D

Figure 3.17 Continued. C. Cell (center) in anaphase. D. Cell (center) in telophase.

giving them the appearance of having extra knobs; these knobs are referred to as satellites. The constrictions at the base of the satellites have no known function, but the satellites themselves are useful in helping to distinguish certain chromosomes from others in a nucleus.

As prophase progresses, the nucleolus gradually becomes less distinct and eventually disintegrates. By the end of prophase, spindle fibers consisting of microtubules have developed; these spindle fibers extend in arcs between two invisible poles located toward the ends of the cell. The tips of the spindle fibers become anchored at the poles. Other spindle fibers grow from each pole to the center of the cell where they become attached to a centromere. At the conclusion of prophase, the nuclear envelope has been reabsorbed into the endoplasmic reticulum and has totally fragmented.

Figure 3.18 Diagram of a chromosome at metaphase. Spindle fibers from opposite ends of the cell become attached at the centromere.

Cells

In certain simpler organisms, such as fungi and algae, and in animal cells, the cytoplasm just outside the nucleus contains pairs of tiny keg-shaped structures called centrioles. The centrioles are surrounded by microtubules that radiate out from them and arrange cytoplasmic particles in the vicinity into starlike rays, each group of rays collectively called an aster. At the beginning of prophase, the aster divides into two parts; one part remains in its original location, while the other part migrates around the nuclear envelope to the opposite side. Centrioles have not been detected in the cells of most of the more complex members of the Plant Kingdom.

Metaph ase

The main feature of metaphase (Fig. 3.17B) is the alignment of the chromosomes in a circle midway between the two poles around the circumference of the spindle and in the same plane as that previously occupied by the preprophase band. This invisible circular plate is perpendicular to the axis of the spindle and is something like the equator of a globe.

As indicated in our discussion of prophase, spindle fibers can be seen in the area previously occupied by the nucleus after the nuclear envelope has disassociated. They form a structure that looks like an old-fashioned spinning top made of fine threads. Collectively, the spindle fibers are referred to as the spindle. The chromosomes become aligned so that their centromeres are in a plane roughly in the center of the cell. This invisible circular plate, called the equator, is analogous to the equator of the earth. At the end of metaphase, the centromeres holding the two strands (sister chromatids) of each chromosome together separate lengthwise.

Anaph ase

Anaphase—the briefest of the phases—involves the sister chromatids of each chromosome separating and appearing to be pulled to opposite poles (Fig. 3.17C).

Until the end of metaphase, the sister chromatids of each chromosome have been united at their centromeres. Anaphase begins with all the sister chromatids separating in unison and moving toward the poles. The chromatids, which after separation at their centromeres are called daughter chromosomes, appear to be pulled toward the poles as their spindle fibers gradually shorten. The shortening occurs as a result of material continuously being removed from the polar ends of the spindle fibers. The centromeres of the daughter chromosomes lead the way, with the chromosomes assuming V shapes as they appear to drag in the cytoplasm.

All of the chromosomes separate and move at the same time. Although experiments have shown that a chromosome will not migrate to a pole if the fiber attached to its centromere is severed, other experiments have shown that the chromosomes will separate from one another but not move to the poles, even if no spindle is present. The force or forces involved in this initial separation phenomenon have not yet been identified. It appears, however, that the main movement of the chromosomes to the poles results from a shortening of the spindle fibers.

Telophase

The five main features of telophase (Fig. 3.17D) are (1) each group of daughter chromosomes becomes surrounded by a reformed nuclear envelope; (2) the daughter chromosomes become longer and thinner and finally become indistinguishable; (3) nucleoli reappear; (4) many of the spindle fibers disintegrate; and (5) a cell plate forms.

The transition from anaphase to telophase is not distinct, but telophase is definitely in progress when elements of new nuclear envelopes appear around each group of daughter chromosomes at the poles. These elements gradually form intact envelopes as the daughter chromosomes return to the diffuse, indistinct threads seen at the onset of prophase. The new nucleoli appear on specific regions of certain chromosomes.

Phragmoplast

Figure 3.19 How a cell plate is formed. A. During telophase a phragmoplast (a complex of microtubles and endoplasmic reticu-lum) develops between the two daughter cell nuclei. The micro-tubules trap dictyosome vesicles along a central plane. B. The vesicles fuse into a flattened, hollow structure that becomes a cell plate. C. The cell plate grows outward; two primary cell walls and two plasma membranes form. When the cell plate reaches the mother cell walls, the plasma membranes unite with the existing plasma membrane, and the production of two daughter cells is complete.

Figure 3.19 How a cell plate is formed. A. During telophase a phragmoplast (a complex of microtubles and endoplasmic reticu-lum) develops between the two daughter cell nuclei. The micro-tubules trap dictyosome vesicles along a central plane. B. The vesicles fuse into a flattened, hollow structure that becomes a cell plate. C. The cell plate grows outward; two primary cell walls and two plasma membranes form. When the cell plate reaches the mother cell walls, the plasma membranes unite with the existing plasma membrane, and the production of two daughter cells is complete.

Chapter 3

middle primary cell 1 lamella cell wall cell 2

middle lamella cell wall middle primary cell 1 lamella cell wall cell 2

middle lamella cell wall

Images Middle Lamella Cell Wall

Figure 3.20 A. A diagram of two adjacent cells connected by a plasmodesma. B. A diagram of adjacent cells depicting the relative locations of the nucleus, endoplasmic reticulum, and a desmotubule. (A. © Biophoto Assoc/Photo Researchers, Inc.)

tubule plasmodesma

Figure 3.20 A. A diagram of two adjacent cells connected by a plasmodesma. B. A diagram of adjacent cells depicting the relative locations of the nucleus, endoplasmic reticulum, and a desmotubule. (A. © Biophoto Assoc/Photo Researchers, Inc.)

During telophase, the spindle microtubules gradually break down, and a set of shorter fibers (fibrils), composed of microtubules, develops in the region of the equator between the daughter nuclei. This set of fibrils, which appears somewhat keg-shaped, is called a phragmoplast. Dictyosomes produce small vesicles containing raw materials for the cell wall and membranes. Some of these vesicles, which resemble tiny droplets of fluid when viewed with a light microscope, are directed toward the center of the spindle (equator) by the remaining spindle fibers.

The microtubules apparently trap the dictyosome-derived vesicles, which then fuse together into one large, flattened but hollow structure called a cell plate (Fig. 3.19). Carbohydrates in the vesicles are synthesized into two new primary cell walls and a middle lamella. The middle lamella is shared by what now have become two new daughter cells. The cell plate grows outward until it contacts and unites with the plasma membrane of the mother cell. Plasmodesmata (minute strands of protoplasm that extend via tiny desmotubules through the walls between cells—Fig. 3.20) are formed apparently as portions of the endoplasmic reticulum are trapped between fusing vesicles of the cell plate.

New plasma membranes develop on either side of the cell plate as it forms, and new cell-wall materials are deposited between the middle lamella and the plasma membranes. These new walls are relatively flexible and remain so until the cells increase to their mature size. At that time, additional cellulose and other substances may be added, forming a secondary cell wall interior to the primary wall. In some instances, cell-plate formation does not accompany division of the nucleus.

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