84 



J. IIKRHKRT TAYLOR 



DUPLICATION 

 with Ihymidine-H^ 



1st DIVISION 



DUPLICATION 

 wi Ihout t hymidine-H ■ 



2nd METAPHASE 



Vrt 



1/2 breoks, reunion ond seporotion 



single 



complete breoks, exchange 

 and restricted reunion 



singles at 2nd metaphase 



connplete breaks, 



exchange and random reunion 



Fig. 5. DitiKnims sliowing the predicted results of chromosome reproduction and 

 sister chromatid exchange in which DNA was labeled during one replication with 

 thymidine-H^; the dashed lines represent labeled subunits and the chromosomes 

 shown in solid black represent regions showing grains in autoradiograms. The 

 unbroken lines represent unlabeled subunits and the chromosome regions shown in 

 outline represent regions without grains in the autoradiograms as shown in Fig. 2. 

 (a) The predicted results of exchanges involving only one subunit in each chromatid 

 (these were undetected in the experiments and perhaps do not occur). (6) The 

 predicted results of exchanges when the two subunits are restricted in their reunion 

 by a difference in sense analogous to the two chains of a DX.\ double helix. 



