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Figure 3-25 Fluorescent label of the nuclear spindle (green) and chromosomes (blue) in mitosis: (a) before the chromatids are pulled apart; (b) during the pulling apart. [From J. C. Waters, R. W. Cole, and C. L. Rieder, J. Cell Biol. 122, 1993, 361; courtesy of C. L. Rieder.]

Figure 3-25 Fluorescent label of the nuclear spindle (green) and chromosomes (blue) in mitosis: (a) before the chromatids are pulled apart; (b) during the pulling apart. [From J. C. Waters, R. W. Cole, and C. L. Rieder, J. Cell Biol. 122, 1993, 361; courtesy of C. L. Rieder.]

3. Anaphase: The sister chromatids are pulled to opposite ends of the cell by microtubules that attach to the centromeres. The microtubules are part of the nuclear spindle, a set of parallel fibers running from one pole of the cell to the other.

Nuclear spindle fibers provide the motive force that pulls apart the chromosomes or chromatids in mitosis and meiosis (Figure 3-25). In nuclear division, spindle fibers form parallel to the cell axis, connected to one of the cell poles. These spindle fibers are polymers of a protein called tubulin. Each centromere acts as a site to which a multiprotein complex called the kinetochore binds (Figure 3-26). The kinetochore acts as the site for attachment to spindle fiber microtubules. From one to many microtubules from one pole attach to one kinetochore, and a similar number from the opposite pole attach to the kinetochore on the homologous chromatid. Although the microtubules of the spindle appear like ropes, their action is not ropelike. Instead the tubulin depolymerizes at the kinetochores, shortening the microtubule and thereby pulling the sister chromatids apart (Figure 3-27). Later, the chromatids are further separated by the action of molecular motor proteins acting on another set of microtubules not connected to the kinetochore but running from pole to pole. The spindle apparatus and the complex of kinetochores and centromeres are what determine the fidelity of nuclear division.

Early anaphase

Pole

Early anaphase

Pole

Kinetochore Microtubules

Kinetochore microtubules

Kinetochore microtubules depolymerize at kinetochore ends, and kinetochores move toward poles

Kinetochore microtubules

Kinetochore microtubules depolymerize at kinetochore ends, and kinetochores move toward poles

Chromatids

Centromeric chromatin Fibrous corona

Microtubule

Kinetochore

Outer plate

Outer plate

Inner plate

Figure 3-26 Microtubule attachment to the kinetochore.

Microtubules are attached to the kinetochore at the centromere region of the chromatid in animal cells. The kinetochore is composed of an inner and outer plate and a fibrous corona. [Adapted from A. G. Pluta et al., Science 270, 1995, 1592; taken from H. Lodish, A. Berk, S. L. Zipursky, P. Matsudaira, D. Baltimore, and J. Darnell, Molecular Cell Biology, 4th ed. Copyright 2000 by W. H. Freeman and Company.]

Kinetochore Fibrous Corona

Region of depolymerization

Figure 3-27 Microtubule action. The microtubules exert pulling force on the chromatids by depolymerizing into tubulin subunits at the kinetochores. [Adapted from G. J. Gorbsky, P. J. Sammak, and G. Borisy, J. Cell Biol. 104, 1987, 9; and G. J. Gorbsky, P. J. Sammak, and G. Borisy, J. Cell Biol. 106, 1988, 1185; modified from H. Lodish, A. Berk, S. L. Zipursky, P. Matsudaira, D. Baltimore, and J. Darnell, Molecular Cell Biology, 4th ed. Copyright 2000 by W. H. Freeman and Company.]

Region of depolymerization

Figure 3-27 Microtubule action. The microtubules exert pulling force on the chromatids by depolymerizing into tubulin subunits at the kinetochores. [Adapted from G. J. Gorbsky, P. J. Sammak, and G. Borisy, J. Cell Biol. 104, 1987, 9; and G. J. Gorbsky, P. J. Sammak, and G. Borisy, J. Cell Biol. 106, 1988, 1185; modified from H. Lodish, A. Berk, S. L. Zipursky, P. Matsudaira, D. Baltimore, and J. Darnell, Molecular Cell Biology, 4th ed. Copyright 2000 by W. H. Freeman and Company.]

4. Telophase: Chromatids have arrived at the poles and the pulling-apart process is complete. A nuclear membrane reforms around each nucleus, and the cell divides into two daughter cells. Each daughter cell inherits one of each pair of sister chromatids, which now become chromosomes in their own right.

