292 



R. H. HOHKHTS. H. .1. HKnTHN, AND H. .1. MCCAKIIIY 



1). KNA Classes 



E. Mechanisms for the Change in Apiiaicnt ( "oiniiosition 



IX. Discussion 



A. Rolo of Ribosonios in Information Traiisl'i r 

 H. Correlation with Enzyme Induction 



C. Messenger Theory 



D. Homogeneity of Ribosomes 



E. Site of Ribosome Synthesis 



F. DXA-RXA Hybrids 



(!. SeU-Replication of Ribosomes .... 

 H. Cell-Free Synthesis of Ribo.somes 



I. State of Ril)osomes in Living Cells 



X. Epilogue 



References 



340 

 341 

 342 

 342 

 343 

 344 

 345 

 346 

 347 

 347 

 348 

 348 

 350 

 350 



I. Introduction 



A. MICROSOMES AND RIBOSOMES 



The term "microsome" was originated by Claude in 1943 to describe 

 a fraction obtained by differential centrifugation of disrupted vertebrate 

 cells (Claude, 1943). After breakage, the cells were centrifuged at low 

 g to sediment nuclei, cell walls, and mitochondria. The supernatant 

 fluid, devoid of structures visible in the microscope, was then centrifuged 

 at 100,000 g for 2 hours and the pellet so obtained was designated the 

 microsomal fraction. In it appeared much of tlie cytoplasmic ribonucleic 

 acid (RNA) and lipid. 



Electron microscopy of this pellet showed that it consisted mainly 

 of membranes to which were attached dense particles of about 200 A 

 diameter. Electron micrographs of the same cells showed an extensive 

 network of membranes designated the endoplasmic reticulum. In addi- 

 tion, dense particles could be seen sometimes free and sometimes at- 

 tached to the membranes. The RNA content of the pellet was attributed 

 to its content of particles (see review, Palade, 1958). Particles from 

 bacteria had been observed earlier (Luria et al., 1943.) 



Analysis of the microsome fraction in the analytical centrifuge, 

 especially after treatment with deoxycholate to dissolve the mem- 

 branous material, showed the jiresence of objects having sedimentation 

 coefficients in the range 20-lOOS (Petcrman and Hamilton, 19o7). 



Similar fractions of high RNA content were obtained from bacteria, 

 yeast, and plant cells. All showed the presence of a number of discrete 

 peaks in the analytical centrifuge having sedimentation coefficients of 

 20-1008 (Schachman et al, 1952; Chao and Schachman, 1956; Ts'O 

 et nl, 1956). 



Since the microsomal fraction included l)()(li the membrane matei'ial 



