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The cell cycle multiplies cells


									9.2 The cell cycle multiplies
• Objectives
• Describe the structure of a chromosome.
• Name the stages of the cell cycle and explain what
  happens during each stage.
• Key Terms
   –   chromatin
   –   chromosome
   –   sister chromatid
   –   centromere
   –   cell cycle
   –   interphase
   –   mitotic phase
   –   mitosis
   –   cytokinesis
• At this moment, millions of cells in your
  body are dividing, each forming two new
  cells. However, the vast majority of your
  cells (about 200 trillion) aren't dividing but
  are going about other cell activities—
  building proteins, breaking down food,
  consuming energy, and so on. How does
  cell division fit into the life of a cell?
• Chromosomes and Cell Division
  Almost all the genes of a eukaryotic cell are
  located in the cell nucleus. Most of the time, this
  genetic material exists as a mass of very long
  fibers that are too thin to be seen under a light
  microscope. These fibers consist of chromatin, a
  combination of DNA and protein molecules. As a
  cell prepares to divide, its chromatin fibers
  condense, becoming visible as the compact
  structures called chromosomes.
• The number of chromosomes in a
  eukaryotic cell depends on the species.
  For example, human body cells generally
  each have 46 chromosomes. Each
  chromosome may contain many hundreds
  of genes.
• Before cell division begins, a cell
  duplicates all of its chromosomes. Each
  chromosome now consists of two identical
  joined copies called sister chromatids.
  (Biologists use the term "sister" to identify
  joined chromatids, but it does not imply
  female gender.) The region where the two
  chromatids are joined tightly together is
  called the centromere.
• Each chromosome in a reproducing
  cell undergoes the duplication and
  separation process shown here.
  Duplication occurs during interphase,
  in preparation for cell division.
• The Cell Cycle
  How often a cell divides depends on the type of
  cell. Some cells divide once a day. Some divide
  more often; others, less often. Some highly
  specialized cells, such as mature muscle cells,
  do not divide at all. Eukaryotic cells that do
  divide undergo an orderly sequence of events
  known as the cell cycle. The cell cycle extends
  from the "birth" of a cell as a result of cell
  reproduction to the time the cell itself reproduces
• Interphase The cell may spend as much as 90
  percent of the cell cycle in interphase.
  Interphase is the stage during which a cell
  carries out its metabolic processes and performs
  its functions. For example, a cell in your stomach
  lining might be making and releasing enzyme
  molecules that help digest your food. During
  interphase, a cell increases its supply of
  proteins, increases the number of many of its
  organelles (such as mitochondria and
  ribosomes), and grows in size.
• Interphase also includes cellular functions
  leading up to cell division.
• One key event is the duplication of the DNA in
  the cell's chromosomes. This period is called the
  S phase (S stands for DNA synthesis).
• The interphase periods before and after the S
  phase are called the G1 and G2 phases (G
  stands for gap). During the G2 phase, each
  duplicated chromosome remains loosely packed
  as chromatin fibers. The cell is now ready to
  begin mitosis.
• Mitotic Phase The stage of the cell cycle when
  the cell is actually dividing is called the mitotic
  phase (M phase). The mitotic phase includes
  two processes, mitosis and cytokinesis
• During mitosis, the nucleus and the duplicated
  chromosomes divide and are evenly distributed,
  forming two "daughter" nuclei.
• Cytokinesis is the process by which the
  cytoplasm is divided in two. Cytokinesis usually
  begins before mitosis is completed..
• The combination of mitosis and
  cytokinesis produces two genetically
  identical daughter cells since the
  chromosomes were duplicated precisely in
  the S phase.
• Each daughter cell has a single nucleus,
  some surrounding cytoplasm, and a
  plasma membrane
• This diagram of the cell cycle, showing
  only one chromosome, highlights the
  mitotic phase (M). The duplicated
  chromosomes are separated during
  mitosis and distributed into daughter cells
  that form through cytokinesis. (The term
  "daughter" refers to offspring cells and
  does not imply female gender.)
• Mitosis is a very accurate way of
  distributing identical copies of a large
  amount of genetic material to two daughter
  cells. Experiments with yeast cells, for
  example, indicate that an error in
  chromosome distribution occurs only once
  in about 100,000 cell divisions. Mitosis is
  unique to eukaryotes

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