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Cell Reproduction--Mitosis Section 8.2 Why do cells reproduce at all? Couldn’t they grow really, really big? • Cells are limited to small sizes, therefore they must reproduce to make many cells instead of one big cell. • Reasons why big cells are bad include: – Diffusion would be too slow. – Too many proteins would be needed. – Cell membrane (surface area of cell) would not be able to increase as fast as the cell volume and would not work properly. (volume = l x w x h; surface area = l x w x 6) Review: all cells come from preexisting cells • This means that when a cell reproduces or divides, it creates two cells that are identical to the original. • This occurs not only because it is better to have many small cells, but also because an organism needs to grow or replace dead tissue. (This also will cause population growth in unicellular organisms.) Chromosomes carry the coiled DNA. The Cell Cycle • The cell cycle is the steps a cell goes through during its life. • First, there is a period of growth and DNA replication called interphase. Most of a cell’s life is spent in this stage. • After this, the cell divides, or reproduces, in a series of steps called mitosis. After this 2 identical cells are formed, each with complete set of chromosomes. Steps of Mitosis • • • • Prophase--prepares Metaphase--middle Anaphase--apart Telophase--two • **use the terms to provide clues for what happens in each step** Prophase • First and longest step. • Chromatin (DNA) coils into chromosomes. • The two halves are identical, and are called sister chromatids. They are held together by the centromere. • By the end of prophase the nuclear membrane is disappearing. • In animal cells, centrioles and spindle fiber have formed and are lining up in the cell. Metaphase • Chromosomes become attached to the spindle fibers at the centromere. • Spindle fibers help to position chromosomes in the middle of the cell, so each sister chromatid is facing a pole. Anaphase • Sister chromatids are pulled apart towards opposite poles. Telophase • Chromatids reach opposite poles and begin to uncoil. • Two new nuclei are formed at the poles. • A new cell membrane begins to form down the middle of cell. This divides the cytoplasm and organelles and is called cytokinesis.