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Cell Division MITOSIS When a living thing grows are its cells getting bigger or is the organism producing more of them? It is producing more cells. Cells of an adult are usually no larger than cells of a younger animal. Why don’t our cells just grow larger? Cell Growth There are many reasons cells just can’t grow to be huge . . . DNA overload – DNA controls the cell’s functions, but if the cell were much larger the nucleus could not direct all of that activity Material Exchange – if our cells were larger it would take materials a long time to reach their destinations (cell membrane) Cell Death – if we only had a few large cells and one died it would be very detrimental to the entire organism So, when cells reach a critical size they . . . Cell Cycles Cells go through a cycle that determine when they should grow and when they should divide. There are four stage to the cell cycle: G1 (Gap 1) S Phase (DNA Synthesis) G2 (Gap 2) M-Phase (Mitotic phase) Gap 1 The cell grows They make new proteins and organelles S-Phase During this phase the DNA in the nucleus is replicated That means each leg of the chromosome is copied Gap 2 Organelles required for cell division are created for example centroile The centrioles are organelles that play a very important role in mitosis M-Phase This is the phase the cell goes through to divide Cell Division – the process cells go through to split into two new daughter cells There are two important parts to this: Mitosis - the process eukaryotes go through to replicate and divide their nucleus Cytokinesis – the division of the cytoplasm during cell division DNA One of the most important parts of cell division is splitting up the DNA so that all cell functions can be performed There two different forms of DNA Chromatin – DNA unwound with no particular shaoe Chromosomes – DNA that is tightly wound into an X shape Chromosomes Sister Chromatids Centromere Mitosis There are four stages to mitosis Prophase Metaphase Anaphase Telophase Prophase During prophase the chromatin starts to condense (thicken and shorten) forming chromosomes The nuclear membrane begins to break down The centrioles move to opposite end of the cell The centrioles start to create the spindle fibers Prophase Metaphase During metaphase the chromosomes line up along the middle of the cell The spindle fibers lengthen and attach to the centromere on each chromosome Metaphase Anaphase During anaphase the centromeres separate splitting up the sister chromatids The spindle fibers shorten pulling the sister chromatids towards the centrioles at the opposite ends of the cell. Anaphase ends when the chromatids reach the opposite ends of the cell and stop moving Anaphase Telophase During telophase the nuclear envelop reforms around the chromatids As the this happens the chromatids relax becoming chromatin again The nucleolus also reappears Telophase Cytokinesis This usually happens about the same time as telophase The cytoplasm separates and all of the organelles separate into the new cells Cytokinesis in Animal Cells The cell membrane pinches inward until it touches. The cells break off and become separate cells Cytokinesis in Plant Cells A cell plate forms down the middle of the cell The cell membranes separates at the cell plate The cell wall begins to form in the middle of the cell plate Mitosis Animation http://www.teachersdomain.org/asset/lsps07 _int_celldivision/