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Cell Nucleus
The defining organelle that differentiates the eukaryotes from the prokaryotes is the nucleus. The nucleus is a membrane-enclosed organelle that stores, protects and expresses most (but not all) of the genetic information (DNA) in the cell. A tiny portion of the genetic information of the cell is also found in the mitochondria. Since most of the genetic information is in the nucleus and the nucleus expresses that genetic information, we sometimes refer to the nucleus as the control center of the cell. The nucleus contains its own lipid bilayer membrane known as the nuclear membrane or nuclear envelope. This membrane is perforated with many small openings called nuclear pores, which are basically protein complexes that allow certain materials such as RNA, ribosomal subunits and polymerases to pass through. The outer member is physically connected to the endoplasmic reticulum for easy RNA transfer. The inner membrane encloses the fluid of the nucleus called the nucleoplasm. It is also connected to a network of intermediate filaments known as the nuclear lamina, which gives the nucleus its structure and is involved with gene expression. At the heart of the nucleus is a region called the nucleolus, which basically contains RNA and proteins that are involved in forming rRNA units that are found ribosomes throughout the cell. The DNA inside the nucleus is wrapped around structural proteins called histones, which are bunched together and further twisted and coiled into supercoils to form an extremely condensed DNA-protein-RNA complex called the chromatin (or chromatid).
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