Study Guide for Cell Cycle Cell Division Notes: There are 3 main reasons why cells need to divide: 1. To replace worn out cells 2. To replace damaged cells 3. Growth and Development Before a cell can divide, it must do two things: 1. Copy DNA 2. Copy Organelles But why? So that each new cell has a complete set of DNA and organelles Organisms with prokaryotic cells (_bacteria_) do a very simple process of dividing called __binary _fission__. (draw below) However, eukaryotic organisms (_like us!_) go through a much more complicated process called the _cell_cycle_, which is the dividing of non-sex cells. Sex cells divide using a different process called _meiosis_. We can divide this process into _3_ main steps: o _interphase_- cell grow and DNA is copied (90% of time spent here) o _mitosis_- (4 steps) steps in which the nucleus’s DNA actually divides o _cytokinesis_- cytoplasm splits and cell divides making two daughter cells In between each phase, there are _ checkpoints__ to make sure that the cell has done everything it needs to do correctly. If a mistake has happened, the cell usually _fixes_ the mistake, but if it does not catch the mistake, the cell could _malfunction_ (not work correctly). If cells do not divide correctly, _mutations_ or _cancer_ can occur. Steps of the Cell Cycle 1st: Interphase Chromosomes appear as threadlike coils (chromatin) at the start, but then coil up making chromosomes, then are copied making sister chromatids. 2nd: Prophase Mitosis begins Centrioles appear and begin to move to opposite ends Spindle fibers form between poles Nuclear membrane dissolves away 3rd: Metaphase Chromatids (or pairs of chromosomes) attach to the spindle fibers and line up in the middle of the cell. 4th: Anaphase Chromatids (or pairs of chromosomes) separate and begin to move to opposite ends of the cell. 5th: Telophase Two new nuclei form Chromosomes are starting to uncoil back to chromatin Cell membrane starts to pinch inward Mitosis Ends 6th: Cytokinesis Cell membrane completely moves inward to create two new daughter cells- each with its own nucleus with identical chromosomes.