194 



CELL HEREDITY 



Original 



unlabeM Duplication witti 

 chromosome thymidine-H ^ 



(a) Z 



1st Division 



Duplication without 

 thymidine-H 3 2nd Division 



(b) 



Break 



Single exchange 



FIGURE 7.6. Interpretation of sister-chromatid exchanges founcJ in studies of chromo- 

 some duplication with thymidine-H (from Taylor, 1958, Genetics, 43:515). 



(o). The type of chromosome configuration resulting from a half-chromatid break, 

 namely, an unpaired segment with loss of label in one of the two daughter chromo- 

 somes, is not found. 



(b). Reciprocal exchanges are found either in two (so-called single exchanges) or in 

 four (twin exchanges) of the daughter chromatids after two duplications. The pattern 

 of reciprocal exchanges found is evidence of polarity in the rejoining of broken ends 

 after exchange, with half-chromatids (equivalent perhaps to a single DNA strand of the 

 double helix) only rejoining with those of the same polarity. 



These exchanges are also of interest because they permit a test to be 

 made of whether or not the subunits are identical. If exchanges involved 

 only two of the four strands, one new with one old, then patterns like 

 that shown in Figure 1.6a should be found, but in fact they are not. 

 From the labeling patterns which are found, it is concluded that ex- 

 changes involve all four strands, as shown in Figure 1.6b. One may 

 then ask: Do exchanges among the four strands occur at random as would 

 be expected if the strands were identical? If so, the expected frequencies 

 of single and twin exchanges after the second replication would be about 

 10:1. But the ratio of frequencies actually found is about 2:1. How 

 can this be explained:* 



The model which best fits these data is one which considers the pair 

 of DNA subunits as complementary, not as identical strands, and limits 



