Ribosomes, of the endoplasmic reticulum

The answer is e. (Murray, pp 452-467. Scriver, pp 3-45. Sack, pp 1—40. Wilson, pp 101-120.) Protein synthesis occurs in the cytoplasm, on groups of free ribosomes called polysomes, and on ribosomes associated with membranes, termed the rough endoplasmic reticulum. However, proteins destined for secretion are only synthesized on ribosomes of the endoplasmic reticulum and are synthesized in such a manner that they end up inside the lumen of the endoplasmic reticulum. From there the secretory proteins are packaged in vesicles. The Golgi apparatus is involved in the glycosylation and packaging of macromolecules into membranes for secretion. 

Recent work on the molecular organization of eukaryotic cells has shown that the classic notion of information transfer must be extended to include extranuclear cell components since the genetic information of a eukaryotic cell is not confined to the cell nucleus but is also present in the mitochondria and in the chloroplasts of photosynthetic cells. Replication of these extranuclear DNA species alra takes place outside the nucleus. Furthermore, protein synthesis is not confined to ribosomes of the endoplasmic reticulum or the nucleus but may also take place at ribosomes in mitochondria and in chloroplasts. Mitochondria, hke chloroplasts, contain a unique DNA species and the means of repheating it, as well as the machinery for transcription and for translation of the message. They also contain some unique tRNA species. 

In bacterial cells and in yeast, which are, for obvious reasons, much more commonly studied than other animals and plants, amino acids, synthesized de novo or by degradation of proteins of various kinds and sizes, are found in the cytosol. The amino acids from these proteins are used for the synthesis of peptides. Much of the synthesis occurs either on/in the ribosomes of the endoplasmic reticulum (ER) or through nonribosomal pathways using acyl carrier protein-like condensation reactions. 

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