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CELL HEREDITY 



FIGURE 7.5. Chromosome duplication experiment (after Taylor, Woods, and Hughes, 

 1957, Proc. Natl. Acad. So. Wos/i., 43: 122). 



1. Chromosome before duplication; seen as one strand under the microscope, it is 

 revealed to be bipartite in this experiment. 



2. Chromosome after duplication in presence of radioactive thymidine-H' . 



3. Appearance of metaphase chromosomes, showing uniform labeling. 



4. Separation of the uniformly labeled chromosomes. 



5. Duplication in the absence of radioactive thymidine-H '. 



6. Duplicated chromosomes at the next metaphase: one chromosome is radioactive, 

 the other is not. 



Diagrams 2, 4, and 5 provide interpretation of observations shown in 3 and 6. 

 Uniform labeling at the first metaphase but not at the second metaphase results from 

 the bipartite nature of the chromosomal DNA. Solid lines indicate unlabeled strands; 

 dotted lines indicate radioactive strands; shadowing indicates grains seen in the 

 autoradiographs, resulting from the presence of thymidine-H . 



but the DNA-containing chromosome. These experiments by J. H. 

 Taylor are formally similar to the N ^^ experiments in that isotope label- 

 ing of the DNA was used to distinguish old and new strands. 



Chromosomes were labeled with radioactive thymidine, a nucleoside 

 that goes only into DNA. The radioactivity came from tritium (radio- 

 hydrogen H"*), substituted for hydrogen in a stable position where it 

 does not exchange appreciably with hydrogen in the medium. Thus it 

 effectively labels the location of incorporated thymidine, and conse- 

 quently of DNA, in material from which all low molecular weight 

 thymidine has been removed. To observe the location of the radioactive 

 element, a photographic emulsion used to record disintegrations is placed 

 tightly against cells or sections mounted on slides. After a suitable ex- 

 posure time followed by development, the slides are examined together 

 with the overlying emulsion under the microscope. It is possible to 

 determine very accurately the location in the chromosome of the tritium, 

 which is the source of electrons which reduce the silver grains of the 

 emulsion following nuclear disintegration. 



The experimental procedure consisted of soaking root tips in a high 



