Chap 12 The Cell Cycle

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Inquiry:
During anaphase, do kinetochore microtubules shorten at their spindle pole ends or their kinetochore ends? The microtubules of a cell in early anaphase were labeled with a fluorescent dye that glows in the microscope (yellow). EXPERIMENT

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 EXPERIMENT 
A Laser was used to mark the kinetochore mircotubles by eliminating the fluorescnce in a region between one spindle pole and the chromosomes. As anaphase proceeded, researches monitored the changes in the lengths of the microtubles on either side of the mark. RESULTS

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 RESULTS 
As the chromosomes moved toward the poles, the microtubule segments on the kinetochore side of the laser mark shortened, while those on the spindle pole side stayed the same length. CONCLUSION

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 CONCLUSION 
During anaphase, the spindle fibers shorten by depolymerizing and losing tubulin subunits at the kinetochore. A motor protein then moves the kinetochore and its attached chromosome toward the spindle pole.

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Bacterial cell division (binary fission) Chromosome replication begins at the origin of replication where replication begins. One copy of the origin moves toward the other end of the cell; DNA is deuplicated.
When one copy of the origin reaches the other end of the cell, replication is complete.
The plasma membrane grows inward, and new cell wall is deposited.
Two daughter cells result.

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Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism. Growth and development. This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells. Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells.

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  • The average eukaryotic cell is about 100 μm (100 x 10-6 m. = 10-4 m.) long.
  • You have 200 trillion (2 x 1014) somatic cells (p. 220).
  • Total length of your cells lined up end-to-end: (2 x 1014) x (10-4 m.) = 2 x 1010 m. = 2 x 107 km.
  • The earth's circumference is 40 million m. (40 x 106 m. = 4 x 107 m. = 4 x 104 km.
  • 2 x 107 km. / 4 x 104 km. = 500: your cells can wrap around the earth 500 times.

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    To prepare for cell division, DNA is replicated and each duplicated chromosome has 2 sister chromatids, attached by a centromere.
    The chromatids separate during cell division.

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    Eukaryotic chromosomes condense and become visible during cell diviison.

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    The cell cycle. The mitotic (M) phase alternates with interphase, which is composed of 2 growth phases (G1 and G2) and a S (DNA synthesis) phase. In the M phase, mitosis divides the nucleus, and cytokinesis divides the cytoplasm, producing 2 daughter cells.

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    Cytokinesis in animal cells. In animal cells, a cleavage furrow forms, pinching the plasma membrane. Then a contractile ring of microfilaments splits the cell.

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    Cytokinesis in plant cells. In plant cells, vesicles containing cell wall material fuse to form a cell plate. The cell plate grows and fuses with the existing cell wall, dividing the cell in two.

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    DNA in nondividing eukaryotic cells (far left and far right) is complexed with proteins in dispersed fibers called chromatin. The chromatin condenses into visible chromosomes during cell division in a process called mitosis.

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    Anaphase. The 2 sister chromatids part, moving toward opposite ends of the cell, Each chromatid is now a full-fledged chromosome. The kinetochore microtubules shorten, while nonkinetochore microtubules lengthen, stretching the cell.

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    G2 of Interphase.

    1. A nuclear envelope bounds the nucleus.
    2. The nucleus contains one or more nucleoli (singular, nucleolus). where ribosomes are synthesized.
    3. Two centrosomes have formed by replication of a single centrosome.
    4. In animal cells, each centrosome features two centrioles.
    5. Chromosomes, duplicated during S phase, cannot be seen individually because they have not yet condensed.

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    Metaphase. The chromosomes convene on the metaphase (equatorial) plate. For each chromosome, the kinetochores of the sister chromatids are attached to kinetochore microtubules coming from opposite poles.

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    Prometaphase. The nuclear envelope fragments. Each of the 2 chromatids has a kinetochore, which attach the chromatid to the spindle microtubules at the centromere.

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    Prophase. The chromatin fibers condense into discrete chromosomes observable with a light microscope. The nucleoli disappear. The mitotic spindle, composed of microtubules, begins to form.

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    Telophase. Two daughter nuclei begin to form in the cell. Nuclear envelopes reform. The chromosomes become less condensed. Mitosis, the division of one nucleus into two genetically identical nuclei, is now complete.

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    Stages of mitosis. From the G2 subphase of Interphase, the dividing cell proceeds through prophase to prometaphase. continue

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    Stages of mitosis continued. A dividing cell continues through metaphase, anaphase, and finally telophase and cytokinesis.