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.
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:
(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.
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.
B. Anaphase stage:
(Fig. 39.3)
1. Nuclear membrane absent.
2. The spindle elongated and the stem body formed.
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:
(Fig. 39.4)
1. Nuclear membrane absent.
2. Spindle formation complete.
3. Centromere undivided.
4. The bivalent possesses two centromeres.
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.
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.