314 CELL HEREDITY 



Centrifugal force 

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FAT VAC CYT NUC MIT ERG GLY 



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FIGURE 11.4. Centrifuged hyphae of Neurospora (after Zaiokar, 1960, Expt. Cell 

 Res., 19:559). 



1 . ScHiematic diagram showing separation of cellular constituents. FAT, fat; VAC, 

 vacuoles; CYT, encfiylemo; NUC, nuclei; MIT, mitrocfiondria; ERG, ergastoplasm; GLY, 

 glycogen. 



2. Autoradiograph taken after one minute exposure to uridine-H . All detectable 

 radioactivity is associated witfi the nuclear fraction. 



3. Autoradiograph taken after one minute exposure to uridine-H followed by one 

 hour in nonradioactive uridine. Radioactivity is now found associated with the 

 mitochondria and ergastoplasm (endoplasmic reticulum) as well as with the nuclear 

 fraction. 



The localization of RNA within the cell has been studied by means of 

 autoradiography, employing P^~, C^^-nitrogen bases, and, more success- 

 fully, tritium-labeled cytidine and uridine with a variety of cell types. 

 Most of these experiments have given similar results, indicating that the 

 bulk, if not all, of RNA synthesis occurs within the nucleus. Two ex- 

 amples of this kind of experimentation will be presented. 



A method was developed by Zaiokar to centrifuge strands of Neuro- 

 spora mycelium so that the cell constituents are layered within the cell, 

 as if in a centrifuge tube. As shown in Figure 11.4, the microsomes, 



