90 



S. S. COHEN 



1956), who showed that the nucleolus of the starfish oocyte mcorporates 

 adeiime-8-C^* and phenyIalanine-2-C^* at far greater rates than do other 

 parts of the nucleus and cytoplasm (Fig. 12). The use of the specific labels for 



Fig. 12. Incorporation of adenine-S-C^^ into the oocytes of Asterias (Ficq, 1956) 

 — • — nucleus, — o — cytoplasm (radioactivity: x 70), — x — nuclear sap (radioactivity: 

 X 2). Ordinate = number of tracks X 10* per /a^ after 3-day exposure to photographic 

 emulsion. 



nucleic acid and protein, respectively, followed earlier work showing similar 

 results with a relatively nonspecific label, glycine-l-C^* (Ficq, 1953). In this 

 study it was shown that glycine was incorporated into the nucleolus of the 

 starfish oocyte at a rate 100 times greater than its incorporation into cyto- 

 plasm.^ These results and others of Ficq and Brachet (1956) have also served 

 to support the hypothesis that organelles relatively rich in ENA are active 

 in protein synthesis. 



Experimentation on the incorporation of amino acids into cell protein has 

 not yet provided data on the specific activity of the nucleoh, but only of the 

 nuclear fraction and various cytoplasmic fractions. The possibility that very 

 high nucleolar activity is a general phenomenon in all cells has been obscured 

 by the dilution of incorporated isotopic amino acid by the excess of inactive 

 nuclear protein. Most data on incorporation into protein have been obtained 

 in just this way and in general appear to show that in animal tissue the 

 microsomal fraction (Borsook, 1956) and occasionally a supernatant fraction 

 have a Mgher rate of incorporation of labeled amino acid than do mito- 

 chondria and nuclei. 



^ However, it has been reported that, in most mammalian tissues, the nucleus is not 

 appreciably more active than cytoplasm at early stages of amino acid incorporation 

 (Brachet, 1957). 