Thus overall, the main events of mitosis are replication and sister chromatid adhesion, followed by segregation of the sister chromatids into each daughter cell. In a diploid cell, for any chromosomal type the number of copies goes from 2 :4 : 2. A full description of mitosis in a plant is given for reference in Figure 3-28.

Even though early investigators did not know about DNA or that it is replicated during interphase, it was still evident from observing mitosis under the microscope that mitosis is the way in which the chromosome number is maintained during cell division. But the sex cycle still presented a puzzle. Two gametes join in the fertilization event. The early investigators knew that in this process two nuclei fuse but that the chromosome number of the fusion product nevertheless is the stan dard for that species. What prevents the doubling of the chromosome number at each generation? This puzzle was resolved by the prediction of a special kind of nuclear division that halved the chromosome number. This special division, which was eventually discovered in the gamete-producing tissues of plants and animals, is called meiosis. A simplified representation of meiosis is shown in the lower panel of Figure 3-24.

Meiosis is the general name given to two successive nuclear divisions called meiosis I and meiosis II. Meiosis takes place in special diploid cells called meiocytes. Because of the two successive divisions, each meiocyte cell gives rise to four cells, 1 cell: 2 cells: 4 cells. The four cells are called products of meiosis. In animals and plants, the products of meiosis become the haploid gametes. In humans and other animals, meiosis takes place in the gonads, and the products of meiosis are the gametes—sperm (more properly, spermatozoa) and eggs (ova). In flowering plants, meiosis takes place in the anthers and ovaries, and the products of meiosis are meiospores, which eventually give rise to gametes.

Before meiosis, an S phase duplicates each chromosome's DNA to form sister chromatids, just as in mitosis.

Telophase. A nuclear membrane re-forms around each daughter nucleus, the chromosomes uncoil, and the nucleoli reappear-all of which effectively re-form interphase nuclei. By the end of telophase, the spindle has dispersed, and the cytoplasm has been divided into two by a new cell membrane.

Anaphase. The pairs of sister chromatids separate, one of a pair moving to each pole. The centromeres, which now appear to have divided, separate first. As each chromatid moves, its two arms appear to trail its centromere; a set of V-shaped structures results, with the points of the V's directed at the poles.

Telophase. A nuclear membrane re-forms around each daughter nucleus, the chromosomes uncoil, and the nucleoli reappear-all of which effectively re-form interphase nuclei. By the end of telophase, the spindle has dispersed, and the cytoplasm has been divided into two by a new cell membrane.

Anaphase. The pairs of sister chromatids separate, one of a pair moving to each pole. The centromeres, which now appear to have divided, separate first. As each chromatid moves, its two arms appear to trail its centromere; a set of V-shaped structures results, with the points of the V's directed at the poles.

Telophase Lilium Regale

Early prophase. The chromosomes become distinct for the first time. They get progressively shorter through a process of contraction, or condensation, into a series of spirals or coils; the coiling produces structures that are more easily moved.

Late prophase. As the chromosomes become visible, they appear double-stranded, each chromosome being composed of two longitudinal halves called chromatids. These "sister" chromatids are joined at the centromere. The nucleoli-large intranuclear spherical structures-disappear at this stage. The nuclear membrane begins to break down, and the nucleoplasm and cytoplasm become one.

4 Mitotic metaphase

Metaphase. The nuclear spindle becomes prominent. The spindle is a birdcage-like structure that forms in the nuclear area; it consists of a series of parallel fibers that point to each of two cell poles. The chromosomes move to the equatorial plane of the cell, where the centromeres become attached to a spindle fiber from each pole.

Early prophase. The chromosomes become distinct for the first time. They get progressively shorter through a process of contraction, or condensation, into a series of spirals or coils; the coiling produces structures that are more easily moved.

Late prophase. As the chromosomes become visible, they appear double-stranded, each chromosome being composed of two longitudinal halves called chromatids. These "sister" chromatids are joined at the centromere. The nucleoli-large intranuclear spherical structures-disappear at this stage. The nuclear membrane begins to break down, and the nucleoplasm and cytoplasm become one.

4 Mitotic metaphase

Metaphase. The nuclear spindle becomes prominent. The spindle is a birdcage-like structure that forms in the nuclear area; it consists of a series of parallel fibers that point to each of two cell poles. The chromosomes move to the equatorial plane of the cell, where the centromeres become attached to a spindle fiber from each pole.

Figure 3-28 Mitosis. The photographs show nuclei of root-tip cells of Lilium regale. [After J. McLeish and B. Snoad, Looking at Chromosomes. Copyright 1958, St. Martin's, Macmillan.]

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