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Transcript
The Cell Cycle The Key Roles of Cell Division: 1. Cell division functions in reproduction, growth, and repair 2. Cell division distributes identical sets of chromosomes to daughter cells Reproduction, Growth and Repair: The division of a unicellular organism reproduces an entire organism, increasing the population Cell division on a larger scale can produce progeny for some multicellular organisms • this includes organisms that can grow by cuttings or by fission Cell division is also central to the development of a multicellular organism that begins as a fertilized egg or zygote • multicellular organisms also use cell division to repair and renew cells that die from normal wear and tear or accidents Cell division requires the distribution of identical genetic material (DNA) to two daughter cells • a dividing cell duplicates its DNA, moves the two copies to opposite ends of the cell, and then splits into two daughter cells Cell division distributes identical sets of chromosomes to daughter cells: • a cell’s genetic information, packaged as DNA, is called its genome • a human cell must duplicate about 3 m of DNA DNA molecules are packaged into chromosomes Every eukaryotic species has a characteristic number of chromosomes in the nucleus • human somatic cells (body cells) have 46 chromosomes • human gametes (sperm or eggs) have 23 chromosomes Each chromosome has hundreds or thousands of genes, the units that specify an organism’s inherited traits Chromatin is a DNA-protein complex organized into a long thin fiber After the DNA duplication, 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 • the region where the strands connect shrinks to a narrow area called the centromere • the sister chromatids are pulled apart and repackaged into two new nuclei Mitosis - the process of the formation of the two daughter nuclei • mitosis is usually followed by division of the cytoplasm – cytokinesis Gametes (eggs or sperm) are produced only in gonads (ovaries or testes) • cells in the gonads undergo meiosis - which yields four daughter cells - each with half the chromosomes of the parent The Mitotic 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 During interphase the cell grows by producing proteins and cytoplasmic organelles, copies its chromosomes, and prepares for cell division Interphase can by subdivided into three phases: 1. G1 phase - (“first gap”) centered on growth 2. S phase - (“synthesis”), when the chromosomes are copied 3. G2 phase - (“second gap”), the cell completes preparations for cell division After interphase the cell divides - (M) Mitosis is broken into five subphases: 1. 2. 3. 4. 5. Prophase Prometaphase Metaphase Anaphase Telophase Interphase – (late) the chromosomes have been duplicated but are loosely packed • the centrosomes have been duplicated and begin to organize microtubules into an aster Prophase - the chromosomes are tightly coiled, with sister chromatids joined together • the nucleoli disappear • the mitotic spindle begins to form and appears to push the centrosomes away from each other toward opposite ends (poles) of the cell Prometaphase - the nuclear envelope fragments and microtubules from the spindle interact with the chromosomes • microtubules from one pole attach to kinetochores - special regions of the centromere Metaphase - The spindle fibers push the sister chromatids until they are all arranged at the metaphase plate, an imaginary plane equidistant between the poles Anaphase - the centromeres divide, separating the sister chromatids • each is now pulled toward the pole to which it is attached by spindle fibers Telophase - the cell continues to elongate as free spindle fibers from each centrosome push off each other • two nuclei begin to form, surrounded by the fragments of the parent’s nuclear envelope • chromatin becomes less tightly coiled • cytokinesis, division of the cytoplasm, begins Mitotic spindle - is a major driving force in mitosis Spindle fibers composed of microtubules and proteins • come from partial disassembly of the cytoskeleton • assembly of the spindle microtubules starts in the centrosome • as the spindle fibers grow from them, the centrioles are pushed apart Experiments support the hypothesis that spindle fibers shorten during anaphase from the end attached to the chromosome, not the centrosome Cytokinesis: In animals, the first sign of cytokinesis is the appearance of a cleavage furrow in the cell surface near the old metaphase plate Cytokinesis in plants, which have cell walls, involves a completely different mechanism • during telophase, vesicles from the Golgi coalesce at the metaphase plate, forming a cell plate Regulation of the Cell Cycle: • the timing and rates of cell division in different parts of an animal or plant are crucial for normal growth, development, and maintenance • the frequency of cell division varies with cell type Some human cells divide frequently throughout life (skin cells), others have the ability to divide, but keep it in reserve (liver cells), and mature nerve and muscle cells do not appear to divide at all after maturity Cancer cells have escaped from cell cycle controls • cancer cells divide excessively and invade other tissues because they are free of the body’s control mechanisms • cancer cells do not stop dividing when growth factors are depleted either because they manufacture their own, have an abnormality in the signaling pathway, or have a problem in the cell cycle control system • if and when cancer cells stop dividing, they do so at random points, not at the normal checkpoints in the cell cycle • cancer cell may divide indefinitely if they have a continual supply of nutrients (In contrast, nearly all mammalian cells divide 20 to 50 times under culture conditions before they stop, age, and die) Cancer cells may be “immortal” Example: Cells (HeLa) from a tumor removed from a woman (Henrietta Lacks) in 1951 are still reproducing in culture The abnormal behavior of cancer cells begins when a single cell in a tissue undergoes a transformation that converts it from a normal cell to a cancer cell • normally, the immune system recognizes and destroys transformed cells • however, cells that evade destruction proliferate to form a tumor - a mass of abnormal cells If the abnormal cells remain at the originating site, the lump is called a benign tumor • most do not cause serious problems and can be removed by surgery In a malignant tumor, the cells leave the original site to impair the functions of one or more organs • this typically fits the definition of cancer In addition to chromosomal and metabolic abnormalities, cancer cells often lose attachment to nearby cells, are carried by the blood and lymph system to other tissues, and start more tumors in an event called metastasis • treatments for metastasizing cancers include high-energy radiation and chemotherapy with toxic drugs