GENETIC RECOMBINATION 193 



concentration of tritiated thymidine for a time interval such that at 

 most one cycle of DNA duplication had occurred in some cells. Then 

 the material was mounted and autoradiographs were prepared. As 

 shown in Figure 7.5, uniform labeling of the two daughter chromosomes 

 was observed after one cycle of duplication. In the next experiment, 

 cells were allowed to undergo two successive divisions, one in the 

 presence of labeled thymidine and one in its absence, and the progeny 

 chromosomes were kept together by the use of colchicine. At the sec- 

 ond division, segregation of labeled and unlabeled strands occurred, as 

 diagrammed in Figure 7.5. When chromosomes were allowed to un- 

 dergo a third division before observation, one labeled strand was still 

 found as well as seven unlabeled ones, representing the eight progeny 

 of an individual chromosome labeled before its first division. 



These results demonstrate that the chromosome is (at least) double, 

 consisting of two DNA subunits, both of which are replicated and then 

 segregated from one another at the first division, resulting in equal 

 labeling of the daughter chromosomes which are themselves double. 

 Their doubleness becomes apparent at the second division when the old 

 unlabeled and new labeled subunits segregate from each other. 



This experiment also shows that each subunit remains intact in the repli- 

 cation process, so that an old and a new subunit can be distinguished. 

 This is extremely important, for it represents direct evidence that the 

 linear structural organization of the many DNA molecules in the chromo- 

 some is kept intact during replication (except for sister-strand exchange; 

 see below). 



The N'^DNA experiment beautifully complements this one. Both 

 systems provide evidence of the same mechanism of DNA replication: 

 namelv, the doubling of a duplex structure in such a way that two 

 duplexes are produced, each containing one old and one new monoplex. 



In the plant chromosome experiments, additional information was ob- 

 tained about the organization of the DNA, because observations were 

 made of DNA segregation in intact chromosomes. It was noted that the 

 linear organization of the DNA does not always remain intact for the 

 entire length of the chromosome. Occasionally a chromosome is labeled 

 for only part of its length, and the sister chromosome is labeled recipro- 

 cally. In these instances, each pair of chromosomes has the equivalent 

 of one labeled and one unlabeled strand, but reciprocal exchanges have 

 occurred between sister half-chromatids. These exchanges may be a 

 regular feature of chromosome replication, occurring in the plant Bella- 

 valia with an average frequency of once per chromosome per replication. 

 As yet nothing is known of the mechanism of the exchange, or even 

 whether it is an essential concomitant of the replication process. 



