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The nucleus is typically the largest single organized cellular component. It appears as a distinct spherical, or oval, structure and is usually located near the center of the cell. The nucleus is surrounded by a double membrane barrier, the nuclear envelope. Each layer of the nuclear envelope is a phospholipid bilayer similar to the plasma membrane. Although the nuclear envelope encloses the nuclear contents, it does not completely separate these contents from the surrounding cytoplasm. The nuclear envelope is penetrated by numerous water-filled nuclear pores that readily allow ions and most water-soluble molecules to enter and leave the nucleus. These large pores even allow some fairly large molecules such as RNA and certain proteins to pass through the nuclear boundary. The nucleus houses deoxyribonucleic acid (DNA), which has two important functions: directing protein synthesis and serving as a genetic blueprint during cell replication. By directing the kinds and amounts of various enzymes and other proteins that are produced, the nucleus indirectly governs most cellular activities and serves as the cell’s control center. Typically, one or more nucleoli (nucleolus, singular) are present in the nucleus. Nucleoli are the sites of production of the subunits that make up ribosomes. Nucleoli are not enclosed by membrane. Once produced, the two protein subunits along with ribosomal RNA exit the nucleus through the nuclear pores. At the time when proteins are being synthesized within the cytoplasm, the subunits join together as intact, functional ribosomes.