Subcellular Particles: A Symposium Held during the Meeting of the Society of General Physiologists at the Marine Biological Laboratory, Woods Hole, Massachusetts, June 9-11, 1958

By Teru Hayashi | Go to book overview

In Situ Studies of Polynucleotide Synthesis in Nucleolus and Chromosomes1

J. HERBERT TAYLOR AND PHILIP S. WOODS

Columbia University and Brookhaven National Laboratory New York

IN SITU STUDIES AT THE INTRACELLULAR LEVEL by means of autoradiography have increased as higher resolution has been attained and appropriately labeled compounds have been prepared. In principle, the autoradiographic method is a simple one in which the wet emulsion is placed in contact with the specimen on a microscope slide, dried, exposed for an appropriate period, developed, fixed, washed and mounted for microscopic examination while still in contact with the specimen (see review by Talyor, ref. 11, for details). Significant improvement in resolution has resulted from the use of radiohydrogen (tritium) as a label. Since most of the beta particles emitted by tritium are stopped by the first micron of a photographic emulsion, the grains produced are very close to the object being exposed. Another improvement comes through advances in biochemistry which allow the selection of precursors which label a limited number of components of the cell. Likewise development of cytochemical techniques for differential extraction when only relatively nonselective labels are available makes possible similar studies in situ.


REPLICATION OF DNA

Although various types of cells may be capable of degrading thymidine, many of them utilize it almost exclusively for DNA (deoxyribonucleic acid) synthesis (8, 2). Thymidine was labeled with tritium and used as a selective label for chromosomes. Resolution was good enough to show labeled and unlabeled segments of single large chromosomes (18). When roots are grown in solutions containing tritium-labeled thymidine, the isotope is incorporated almost exclusively into nuclei. After 6-8 hours of growth, about one-third of the non- dividing nuclei are labeled. The remainder of the cells in that period show no detectable amount of the isotope. Within 10 hours after the cells begin incor-

____________________
1
The work of the senior author was supported in part by grants from the Atomic Energy Commission, Contract AT (30-1) 1304, and the Higgins Fund, Columbia University. The technical assistance of Mrs. Toni Simon is gratefully acknowledged.

-172-

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Subcellular Particles: A Symposium Held during the Meeting of the Society of General Physiologists at the Marine Biological Laboratory, Woods Hole, Massachusetts, June 9-11, 1958
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