the two groups of daughter chromosomes. The spiral chromatin
despiralizes, the nucleolus appears and mitosis is accompliched.
Cytokinesis. The preparation of cytokinesis begins as early as during
the late anaphase or during telophase. A concavity appears on the cell
membrane, a furrow is formed, gradually it wedges into the cell and divides
the cytoplasm. As a result, two daughter cells are formed. During
cytokinesis the cell organelles of newly formed cells are also formed.
Cytological pictures of mitotic division in plant and animal cell are given
in Figures 3–12 and 3–13.
image
Figure 3-12. Mitotic division of cells from the root tip of onion (Allium cepa). (After Dubinin, 1976). A — interphase: all chromosomes are despiralized; in phase S, i.e. synthesis of DNA, an autoreproduction of the chromosomes occurs. B, C, D, E — prophase of the mitosis: each chromosome represents two chromatids; a condensation of chromosomes occurs through their package and spirallization; the nuclear envelope disappear and the division spindle appears (E). F, G — metaphase: the chromosomes settle down on the equatorial plane; the two chromatids in each chromosome are still connected through centromeres. H, I — anaphase: separation of the new chromosomes (former chromatids) after dividing the centromeres. J — telophase: forming a cell barrier between the two daughter cells begins. K, L — formation of two daughter cells with equal sets of chromosomes.
image
Figure 3-13. Mitotic division in a typical animal cell (of fish). (Courtesy of J. D. Pickett-Heaps; From Alberts et al., 1986). (A) Interphase: the cell centre, where the centrioles are located, is small in size and separated from the nucleus. (B) Early prophase: the cell centre is doubled and approaches the nucleus; the number of microtubules coming out of it has increased. (C) Middle prophase: the two distant from each other stars settle down around the nuclear surface. (D) Prometaphase: the nuclear envelope is destroyed and the fibres of spindle already interact with the chromosomes. (E) Metaphase: the bipolar structure of the spindle is clearly seen; all chromosomes are settled down on its equatorial plane. (F) Anaphase: simultaneously (as “at the order”) chromatids move to the poles, carried by the fibres. (G) Early telophase: the chromatids are located on the poles, and the dividing furrow (shown by arrows) divides the spindle and shortens the distance between the chromatids. (H) Telophase: completed formation of the daughter nuclei, though they are still compact; almost completed cytokinesis; the residual small body is kept between the daughter nuclei.
Sometimes mitosis is realized with a slow cytokinesis characterized by multiple
division of the nucleus and after undefinite time a dividing of the cell occurs. Thus
multinuclear cells are formed, as are the cases with symplasts of plant and animal
organisms, HeLa cells, etc. This question will be considered further.