Cell Division and Mitosis

  1. Dividing Cells: The Bridge Between Generations

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    1. Overview of Division Mechanisms
      1. Before cells are able to reproduce, there must be a division of the nucleus and its DNA.
      2. Mitosis and meiosis are eukaryotic nuclear division mechanisms that lead to the distribution of DNA to new nuclei in forthcoming daughter cells.
        1. Mitosis is used by multicelled organisms for growth by repeated divisions of somatic cells.
        2. Meiosis occurs only in germ cells that divide to form gametes.
    2. Some Key Points About Chromosomes
      1. Each chromosomes is a molecule of DNA complexed with proteins.
      2. Prior to division, each threadlike chromosome is duplicated to form two sister chromatids held together by a centromere.
      3. The centromere is also the region where the duplicated chromosome will attach to the microtubules of the spindle during nuclear division.
    1. Mitosis and the Chromosome Number
      1. Each species has a characteristic chromosome number (for example: human somatic cells contain 46 chromosomes).
      2. Chromosomes exist as pairs: one member of each pair from each parent.
        1. Somatic cells are diploid; that is, they have two of each type of chromosome.
        2. Germ cells (egg and sperm) are haploid—only one chromosome of each type.
      3. Mitosis maintains the chromosome number of the species through all the divisions of development, growth, and repair.
  1. The Cell Cycle
    1. The cell cycle is a recurring sequence of events that extends from the time of a cell’s formation until each division is completed.
    2. Most of a cell’s existence (about 90 percent) is spent in interphase; mitosis occupies only a small part.
      1. During interphase the cell’s mass increases, the cytoplasmic components approximately double in number, and the DNA is duplicated (S).
      2. Some cells are arrested in interphase and never divide again (example: brain cells).

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  1. Stages of Mitosis—An Overview
    1. The four sequential stages of mitosis are: prophase, metaphase, anaphase, and telophase.
    2. The spindle apparatus moves the chromosomes.
      1. The spindle apparatus is composed of two sets of microtubules.
      2. Microtubules (components of the cytoskeleton) extend from the two "poles" of the cell and overlap at the cell equator.
    3. Prophase: Mitosis Begins
      1. Chromosomes become visible as rodlike units, each consisting of two sister chromatids.
      2. In the cytoplasm, the microtubules of the cytoskeleton break apart and begin reassembling near the nucleus.
        1. Microtubules are composed of numerous subunits called tubulins.
        2. Some microtubules extend from the centromeres to one of the two poles; others extend from the poles, overlap in the middle of the cell, but do not contact the chromosomes.
      3. The nuclear envelope begins to disintegrate.
      4. The centrioles, which have duplicated by the time prophase is underway, are now moved by the microtubules to the opposite poles of the cell
    4. Transition to Metaphase
      1. The nuclear membrane now breaks up completely in the transition between pro- and metaphase.
      2. Sister chromatids, each attached to microtubules, become oriented toward opposite poles.
      3. When all the chromosomes are aligned at the cell’s equator, halfway between the poles, we call the stage metaphase.
    5. From Anaphase Through Telophase
      1. Sister chromatids separate and move toward opposite poles.
        1. Microtubules attached to the centromeres shorten and pull the chromosomes toward the poles.
        2. Other microtubules at the spindle poles ratchet past each other to push the two spindle poles apart.
        3. Once separated, each chromatid is now an independent chromosome.
      2. Telophase begins when the two daughter chromosomes of each original chromatid pair arrive at opposite poles.
        1. Chromosomes return to the threadlike form typical of interphase.
        2. The nuclear envelope reforms from the fusion of small vesicles
        3. Each daughter cell has the same number of chromosomes as the parent cell; mitosis is complete.
  2. A Closer Look at the Cell Cycle
    1. The Wonder of Interphase
      1. During G1, most of the carbohydrates, lipids, and proteins for a cell’s own use and for export are assembled.
      2. During the S phase, the cell copies its DNA and synthesizes proteins used in organizing the condensed chromosomes.
      3. During G2, the proteins that will drive mitosis to completion are produced.
    2. Chromosomes, Microtubules, and the Precision of Mitosis.
      1. Metaphase chromosomes are highly organized.

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        1. Proteins called histones tightly bind to DNA and cause spooling into a structural unit called a nucleosome.
        2. By late prophase, each chromatid has a constricted region called the centromere, on the surface of which is the kinetochore—a docking site for the spindle microtubules.
        3. To prevent tangling of the DNA molecules, an enzyme called DNA topoisomerase recognizes possible tangling and corrects it.
      1. The microtubules of the spindle are continually assembling and disassembling.
        1. At anaphase, the kinetochores slide over the microtubules on their way to the poles; the microtubules shorten behind them by disassembling themselves.
        2. Two proteins, dynein and kinesin, drive the sliding motion.
  1. Division of the Cytoplasm
    1. Cell Plate Formation in Plants
      1. Because of the rather rigid cell wall, the cytoplasm of plant cells cannot just be pinched in two.
      2. Instead vesicles containing remnants of the microtubular spindle form a disklike structure during cell plate formation.
    2. Cleavage of Animal Cells
      1. The flexible plasma membrane of animal cells can be squeezed in the middle to separate the two daughter cells—a process called cleavage.
      2. Parallel arrays of contractile microfilaments slide past one another at the cleavage furrow, pulling the plasma membrane inward.