. Principi di biologia moderna. Biologia. CENTRIOLES ASTER- PROPHASES metafasi precoce tardivo. Membrana nucleare ri-formazione ANAPHASE TELOPHASES Fig. 3-2. Diagramma che illustra un concetto moderno di mitosi come osservato in una tipica cellula animale sistema di protoplasmic fibre, chiamati collettivamente il mandrino. I centri mitotico sono considerati come parti specializzate del citomegalovirus- plasm, situate alle estremità o pali, del mandrino, sebbene queste regioni a volte non sono molto chiaramente delineate, soprattutto nelle cellule vegetali. Le fibre verso ciascuna estremità del mandrino sempre convergenti, tuttavia, verso un sicuramente lo

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. Principles of modern biology. Biology. CENTRIOLES ASTER- PROPHASES METAPHASES EARLY LATE. NUCLEAR MEMBRANE RE-FORMING ANAPHASE TELOPHASES Fig. 3-2. Diagram depicting modern concept of mitosis as observed in a typical animal cell system of protoplasmic fibers, collectively called the spindle. The mitotic centers are regarded as specialized parts of the cyto- plasm, situated at the ends, or poles, of the spindle, although these regions sometimes are not very clearly delineated, especially in plant cells. The fibers toward each end of the spindle always converge, however, toward a definitely localized center. Moreover, in larger animal cells at least, radiating from each mitotic center there is a stellate system of protoplasmic fibers, collectively designated as the aster (Fig. 3-2). Also in most, if not all, animal cells, a pair of short rodlike bodies, the centrioles, can be demonstrated occupying a focal position in the mitotic center (Figs. 3-2 and 3-5). The centrioles are "self-replicating" struc- tures. They appear to focalize the dynamic activities of the mitotic center during the divi- sion of animal cells generally. The spindle always stretches between the centriolar re- gions, and the plane of division regularly cuts across the center of the spindle at right angles to the spindle axis (Fig. 3-2). More- over, the two short rodlike units that consti- tute each pair of centrioles always lie at right angles to one another. Each unit of the pair, as seen at high magnifications in favorable electronmicrographs, consists of a compact bundle of 27 fibers (perhaps tubules), ar- ranged concentrically in groups of three (Fig. 3-5). Sometimes the units of each pair of centriolar bodies replicate themselves very early, toward the end of one mitosis, in prepa- ration for the next; but in other cases replica- tion is delayed until early prophase of the next division. In any event, the sister pairs of centrioles, after replication has occurred, move to opposite s

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• bookcentury1900
• bookcollectionbiodiversity
• booksubjectbiology