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PinkMonkey Online Study Guide-Biology

(4) Telophase : This is the last phase in karyokinesis. The two sets of daughter chromosomes reach the opposite poles. The chromosomes again become long and thin. A new nucleolus is organized. A Nuclear envelope is formed around each set of chromosomes. In other words, a daughter nucleus is organized at each pole in the parent cell. Each daughter nucleus has the same number of chromosomes as that of the mother cell. The spindle fibers also dissolve and disappear gradually.

The two daughter nuclei are identical in structure and characters. They are also exact copies of the original parent nucleus.

Cytokinesis : The division of the cell cytoplasm is called cytokinesis. It starts towards the end of telophase.

In plant cells, cytokinesis usually begins with centrifugal formation of a cell plate along the equatorial plane which is followed by a new wall formation. This divides the mother cell into two equal daughter cells.

In animal cells, cytokinesis takes place by the cleavage constriction of the cell cytoplasm. It begins peripherally and progresses centripetally.

Figure 6.3 Aster and the mitotic apparatus
in an animal cell

Astral and Anastral mitosis. In the cell of higher animals and lower plants, the centriole is present just outside the nucleus. It plays a definite role in mitosis. During interphase, the centriole divides into two. The two centrioles then move to the opposite poles during prophase and later on help to organize the bipolar spindle body during the early metaphase.

From each centriole at the pole, radiating fibers extend into the cytoplasm. These are called astral rays and form the aster (Figure 6.3).

Astral mitosis : "The mitosis in which asters are formed from the centrioles is called astral mitosis."

Anastral mitosis : The centriole is absent in the cells of the higher plants and some animals. In such cells, the astral rays and the asters are not formed at the poles of the spindle body during the metaphase. "The mitosis in which asters are not formed at the poles of the spindle body is called anastral mitosis." It is common in the higher plants.

Table of Contents

6.0 - Introduction
6.1 The Cell Cycle
6.2 Mitosis
6.3 Meiosis
6.4 Comparison between Mitosis & Meiosis

Chapter 7


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