The Cell Cycle and Mitosis

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The Cell Cycle and Mitosis

1. The eukaryotic cell cycle
2. Regulation of the cell cycle
3. Mitosis an overview
4. Stages of mitosis
5. The spindle apparatus
6. Cytokinesis

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A. The Eukaryotic Cell Cycle

• Eukaryotic cell cycle
• A series of stages through which most eukaryotic
  cells pass during some time in their development
• In single-celled eukaryotes, the cell cycle often
  represents the major mechanism for asexual
  reproduction of the species
• In multicellular eukaryotes, the cell cycle is
  used for cellular reproduction in embryonic cells
  and stem cells

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A. The Eukaryotic Cell Cycle

• Key events of the cell cycle
• Interphase a period of extensive metabolic
  synthetic activity
• During this time the DNA (chromosomes) of the
  cell are replicated
• Mitosis the nucleus of the cell divides to form
  two identical nuclei, with each new nucleus
  having the same number of chromosomes as the
  parent nucleus
• Cytokinesis in most cases (with some
  exceptions), the cells cytoplasm divides to form
  two cells, each containing one of the new nuclei

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A. The Eukaryotic Cell Cycle

• Stages of the cell cycle
• G1 (gap-1) is the period before chromosomal
  replication takes place
• S (synthesis) is the period when the chromosomes
  are replicated
• G2 (gap-2) is the period following chromosomal
  replication
• M (mitosis) is the period of nuclear division

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A. The Eukaryotic Cell Cycle

• G1, S, and G2 are collectively called
  interphase, characterized by
• The presence of a well-defined nucleus
• Individual chromosomes are not visible
• Chromatin chromosomes are dispersed in an
  uncondensed state called chromatin Grainy or
  sandy appearance
• Nucleolus a dark-staining spot sometimes (but
  not always) seen in the interphase nucleus The
  site of ribosome synthesis

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B. Regulation of the Cell Cycle

• In multicellular eukaryotes, dividing cells are
  found
• In embryonic tissue
• In certain tissues that are constantly renewed,
  such as bone marrow stem cells or plant
  meristems
• In damaged tissue, such as cell division in
  damaged liver

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B. Regulation of the Cell Cycle

• Go (stationary) cells cells that have reached
  terminal differentiation and no longer divide
• Cells stop dividing before they enter S phase
  Therefore, Go is similar in some respects to G1
  except that the cell does not enter S phase
• The cell cycle is regulated by several types of
  regulatory proteins that act at two major points
  the G1 S junction, and the G2 M junction

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C. Mitosis An Overview

1. During G1 each chromosome is uncondensed and in
   an unreplicated state.

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C. Mitosis An Overview

1. In the S phase, each chromosome is replicated.
   The replicated chromosome consists of two sister
   chromatids attached at the centromere.

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C. Mitosis An Overview

1. At the beginning of mitosis, the chromosomes
   condense and become visible. Note that the
   chromosome is still in a replicated state, with a
   pair of chromatids attached at the centromere.

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C. Mitosis An Overview

1. During mitosis the centromere splits, the
   chromatids separate and become daughter
   chromosomes, and the daughter chromosomes move to
   opposite sides of the cell.

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C. Mitosis An Overview

1. At the end of mitosis, the chromosomes on each
   side of the cell decondense and form two new,
   identical nuclei.

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D. Stages of Mitosis

• Prophase
• The nuclear membrane and nucleolus disperse
• The chromatin condenses to form visible
  chromosomes
• The mitotic spindle apparatus begins to form

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D. Stages of Mitosis

• Metaphase
• The chromosomes line up along the equator of the
  spindle apparatus
• Some writers use the term prometaphase to
  represent the transitional state between prophase
  metaphase
• Anaphase
• The centromeres split and the daughter
  chromosomes move to opposite poles of the spindle
  apparatus

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D. Stages of Mitosis

• Telophase
• At each pole of the spindle, the chromosomes
  decondense and return to the state of chromatin
• New nuclear membranes and nucleoli form at each
  spindle pole, completing the formation of two new
  identical nuclei

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E. The Spindle Apparatus

• Composed of microtubules fibers that are
  dynamically assembled from the protein tubulin
• Centrosomes
• Found at the poles of the spindle in animal cells
• Each centrosome consists of two centrioles short
  tubulin structures that help to regulate tubulin
  polymerization
• In plant cells plant cells have
  microtubule-organizing centers (instead of
  centrosomes) at each pole

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E. The Spindle Apparatus

• Some fibers are attached to chromatids
• Each attached fiber is connected to the
  kinetochore of the chromatid a region near (or
  at) the centromere
• The movement of chromatids is accomplished by
  shortening of the kinetochore fibers
• This is done by depolymerization of the
  microtubules at the spindle poles

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E. The Spindle Apparatus

• Chemicals that inhibit microtubule formation
  (such as colchicine) cause mitosis to stop at
  metaphase
• Colchicine and similar chemicals are used to
  arrest mitosis at metaphase for the purpose of
  chromosomal analysis (karyotyping)

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F. Cytokinesis

• In animal cells
• Cytokinesis is accomplished by the formation of
  the cleavage furrow
• A contractile ring or girdle of actin fibers
  and myosin (a transient muscle-like assembly)
  forms just inside the plasma membrane, around the
  equatorial plate
• The contractile ring pulls the plasma membrane
  together to pinch the cell into two cells

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F. Cytokinesis

• In plant cells
• Cytokinesis is accomplished by the formation of
  the cell plate
• Cell plate A new cell wall forming at the
  equatorial plate
• The cell plate enlarges until it completely
  separates the cell into two cells