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CHAPTER 12 THE CELL CYCLE The Key Roles of Cell Division Cell division results in genetically identical daughter cells The Mitotic phase alternates with interphase in the cell cycle Introduction • The continuity of life from one cell to another is based on the reproduction of cells via cell division. • This division process occurs as part of the cell cycle (the life of a cell from its origin in the division of a parent cell until its own division into two). two). Fig. 12.1, Page 216 • The division of a unicellular وﺣﯿﺪ اﻟﺨﻠﯿﺔorganism (e.g. (e.g. Amoeba)) reproduces an entire organism, increasing Amoeba the population. • Cell division is also central to the development of a multicellular ﻋﺪﯾﺪ اﻟﺨﻠﯿﺔorganism that begins as a fertilized egg or zygote. Figs. 12 12..1, Page 216 2 Multicellular organisms also use cell division to repair and renew cells that die normally or by accidents (blood ( cells from bone marrow). ). Cell division distributes the genetic material (DNA) to two daughter cells. Figs. 12 12..1, Page 216 Division is differ among cells:. - Skin cells divide frequently. - Liver cells divide when needed (damage repair). - Nerve cells and muscle cells do not divide at all. 3 Cell division results in genetically identical daughter cells A cell’s genetic information (genome )اﻟﺒﻨﻚ اﻟـﭽﯾﻨﻰis packaged as DNA. In prokaryotes, the genome is often a single long DNA molecule. In eukaryotes, the genome consists of several DNA molecules. A human cell must duplicate about 3 m of DNA and separate the two copies such that each daughter cell ends up with a complete genome. DNA molecules are packaged into chromosomes. Every eukaryotic species has a characteristic chromosomes in the nucleus. Human somatic cells (body (body cells) cells) have chromosomes. Human gametes ( أﻣﺸﺎجsperm or eggs) have chromosomes, half the number in a somatic cell اﻟﺨﻠﯿﺔ اﻟﺠﺴﺪﯾﺔ.. Each eukaryotic chromosome consists linear DNA molecule. number of 46 23 of a long, Fig. 12.2, Page 216 4 Each chromosome has hundreds or thousands of genes, the units that specify an organism’s inherited traits. Associated with DNA are proteins that maintain its structure and help control gene activity. This DNA-protein complex, chromatin, is organized into a long thin fiber. After the DNA duplication in preparation for cell division, chromatin condenses, coiling and folding to make a smaller package. Each duplicated chromosome consists of two sister chromatids which contain identical copies of the chromosome’s DNA. Asthey condense, the region where the strands connect shrinks to a narrow area, is the centromere. Fig. 12.3 Later, the sister chromatids are pulled apart and repackaged into two new nuclei at opposite ends of the parent cell during cell division. The process of the formation of the two daughter nuclei called (mitosis) and is usually followed by division of the cytoplasm (cytokinesis ) اﻹﻧﺸﻄﺎر اﻟﺨﻠﻮى. It occurs in somatic cells اﻟﺨﻼﯾﺎ اﻟﺠﺴﺪﯾﺔ Chromatid Chromatin + DNA Sister chromatid Centromere Homologous Chromosome Chromosome اﻟﺼﺒﻎ 6 In the gonads اﻟﻤﻨﺎﺳﻞ اﻟﻤﻨﺎﺳﻞ,, cells undergo a meiosis division, which yields four daughter cells, each with half the chromosomes number of the parent cell. In humans, meiosis reduces the number of chromosomes from 46 to 23 23.. Each of us inherited 23 chromosomes from each parent: one set in an egg and one set in a sperm during meiosis. gametes اﻷﻣﺸﺎج ((اﻷﻣﺸﺎجeggs or sperm) are produced only in gonads ( اﻟﻤﻨﺎﺳﻞovaries or testes). The fertilized egg undergoes trillions of cycles of mitosis and cytokinesis to produce a fully developed multicellular human. These processes continue every day to replace dead and damaged cell. Fertilization fuses two gametes together and doubles the number of chromosomes to 46 again. Definitions Genes: Small segments of DNA that specify an organism’s inherited characters. Genes: characters. Chromatin:: A DNAChromatin DNA-protein complex which is organized into a long thin fiber Chromosome::In the nucleus of each cell, the DNA molecule is packaged into Chromosome thread-like structures called chromosomes. