II. REPLICATION OF DNA IN CHROMOSOMES 85 



very nearly the predicted one, and furthermore, the absolute frequency 

 of exchanges decreased in the first interj:)hase as colchicine was added 

 at earlier times. The frequency of exchanges at the second interphase 

 remained nearly constant in all tliree experiments. 



TABLE I 

 Frequency of Single and Twin Exchanges in the 

 Tetraploid Chromosome Sets of Bellevalia romana'^ 



" Data for only the largest chromosome are given. 



'' Data for the first two experiments are from Taylor (1958a, b). 



The conclusion reached ^vas that the t\vo subunits of a chromatid 

 are unlike. In addition, the exchanges appear to occur during DNA 

 replication, for colchicine must be applied at that time in order to get 

 the maximum effect on their frequency. While these results strongly 

 suggest a chromosome model in which the subunit is a single polynucleo- 

 tide chain of the DNA double helix, they do not exclude models in which 

 the axis of the chromosome is a tandemly linked sequence of DNA 

 helices (Taylor, 1959a,b). 



Sister chromatid exchanges have been observed in all species analyzed 

 to date. These include several genera of plants, Vicia, Bellevalia, Crepis, 

 Allium, two mammalian species, Chinese hamster, and human cells in 

 culture, and an insect, the grasshopper (Taylor, 1958a,b; 1959a; 1960b; 

 and unpublished). None of these except Bellevalia have been analyzed 

 for the occurrence of twin exchanges. From what is known of the survival 

 of ring chromosomes in the Diptera, it appears that sister chromatid 

 exchanges may be absent or occur at a very low frequency in this group. 



(Exchanges are assumed to occur with an efjual frequency during the first duplica- 

 tion with thymidine-H'' and during the second duplication without thymidine-H^. 

 All exchanges in the first duplication produce twins and the frequency of single 

 exchanges to twin pairs is 2:1; see text for explanation.) (c) The predicted 

 frequency of single and twin exchanges if the two subunits reunite at random. 

 (Since single exchanges could result from events at both the first and second 

 duphcation, the frequency of single exchanges would be higher, at least 10:1.) 

 (From Taylor, 1958a.) 



