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The Cell Cycle If we are the product of one fertilized egg, a single cell, how is it we have become living, functioning organisms of over a trillion cells? Without cell division, living organisms cannot grow and develop. Each new cell must have a complete nucleus with a complete copy of all genetic instructions (DNA chromosomes) The process which ensures this is called MITOSIS. Replication - Before mitosis can begin, the nucleus must make a copy of its chromatin so that there are two complete sets of DNA. We have 6 feet of DNA in every nucleus! The phases of Mitosis: End result: replication and then separation of DNA into two complete identical sets, one for each new nucleus. There are four major phases of mitosis: Prophase Metaphase Anaphase Telophase PROPHASE: Double stranded chromosomes are now large and dense enough to be visible with light microscope. Nucleus and nuclear membrane disappear. In animal cells, spindle fibres begin to form and stretch across the cell from centrioles. Centrioles move to opposite ends of the cell (no centrioles in plant cells) Spindle fibres attach to one side of each centromere METAPHASE Tugging action of spindle fibres pulls double-stranded chromosomes into a line across the middle of the cell ANAPHASE spindle fibres contract and shorten, pulling centromere apart, allowing one of each replicated strands to move to opposite ends of the cell. TELOPHASE final phase of mitosis one complete set of chromosomes at each pole of cell spindle fibres disappear nuclear membrane forms around each set of chromosomes chromosomes uncoil back into chromatin Two nuclei on one cell Mitosis is over, there is still only one cell but with two sets of genetic instructions. Cell Division: In animal cells, cell membrane pinches in near the middle of the cell, dividing cytoplasm into two new cells. This is call cytokinesis. In plant cells, a cell plate develops across the centre of the cell, forming a new cell wall between the two new cells. Interphase: Makes up most of the cells life Cell grows, replicates its DNA, and becomes prepared for the first phase of mitosis.