The Cellular Basis of Reproduction and Inheritance 130 8.5 Cell division is a continuum of dynamic changes Figure 8.5 illustrates the cell cycle for an animal cell using micro- graphs, drawings (simplified to include just four chromosomes), and text. The micrographs show cells from a newt, with chromo- somes in blue and the mitotic spindle in green. Interphase is included, but the emphasis is on the dramatic changes that occur during cell division, the mitotic phase. Mitosis is a continuum, but biologists distinguish five main stages: prophase , prometaphase , metaphase , anaphase , and telophase . LM 250 G 1 G 2 S G 1 G 2 S Prophase Prometaphase Mitosis Interphase I NTERPHASE Prophase Prometaphase M ITOSIS Interphase Interphase is the period of cell growth, when the cell synthesizes new molecules and organelles. At the point shown here, during late interphase (G 2 ), the cell has doubled much of its earlier contents and the cytoplasm contains two centro- somes. Each centrosome has a pair of centrioles, can-shaped structures made of microtubules. Within the nucleus, the chromosomes are duplicated, but they cannot be distinguished individually because they are still in the form of loosely packed chromatin. Prophase During prophase, changes occur in both the nucleus and the cytoplasm. Within the nucleus, the chromatin fibers become more tightly coiled and folded, forming discrete chromosomes that can be seen with the light microscope. Each duplicated chromosome appears as two identical sister chromatids joined together, with a narrow “ waist ” at the centromere. In the cytoplasm, the mitotic spindle begins to form as microtubules rapidly grow out from the centrosomes, which begin to move away from each other. Prometaphase The nuclear envelope breaks into fragments and disappears. Microtubules emerging from the centrosomes at the poles (ends) of the mitotic spindle reach the chromo- somes, now highly condensed. At the centro- mere region, each sister chro matid has a protein structure called a kineto chore (shown as a black dot). Some of the spindle microtubules attach to the kinetochores, throwing the chromo- somes into agitated motion. Other spindle microtubules make contact with microtubules coming from the opposite pole. Forces exerted by protein “ motors ” associated with spindle microtubules move the chromosomes toward the center of the cell. Centrosomes (with centriole pairs) Centrioles Early mitotic spindle Fragments of the nuclear envelope Centromere Spindle microtubules Kinetochore Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Chromatin Centrosome M Figure 8.5 The stages of cell division by mitosis
131 The Eukaryotic Cell Cycle and Mitosis The chromosomes are the stars of the mitotic drama. Their movements depend on the mitotic spindle , a football-shaped structure of microtubules that guides the separation of the two sets of daughter chromosomes. The spindle microtubules emerge from two centrosomes , clouds of cytoplasmic material that in animal cells contain pairs of centrioles (see Module 4.16 for more information on centrioles). Centrosomes are also
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