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ch12im - The Nucleus of a Eukaryotic Cell An Overview...

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The Nucleus of a Eukaryotic Cell: An Overview I. Cells possess mechanisms that allow them to express their genetic information selectively, following only those instructions that are relevant for that cell at that particular time II. Nuclear contents - viscous, amorphous mass of material enclosed by a complex nuclear envelope that forms a boundary between the nucleus & cytoplasm; its morphology is relatively undistinguished III. Included within a typical interphase (nonmitotic) cell nucleus are: A. Chromosomes – present as highly extended nucleoprotein fibers called chromatin B. Nuclear matrix - protein-containing fibrillar network C. Nucleolus (nucleoli - plural ) - one or more; irregularly-shaped, electron-dense, amorphous structures that function in the synthesis of rRNA & the assembly of the ribosomes D. Nucleoplasm - the fluid substance in which the solutes of the nucleus are dissolved The Nuclear Envelope I. The nuclear envelope is complex; consists of several distinct components; core of nuclear envelope consists of 2 cellular membranes arranged parallel to one another & separated by 10 - 50 nm A. Separates genetic material in nucleus from cytoplasm - important distinction (maybe the single, most important one) between prokaryotes & eukaryotes; its appearance is an evolutionary landmark 1. Serves as barrier; keeps ions, solutes, macromolecules from passing between nucleus & cytoplasm B. Fused at sites forming circular pores that contain complex assemblies of proteins 1. Average mammalian cell: contains ~3000 nuclear pores 2. Pore density: ~2-4/μm 2 (metabolically inactive bird erythrocyte) to >60/μm 2 (active oocyte) C. Outer membrane is generally studded with ribosomes & is often seen to be continuous with RER membrane; the space between the membranes is continuous with the ER lumen D. Inner surface of the nuclear envelope of animal cells is bound by integral membrane proteins to a thin filamentous meshwork ( nuclear lamina ) 1. Provides mechanical support to nuclear envelope, serves as a site of attachment for chromatin fibers at the nuclear periphery & has a poorly understood role in DNA replication & transcription 2. Filaments of nuclear lamina are ~10 nm in diameter, made of polypeptides called lamins (members of the same superfamily of polypeptides that assemble into the 10-nm intermediate filaments) 3. As in cytoplasm, the integrity of the intermediate filaments that make up the nuclear lamina is regulated by phosphorylation & dephosphorylation 4. Nuclear lamina disassembly prior to mitosis is thought to be induced by phosphorylation of the lamins by a specific protein kinase E. Mutations in one of lamin genes ( LMNA ) are responsible for several distinct human diseases, including rare form of muscular dystrophy (EDMD2) in which muscle cells contain exceptionally fragile nuclei 1. Lamin A mutations are also linked to a disease (Hutchinson-Gilford progeria syndrome (HGPS) that is characterized by premature aging & death during teenage years from heart attack or stroke 2.
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