The Cell Cycle

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Mark Huyler

The Cell Cycle  Four parts of the cycle  In eukaryotes, most dividing cells go through a cycle that consists of four phases  After chromosomes are copied during S phase, they are moved to the middle of the cell during M phase (mitosis). One chromosome copy is distributed to each of the two daughter cells. Mitosis and cytokinesis produce two cells that are genetically identical to the parents  Progression through the cell cycle is carefully controlled  In multicellular organisms, uncontrolled cell division may lead to cancer. Different types of cancer results from different types of defects in control over the cell cycle  Cells arise through the division of preexisting cells. There are two types of cell division: meiosis and mitosis  Both forms of cell division are usually accompanied by cytokinesis, in which the cytoplasm of the cell divides into two distinct daughter cells.  Meiosis leads to the production of gametes (eggs and sperm) o Daughter cells have half the amount of genetic material as the parent cell  Mitosis leads to the production of all other cell types, referred to as somatic cells o Genetic material contained in the chromosomes is copied and then divided equally o Daughter cells are genetically identical to the parent  Chromosomes contain a single long double helix of DNA wrapped around proteins o DNA encodes the cell’s genetic information o A gene is a section of DNA that encodes a specific protein or RNA  Chromosomes can be stained with dyes and observed under light microscopes  The purpose of mitosis is to distribute chromosomes to daughter cells during cell division o To this end, each chromosome is replicated prior to mitosis  As mitosis starts, the chromosomes condense from long, thin filaments into compact structures that can be moved around the cell efficiently  At the end of mitosis, one of the chromosome copies is distributed to each of the daughter cells Chromosome Replication  Prior to mitosis, each chromosome is replicated o Each of the DNA copies in a replicated chromosome is called a chromatid o Chromatids are joined together along their entire length as well as at specialized region of the chromosome called the centromere o Chromatids from the same chromosome are referred to as sister chromatids  Even though a replicated chromosome consists of two chromatids, it is still considered a single chromosome  Growing cells cycle between a dividing phase called the mitotic phase (M) phase and a non-dividing phase called interphase  The cell cycle is the orderly sequence of events that occurs from the formation of a eukaryotic cell through the duplication of its chromosome to the time it undergoes cell division  Chromosome replication occurs only during interphase and not during M phase o The stage in which DNA replication occurs is called the synthesis phase (S) Interphase – Gap Phases  Interphase also includes two gap phases, during which no DNA synthesis occurs o The first gap, G1 phase occurs before the S phase o The second gap, G2 phase, occurs between S phase and mitosis  During the gap phase organelles replicated and additional cytoplasm are made in preparation for cell division  It takes a cell about twenty-four hours to complete one cell cycle  G-zero holds terminally differentiating cells (neural/blood cells)  There are a total of four phases in the cell cycle: M phase and an interphase consisting of the G 1 S, and G 2 phases.  Gap phases allow t
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