General physiologic Processes CELL STRUCTURE AND FUNCTION Three structural features of human cells( Figure 1-1)identify them as eukaryotic cells. They are 1. a distinct membrane surrounding a central nucleus, 2. several membrane-lined intracellular structures and organelles, and 3. a number of well-defined subcellular domains in which different nicroenvironments are maintained so that several chemical reactions can occur simultaneously and optimally because the properties of the membranes defining these domains permit precise regulation of Cytosolic Membrane Systems, Organelles, and Inclusions Nucl The nucleus is the site where that portion of the human genome that rep- resents"meaningful"deoxyribonucleic acid(DNA) is transcribed into ribonucleic acid(RNA)by olymerization. Of the transcribed RNA, the majority is heterogeneous nuclear RNA that is either destroyed or further modified by capping, polyadenylation, or splicing. A small portion is messenger RNA (mRNA), which leaves the nucleus in that form and reaches the cytosol and ribosomes to be translated into proteins The nucleus is the largest intracellular organelle. It is surrounded by the nuclear membrane and contains chromatin(densely packed DNA)andGeneral Physiologic Processes CELL STRUCTURE AND FUNCTION Three structural features of human cells (Figure 1–1) identify them as eukaryotic cells. They are 1. a distinct membrane surrounding a central nucleus, 2. several membrane-lined intracellular structures and organelles, and 3. a number of well-defined subcellular domains in which different microenvironments are maintained so that several chemical reactions can occur simultaneously and optimally because the properties of the membranes defining these domains permit precise regulation of regional milieus. Cytosolic Membrane Systems, Organelles, and Inclusions Nucleus The nucleus is the site where that portion of the human genome that rep￾resents “meaningful” deoxyribonucleic acid (DNA) is transcribed into ribonucleic acid (RNA) by a process of regulated polymerization. Of the transcribed RNA, the majority is heterogeneous nuclear RNA that is either destroyed or further modified by capping, polyadenylation, or splicing. A small portion is messenger RNA (mRNA), which leaves the nucleus in that form and reaches the cytosol and ribosomes to be translated into proteins. The nucleus is the largest intracellular organelle. It is surrounded by the nuclear membrane and contains chromatin (densely packed DNA) and one or two nucleoli. 1 1