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7th lesson Biology The Cell Nucleus The cell nucleus is a membrane bound structure that contains the cell's genetic information and controls the cell's growth and reproduction. It is the command center of a eukaryotic cell and is commonly the most prominent organelle in a cell. The Nucleus consists of: Nuclear envelope: The cell nucleus is bound by a double membrane called the nuclear envelope. This membrane separates the contents of the nucleus from the cytoplasm. Like the cell membrane, the nuclear envelope consists of phospholipids that form a lipid bilayer. The envelope helps to maintain the shape of the nucleus and assists in regulating the flow of molecules into and out of the nucleus through nuclear pores. i Nuclear Pores: The nuclear envelope is perforated with holes called nuclear pores. These pores regulate the passage of molecules between the nucleus and cytoplasm, permitting some to pass through the membrane, but not others. Building blocks for building DNA and RNA are allowed into the nucleus as well as molecules that provide the energy for constructing genetic material. Chromosomes: are located within the nucleus. Chromosomes consists of DNA, which contains genetic information and instructions for cell growth, development, and reproduction. the chromosomes are organized into long entangled structures called chromatin. The Nucleolus: Contained within the nucleus is a dense structure composed of RNA and proteins called the nucleolus. The nucleolus contains nucleolar organizers, which are parts of chromosomes with the genes for ribosome synthesis on them. The nucleolus helps to synthesize ribosomes by transcribing and assembling ribosomal RNA. A ribosome is composed of ribosomal RNA and proteins. ii Functions of the Nucleus: It controls the heredity characteristics of an organism. It is responsible for protein synthesis, cell division, growth and differentiation. Stores heredity material in the form of deoxy-ribonucleic acid (DNA) strands. Also stores proteins and ribonucleic acid (RNA) in the nucleolus. It is a site for transcription process in which messenger RNA (m RNA) are produced for protein synthesis. Aids in exchange of DNA and RNA (heredity materials) between the nucleus and the rest of the cell. Nucleolus produces ribosomes and are known as protein factories iii The Lysosomes Lysosomes are spherical membranous sacs of enzymes. These enzymes are acidic hydrolase enzymes that can digest cellular macromolecules. The lysosome membrane helps to keep its internal compartment acidic and separates the digestive enzymes from the rest of the cell. Lysosomes are formed by budding from the Golgi complex. Four types of Lysosomes: 1. Primary lysosome (storage granules) 2. Secondary lysosomes (digestive vacuole or heterophagosome) 3. Residual bodies 4. Autophagic vacuole (cytolysosome or autophagosome) Lysosome Function: Lysosomes act as the "garbage disposal" of a cell. They are active in recycling the cell's organic material and in the intracellular digestion of macromolecules. Some cells, such as white blood cells, have many more lysosomes than others. These cells destroy bacteria, dead cells, cancerous cells, and foreign matter through cell digestion. Lysosomes are also necessary for the degradation of internal cell components such as organelles. In many organisms, lysosomes are also involved in programmed cell death. iv