8 Chapter 1 • Genetics and the Organism the endoplasmic reticulum and Golgi apparatus, and or-ganelles such as mitochondria and chloroplasts. The nu-cleus contains most of the DNA, but note that mitochon-dria and chloroplasts also contain small chromosomes. Each gene encodes a separate protein, each with spe-ciFc functions either within the cell (for example, the purple-rectangle proteins in ±igure 1-9) or for export to other parts of the organism (the purple-circle proteins). The synthesis of proteins for export (secretory proteins) takes place on ribosomes that are located on the surface of the rough endoplasmic reticulum, a system of large, ²attened membrane vesicles. The completed amino acid chains are passed into the lumen of the endoplasmic reticulum, where they fold up spontaneously to take on their three-dimensional structure. The proteins may be modiFed at this stage, but they eventually enter the chambers of the Golgi apparatus and from there, the se-cretory vessels, which eventually fuse with the cell mem-brane and release their contents to the outside. Proteins destined to function in the cytoplasm and
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