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Organisms reproduce like offspring. There are two types of reproduction: ASEXUAL REPRODUCTION - producing offspring from only 1 parent • offspring look EXACTLY like parent • this type of reproduction is best suited for a stable environment • occurs in unicellular organisms and some plants SEXUAL REPRODUCTION - producing offspring from two parents • offspring have characteristics from BOTH parents • this type of reproduction is suited for either a stable environment OR a changing environment QuickTime™ and a Sorenson Video decompressor are needed to see this picture. Cells are organisms. They must reproduce. • Cells need to divide (reproduce) for ... growth healing and repair to replace dead or damaged cells • New cells formed must be IDENTICAL to original cells WHY ? • So the new cells can perform the same function as the original cell ! QuickTime™ and a Cinepak decompressor are needed to see this picture. •• MITOSIS - the process cells use to make exact copies of the cell (aka - Nuclear Reproduction) • In order for the cells made by mitosis to be identical, they must have the same DNA ! • Mitosis is a process that makes sure each cell gets the same DNA ! • Mitosis occurs in SOMATIC CELLS (body cells) ex) liver cells, skin cells, lung cells • Somatic cells are DIPLOID (having the total number of chromosomes) Human diploid number = 46 • Mitosis begins w/ ONE diploid somatic cell • Mitosis ends w/ TWO diploid somatic cells that are identical ! • It is important that the cells made during Mitosis are identical so that the new cells made will do the same functions as the original cell ! The Cell Cycle • The cell cycle is the series of events a cell goes through during its lifetime • It includes 3 main parts : Interphase (G1, S phase, and G2), Mitosis, and Cytokinesis. • The following is a description of the events that occur during the cell cycle. http://www.iknow.net/CDROMs/cell_cdrom/cell3.html Interphase (not actually part of Mitosis, but included in the Cell Cycle) • G1 - Cell is doing what cell normally does (performing cell functions) • S phase - DNA Replication occurs • G2 - Cell prepares to divide (organelles are doubled in numbers) Prophase • Chromatin condenses into chromosomes Sister chromatids are exact centromere copies of a chromosome. The centromere is a belt-like structure that holds sister chromatids together. Sister chromatids • Nuclear membrane disappears • Centrioles begin to move to opposite ends of cell (aster begins to form) Metaphase • Sister chromatids line up along center (middle) of cell • Spindle fibers attach to each centromere Anaphase • Spindle fibers shorten • Each centromere breaks apart • One member of each sister chromatid pair is pulled toward each end of the cell Telophase • Nuclear membranes form around each set of chromosomes • Chromosomes uncoil into chromatin • The cell now has 2 nuclei and is ready to divide Cytokinesis • The cell divides to form two identical cells • Occurs after telophase • CYTOKINESIS - the actual, physical division of one cell into two cells • Cytokinesis is different in plants and animals (due to plants having a cell wall) QuickTime™ and a Animation decompressor are needed to see this picture.