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure. Chromatids:: Two sister arms (chromatids) formed in each duplicated Chromatids chromosome. They contain identical copies of the chromosome’s DNA Centromere:: The narrow region at which the chromatids are attached to each Centromere other Mitosis::Mitosis is a process of cell division which results in the production of Mitosis two identical daughter cells from a single parent cell. Cyt okinesis okinesis:: اﻹﻧﺸﻄﺎر اﻟﺨﻠﻮىIs the division of the cytoplasm which usually follows mitosis. Meiosis:: Cell division that occurs In the gonads اﻟﻤﻨﺎﺳﻞ Meiosis اﻟﻤﻨﺎﺳﻞ,, and yields four daughter cells, each with half the number of chromosomes of the parent cell. 8 The mitotic phase alternates with interphase in the cell cycle: an overview Phases of the Cell Cycle • The mitotic (M) phase of the cell cycle alternates with the much longer interphase. – The M phase includes mitosis and cytokinesis. – Interphase accounts for 90% of the cell cycle. Fig. 12.5 A. Interphase has three subphases: A. the G1 phase (“first gap”) centered on growth, – the S phase (“synthesis”) when the chromosomes are copied, – the G2 phase (“second gap”) where the cell completes preparations for cell division, – and divides (M). • The daughter cells may then repeat the cycle. B. Mitosis: is usually include five subsub-phases ﻣﺮاﺣﻞ ﻓﺮﻋﯿﺔ: A. B. C. D. Prophase, اﻟﺘﻤﮭﯿﺪﯾﺔ Prometaphase,, ﻗﺒﻞ اﻹﺳﺘﻮاﺋﯿﺔ Prometaphase Metaphase, اﻹﺳﺘﻮاﺋﯿﺔ Anaphase, اﻹﻧﻔﺼﺎﻟﯿﺔ E. Telophase. اﻹﻧﺘﮭﺎﺋﯿﺔ By late interphase (G (G2 2), the chromosomes have been duplicated ﺗﻀﺎﻋﻔﺖbut are loosely packed. The centrosomes have been duplicated and begin to organize microtubules into an aster (“star”). Fig. 12 12..5a, 218 11 1) Prophase, اﻟﺘﻤﮭﯿﺪﯾﺔthe chromosomes are tightly coiled, with sister chromatids joined together, The nucleoli disappear disappear.. The mitotic spindle begins to form and appears to push the centrosomes away from each other towards opposite ends (poles) of the cell cell.. 2) Prometaphase, ﻗﺒﻞ اﻹﺳﺘﻮاﺋﯿﺔthe nuclear envelope fragments and microtubules from one pole attach to one of two kinetochores (special regions of the centromere)) while microtubules from centromere the other pole attach to the other kinetochore.. kinetochore 3) Metaphase, اﻹﺳﺘﻮاﺋﯿﺔthe spindle fibers push the sister chromatids until they are all arranged at the imaginary plane equidistant between the poles, defining Fig. 12 12..6a, b,c,d Anaphase, اﻹﻧﻔﺼﺎﻟﯿﺔthe centromeres Anaphase, divide, result in separating the sister chromatids chromatids.. Each is then pulled toward the pole to which it is attached by spindle fibers fibers.. By the end, the two poles have equivalent collections of chromosomes chromosomes.. Telophase, اﻹﻧﺘﮭﺎﺋﯿﺔthe cell continues to Telophase, elongate as free spindle fibers from each centrosome push off each other.. other 1) 2) Two nuclei begin to form, surrounded by the fragments of the parent’s nuclear envelope envelope.. Chromatin becomes less tightly coiled.. coiled Cytokinesis, begins as the Cytokinesis, division of the cytoplasm occurs. Fig. 12 12..6 e and f Fig. 12.6 Fig. 12.6 B. The cytokinesis cytokinesis:: اﻹﻧﺸﻄﺎر اﻟﺨﻠﻮىdivides the cytoplasm: • Cytokinesis (division of the cytoplasm)) typically follows cytoplasm mitosis. • Contraction إﻧﻘﺒﺎضof the cell pinches the cell into two new cells Fig. 12.8, Page 222 16 Cell Cycle Interphase Division process Cytokinesis Mitosis G1 S Prophase اﻟﺘﻤﮭﯿﺪﯾﺔ G2 Prometaphase ﻗﺒﻞ اﻹﺳﺘﻮاﺋﯿﺔ Metaphase اﻹﺳﺘﻮاﺋﯿﺔ Anaphase اﻹﻧﻔﺼﺎﻟﯿﺔ Telophase اﻹﻧﺘﮭﺎﺋﯿﺔ 17