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Ex8Mitosis(2) - K101 LAB EXERCISE 8 MITOSIS THE CELL CYCLE...

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K101 LAB EXERCISE 8 MITOSIS: THE CELL CYCLE AND CELL DIVISION Multicellular organisms - like humans - start out life as a single cell, or zygote, that is the direct product of fertilization of an egg by a sperm . Very soon after fertilization, cell division or mitosis begins, resulting in the formation of a multicellular organism from a single cell. The zygote divides into two cells, then four, and so on. The rate of mitosis is astounding, and a developing human embryo just a few weeks after conception contains millions of cells specialized for many different functions. A human baby at birth has nearly 100 trillion cells, all the result of mitosis from a single celled zygote! Mitosis is also a very important process for regular renewal of cells that ‘wear out’ quickly (like red blood cells) - over 250 MILLION new red blood cells are made by mitosis in your body every minute of every day ! Wounding is another process where mitosis is important. When an organism is injured, cells around the wound site very quickly initiate mitosis to produce new cells to fill in the gap made by the wound. The ‘ dark side’ of mitosis is the development of cancer - an abnormal pattern of cell division that ignores the typical controls regulating mitosis, leading to the formation of tumors. We will discuss the development and treatment of cancer in lecture. Cells divide according to a certain ‘lifespan’ defined as the cell cycle . Some cells divide very rapidly, like blood cells, and some cells divide only rarely, if ever, like neurons in the brain. Most of the cell cycle (90%) consists of a growth and development stage called interphase . During interphase, the cell first spends time in a phase called G1 or ‘Growth 1’ . Most metabolic activities occur during interphase - glycolysis, building up and breaking down of macromolecules, growth and development. At some point in G1 , the cell initiates the first step in the program that will prepare the cell for mitosis. S-phase or ‘ Synthesis ’ phase is where the cell replicates all the DNA in its nucleus – all 46 chromosomes for humans - so that an identical set of 46 chromosomes can be distributed to each of the two cells that will result from cell division (Chapter 16). After S phase, each chromosome exists in a temporarily duplicated state where identical ‘ sister chromatid’ pairs ( duplicated chromosomes ) are held together by a region of the chromosome called the centromere . The purpose of mitosis is to split these identical sister chromatids into two new cells that will each contain 46 chromosomes. Following S-phase, the cell enters the last stage of interphase, called G2 or Growth 2 . During this time, the cell completes growth and assembles structures necessary for entering mitosis.
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