. Cell heredity. Cytogenetics. 192 CELL HEREDITY. Duplication First c-metaphase Duplication Second c-metaphase with labeled after lat)eling without latKled after labeling thymidine thymidine FIGURE 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 t
. Cell heredity. Cytogenetics. 192 CELL HEREDITY. Duplication First c-metaphase Duplication Second c-metaphase with labeled after lat)eling without latKled after labeling thymidine thymidine FIGURE 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 su
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