Figure 223

Electron micrograph of the rER. This image of rER in a chief cell of the stomach shows the membranous cisternae (C) closely packed in parallel arrays. Polyribosomes are present on the cytoplasmic surface of the membrane surrounding the cisternae. The image of ribosome-studded membrane is the origin of the term "rough endoplasmic reticulum." A few ribosomes are free in the cytoplasm. M, mitochondrion. x 50,000.

tached to the membrane of the rER by ribosomal docking proteins. Ribosomes measure 15 to 20 nm in diameter and contain RNA and protein. In many instances, the rER is continuous with the outer membrane of the nuclear envelope (see below). Groups of ribosomes form short spiral arrays called polyribosomes or polysomes (Fig. 2.24), in which many ribosomes are attached to a thread of messenger RNA (mRNA).

Protein synthesis involves transcription and translation

The production of proteins by the cell begins with transcription, in which the genetic code for a protein is tran-

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