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In this article we will discuss about the structure of different parts of animal cells.
1. Mitochondrion:
(Fig. 39.1)
1. Ovoid or cylindrical in shape.
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2. Number and size of mitochondria variable.
3. Cavity of mitochondrion divided into a number of cristae.
4. Both the walls of the mitochondrion and cristae formed of double lipoprotein membrane.
2. Golgi Apparatus:
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(Fig. 39.2)
1. Made up of a system of membrane bound vesicles.
2. The membranes are arranged more or less in a parallel fashion.
3. The vesicles are communicated with cisternae of endoplasmic reticulum through perforations.
3. Mitosis:
A. Metaphase stage:
(Fig. 39.3)
1. Nuclear membrane absent.
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2. Formation of spindle complete.
3. Chromosomes distinct and arranged along the equator of the spindle, the metaphase plate.
4. Chromatids united only at the centromere.
5. Metacentric chromosomes usually appear V-shaped and acrocentrics as straight or slightly curved rods.
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B. Anaphase stage:
(Fig. 39.3)
1. Nuclear membrane absent.
2. The spindle elongated and the stem body formed.
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3. The centromere divided into two.
4. One centromere and one chromatid of a chromosome on the way to one pole while the other to the opposite pole.
5. Cytoplasmic separation at the middle of the cell begins.
4. Meiosis:
A. Metaphase I stage:
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(Fig. 39.4)
1. Nuclear membrane absent.
2. Spindle formation complete.
3. Centromere undivided.
4. The bivalent possesses two centromeres.
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5. Bivalents remain attached to the spindle by the centromeres.
6. Centromeres arranged at equal distances, one above and the other below the equator.
7. The chromosomes much condensed.
B. Anaphase I stage:
(Fig. 39.4)
1. Nuclear membrane absent.
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2. Chromosomes separate.
3. Centromeres at the opposite poles of the spindle.
4. Separating chromosomes are whole chromosomes.
5. Unterminalised chiasmata at the end, away from the centromere.