Contents References 11 II. A short historical survey 13 References 16 III. Ribosomes within the cell 20 1. Proportion of ribosomes in tissues 21 2. Free and membrane-bound ribo- mes 22 3. Ribosomes in nucleoli 26 4. Structure of polysomes in the cell 26 5. Ribosomal crystals 33 6. References 33 IV. Some general properties of ribosomes 41 1. Physical characteristics 41 2. Chemical characteristics 43 3. References 44 V. Morphology of ribosomes and poly- somes 47 1. Electron microscopy 47 a) The small ribosomal subunit 48 b) The large ribosomal subunit 48 c) The monomeric ribosome 52 d) Three-dimensional models 54 e) Polysomes 55 2. Small-angle X-ray scattering 55 a) The monomeric ribosome 56 b) Polysomes 57 3. References 57 VI. Chemical components 61 1. Ribosomal proteins 61 a) Electrophoretic separation and num- ber 61 b) Preparation of single ribosomal pro- teins 67 c) Molecular weights 68 d) Amino acid composition and amino acid sequences 71 e) Stoichiometry 71 f) Posttranslational modifications 72 g) Comparison of ribosomal proteins of different tissues 73 h) Species specificities and evolution 74 i) Ribosomal proteins of mitochondria and chloroplasts 75 k) Ribosomal mutants with altered pro- teins 75 5 3. Formation of preribosomal 2. Phosphorylation of ribosomal prote- particles 139 ins 76 4. References 141 a) In vitro phosphorylation and de- phosphorylation 76 VIII. Dissociation - reassociation processes b) In vivo phosphorylation 77 of ribosomal particles 151 c) Phosphorylation of ribosomal pro- tein S 6 78 1.