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5 The Nucleus: A Command Center for Cells The nucleus of a cell is a spherical or oval structure averaging about 5
micrometers in diameter, making it the largest cytoplasmic organelle. It is usually located near the center of a cell and is surrounded by a double-layered envelope, consisting of inner and outer lipid bilayer membranes. The nucleus is considered the “command center� of a cell. Its DNA contains the genetic code which has the instructions to produce virtually every protein in the body (recall that mitochondria have their own DNA and produce some proteins not coded for in nuclear DNA). In addition, the nucleus also directs the kinds and amounts of proteins that are synthesized at any given time. The nucleus has three distinct constituents: the nuclear
DID YOU KNOW? Every cell type in the body contains a nucleus, with one notable exception. Mature red blood cells lose their nucleus before entering the blood stream from bone marrow. As a consequence, these anucleate cells cannot synthesize proteins. Therefore, circulating red blood cells do not have the ability to replace enzymes or structural parts that break down. For this reason, they have a limited life span, approximately 3 to 4 months. In contrast, some cells contain many nuclei, such as those in skeletal muscle and the liver. The presence of multiple nuclei usually indicates a relatively large mass of cytoplasm that must be regulated.
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