9.3 Cells Divide during the Mitotic phase

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9.3 Cells Divide during the Mitotic phase
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• Objectives
• Summarize the major events that occur during each
  phase of mitosis.
• Explain how cytokinesis differs in plant and
  animal cells.
• Key Terms
• spindle
• centrosome
• prophase
• metaphase
• anaphase
• telophase
• cell plate

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• You can think of mitosis as a lively "dance" of
  the chromosomes. Before the action begins, the
  chromatin of each chromosome doubles during
  interphase. Then the elaborately "choreographed"
  stages of the mitotic phase take place rapidly,
  distributing the duplicate sets of chromosomes to
  two daughter nuclei. Finally, cytokinesis divides
  the cytoplasm, producing two daughter cells.

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• The Mitosis Dance During mitosis, the
  chromosomes' movements are guided by a
  football-shaped framework of microtubules called
  the spindle. The spindle microtubules grow from
  two centrosomes, regions of cytoplasmic material
  that in animal cells contain structures called
  centrioles.

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• Although mitosis is a continual process,
  biologists divide the mitotic phase into four
  main stages in order to study it prophase,
  metaphase, anaphase, and telophase

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• Mitosis begins after the chromosomes have
  duplicated in interphase and ends when telophase
  is completed

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• Prophase In prophase, the first stage of mitosis,
  the chromosome "dancers" make their appearance on
  the dance floor.
• In the nucleus, the chromatin fibers have
  condensed
• Each chromosome can be clearly seen now to
  consist of a pair of sister chromatids joined at
  the centromere.
• The nucleolus disappears, and the cell stops
  making ribosomes.
• Late in prophase, the nuclear envelope breaks
  down.
• Meanwhile, in the cytoplasm, a football-shaped
  structure called the mitotic spindle forms.
• The chromatids now attach to the microtubules
  that make up the spindle.

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• The spindle starts tugging the chromosomes toward
  the center of the cell for the next step in the
  dance.

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• Metaphase During metaphase, the brief second
  stage, the chromosomes all gather in a plane
  across the middle of the cell. The mitotic
  spindle is now fully formed.
• All the chromosomes are attached to the spindle
  microtubules, with their centromeres lined up
  about halfway between the two ends, or poles, of
  the spindle.

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• Anaphase Anaphase is the third stage of the
  mitosis dance. The sister chromatids suddenly
  separate from their partners. Each chromatid is
  now considered a daughter chromosome. Proteins at
  the centromeres help move the daughter
  chromosomes along the spindle microtubules toward
  the poles.
• At the same time, these microtubules shorten,
  bringing the chromosomes closer to the poles.
  However, spindle microtubules that are not
  attached to centromeres do just the oppositethey
  grow longer, pushing the poles farther apart.

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• Telophase and Cytokinesis The final stage of
  mitosis, telophase, begins when the chromosomes
  reach the poles of the spindle.
• The spindle disappears, two nuclear envelopes
  reform (one around each set of daughter
  chromosomes), the chromosomes uncoil and
  lengthen, and the nucleoli reappear.
• Mitosis, the division of one nucleus into two
  genetically identical daughter nuclei, is now
  finished.
• Cytokinesis completes the cell division process
  by dividing the cytoplasm into two daughter
  cells, each with a nucleus. Usually this process
  occurs along with telophase.

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• Cytokinesis in a plant cell occurs differently.
  A disk containing cell wall material called a
  cell plate forms inside the cell and grows
  outward. Eventually this new piece of cell wall
  divides the cell in two. The result is two
  daughter cells, each bounded by its own
  continuous membrane and its own cell wall.

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