Ribosomes are often called the "protein factories" of the cell, because it is here that proteins are produced according to the genetic information contained in messenger RNA (discussed in a later section). The ribosomes are quite tiny, about 25 nanometers in size, and can be found both free in the cytoplasm and located on the surface of an organelle called the endoplasmic reticulum (discussed in the next section).

Each ribosome consists of two subunits (fig. 3.11) that are designated 30S and 50S, after their sedimentation rate in a centrifuge (this is measured in Svedberg units, from which the "S" is derived). Each of the subunits is composed of both ri-bosomal RNA and proteins. Contrary to earlier expectations of most scientists, it now appears that the ribosomal RNA molecules serve as enzymes (called ribozymes) for many of the reactions in the ribosomes that are required for protein synthesis. Protein synthesis is covered later in this chapter, and the general subject of enzymes and catalysis is discussed in chapter 4.

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