76 .1. IIIlKHl.Hr TAM.UU 



thymidine, the labeled ciiroiiiosoniet> pi'oduced one lahdeil and one un- 

 labeled descendant- (Fig. 2B). This is the result predielfd for senii- 

 conservative icplieation of DNA (Fig. 2C). The examination of other 

 species of plants, Hcllcvcdia (Taylor, 1958a), Crepi-s (Taylor, 1958b), 

 and Allium (Gall, 1958a) confirmed the regular semi-conservative be- 

 havior at the chromosomal level. The fii'st exi)eriments were monitored 

 by the use of colchicine. It was used to accumulate cells at division and 

 to identify the cells which had gone through two cycles. By placing roots 

 in colchicine after labeling and leaving them in this solution until labeled 

 tetraploid cells appeared, we could identify chromosomes which had gone 

 through one cycle of duplication after the one in which they were 

 labeled. These cells regularly showed one labeled chromatid ]:)aire(l with 

 one unlabeled chromatid (Fig. 2B). Chromatid exchanges between the 

 labeled and the unlabeled chromatids tended to obscure the picture, but 

 these later proved of value in analyzing the organization of chromatids 

 (see below). 



A report by LaCour and Pelc (1958) criticized the use of colchicine 

 and reported that it had an effect on the segregation of the labeled DNA 

 in Vicia chromosomes. However, a rc-cxamination of the problem by 

 Woods and Schaircr (1959) failed to show any effect of colchicine. In 

 addition the studies on Bellevalia and Crepis (Taylor, 1958a,b) wei'e 

 carried out with and without colchicine. The results with respect to 

 segregation of labeled DNA were the same in both instances. Rare 

 instances of apparent equal segregation of labeled DNA after the second 

 replication in colchicine-treated cells were reported by Taylor (1958a). 

 These could result from exchanges among the four chromatids which 

 were the descendants of a pair of labeled chromatids. Colchicine, which 

 probably keeps such pairs together, would be expected to increase the 

 frequency of this event. In addition an unequal labeling of sister chro- 

 matids may appear at the first division in the first cells to reach division. 

 Usually these have relatively less radioactivity than the majority of 

 labeled division figures. With a low number of grains the significance 

 of any apparent asymmetiy of labeling in sister chromatids is difficult 

 to assess. Recent studies (Taylor, unpublished) with thymidine-H'' of 

 higher specific activity than was available in early experiments reveal 

 what appears to be a significant asymmetry in the labeling of sister 

 chromatids. This is interpreted as evidence for the out-of-phase replica- 

 tion of the two subunits of a chromatid. 



Examination of two manmialian species (human and Chinese ham- 

 ster) and an insect (grasshopper) has revealed the semi-conservative 

 distribution of tntium-labeled DNA (Taylor, 1960b and unpublished). 



Forro and A\'ertheimer (1960) have also carried out extensive studies 



