(1953). Essentially what these workers found was that by exposing cells 

 in interphase to tritiated thymidine for short intervals, it was possible to 

 obtain chromosomes at metaphase which had both of their daughter 

 chromatids labeled. By allowing such cells to go through a second divi- 

 sion in the absence of the label, it was discovered that when they again 

 reached metaphase the chromosomes had only one of their chromatids 



it 4 





(a) (b) 



Figure 4-24. (a) Autoradiograph of Chromosomes of Vicia faba at the 

 First Mitotic Division After Duplication in the Presence of Tritiated Thymi- 

 dine. The isotope is equally distributed between both chromatids (daughter 

 chromosomes), (b) Autoradiograph of Chromosomes of V. faba at the Sec- 

 ond Mitotic Division After Labeling by One Duplication with Tritiated 

 Thymidine. The isotope is confined to only one daughter chromosome of each 

 sister pair. (From Woods, P. S. and Schairer, M. V., 1959. "Distribution of 

 Newly Synthesized Deoxyribonucleic Acid in Dividing Chromosomes," 

 Nature, 183, Fig. 1. p. 303. Courtesy of Dr. Philip S. Woods, Brookhaven 

 National Laboratory.) 



labeled. The results of this study are diagrammed in Figure 4-25. The 

 sequence begins with two complementary nonlabeled strands in a chro- 

 mosome, each of which separates and forms a complementary labeled 

 strand along its length in the presence of tritiated thymidine. In the sub- 

 sequent metaphase, each chromosome will consist of two chromatids, 

 both of which carry a labeled and a nonlabeled strand. Following repli- 

 cation during the next interphase in the absence of the labeled thymidine, 

 each chromosome at metaphase will show a nonlabeled and a labeled 

 chromatid. If a third replication occurs, only one half of the chromo- 



106 / CHAPTER 4 



