STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 89 



deposition in which protein is found, or if such synthesis occurs j)redomin- 

 antly at a particular organelle from which protein or polypeptide may be 

 transported to its particular site of dej)osition, or if some combination of 

 these more accurately describes the cellular biosynthesis of particular 

 proteins. Present data relates for the most part to two extremes of protein 

 synthesis, i.e., to the site of mcorporation of amino acids into the peptide 

 linkages of protein and to the origm of enzymatic activity. Most current 

 experimentation has not yet dissected the problem in terms of the various 

 levels of protem structure. These levels include: 



(1) Primary structure — the specific order of ammo acids in a polypeptide 

 cham. 



(2) Secondary structure — the folding of a specific polypeptide chain into 

 a helix or other form such that hydrogen bonding between ammo acid 

 residues on the same chain confers stability on this configuration of the chain 

 and produces a specific three dimensional geometry to a single polypeptide 

 chain. 



(3) Tertiary structure — the side to side arrangement of folded polypeptide 

 chains, fixed by a variety of bonds, polar, covalent, etc., between functional 

 groups, e.g., — SH, - — NHg, — COOH, etc., between amino acid side chains. 

 These arrays of peptide chams determine the size and shape of specific 

 proteins and the linkages between chains confer stability to the protein as a 

 whole. If the problems of protein synthesis are posed in terms of these three 

 different levels of structure, it can be seen, for example, that the incorpora- 

 tion of an amino acid into protein-bound peptide in one or another part of 

 the cell may relate only to an early step in the total process of producing 

 functional protein. Thus, this functional unit may really be assembled at a 

 very different site from that indicated by incorporation data. 



b. Sites of Protein Synthesis, i. Incorporation Data. Caspersson (1941) pro- 

 posed that the nucleus is the main center of protein synthesis. He considered 

 that the genetic material of the chromosomes controlled the quahty of pro- 

 tein synthesis. More particularly, heterochromatin and nucleoli were beheved 

 to be responsible for the synthesis of basic proteins, which were thought to 

 accumulate in the latter organelle. From the nucleolus, basic proteins 

 diffused to and through the nuclear membrane and induced the synthesis of 

 cytoplasmic ENA, which subsequently induced the synthesis of cytoplasmic 

 proteins. At approximately the same time Brachet (1941, 1942) presented the 

 hypothesis that RNA was somehow concerned with protein synthesis, which 

 he did not yet relate to nuclear activity. 



As described earher, modern data on the composition of nuclear com- 

 ponents do not fit well with the details of Caspersson's hypotheses. However, 

 that the nucleolus can indeed be an important center of RNA and protein 

 synthesis is strongly supported by the autoradiographic studies of Ficq (1955, 



