/;/ Situ Studies of Polynucleotide Synthesis in 

 Nucleolus and Chromosomes^ 



J. HERBERT TAYLOR and PHILIP S. WOODS 



Columbia University and 

 Brookhaven National Laboratory 



New York 



IN SITU STUDIES AT THE INTRACELLULAR LEVEL by iTieaiis 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. ii, 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 be- 

 ing exposed. Another improvement comes through advances in biochemistry 

 which allow the selection of precursors which label a limited number of com- 

 ponents of the cell. Likewise development of cytochemical techniques for dif- 

 ferential 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 seg- 

 ments of single large chromosomes ( 18). When roots are grown in solutions 

 containing tritium-labeled thymidine, the isotope is incorporated almost ex- 

 clusively 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- 



' 'I'Ik- work of the scnidi' author was MipportctI in [lart by grants from the Atomic Energy 

 C'ommission, C'ontract AT (^0-1)1304, anil the Higgins Fund, Columbia University. The 

 technical assistance of Mrs. 'I'oni Simon is gratefully acknowledged. 



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