Answer:
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Explanation:
The existence and perpetuation of life depend on the functioning of cells and their divisions. There are two important processes of cell division, which are inextricably linked with each other to maintain the progress of life. These processes are Mitosis and Meiosis.
After fertilization, when a sperm cell and an egg cell unite to form a new organism, it is expected to carry a double number of chromosomes because both the parents contribute their whole number of chromosomes as appropriate for the respective species. But the process of meiosis prevents the doubling of the chromosomes.
It reduces the total number of chromosomes in half so that the offspring (newly formed cell) can retain the same number of chromosomes as the parents. On the other hand, mitosis facilitates in forming of two daughter cells from a single cell where each new cell resembles the parent cell in every respect. Unlike meiosis, mitosis involves duplithe cation of chromosomal pairs, which is essential for cellular reproduction, especially during growth.
The Mechanism of Mitosis:
A specific process is required for forming two daughter cells out of a mother cell. This process is known as mitosis and it is usually divided into four stages, prophase, metaphase, anaphase, and, telophase. In some cases, it is convenient to designate the transition from prophase to metaphase by the name of prometaphase. However, the whole process of the cycle of mitosis is a continuous one.
1. Prophase:
At the beginning of the prophase stage, the cell prepares itself to divide the chromosomes. It is the resting stage when the nucleus becomes a little larger. It also involves the condensation of the previously diffuse, invisible, or dispersed threads into visible chromosomes.
Each chromosome is optically double, which is composed of two identical and closely parallel strands or chromatids lying throughout their length. A chromosome bears a constriction or a narrow region of attachment to the spindle, which is called a centromere.
Constrictions other than the centromere are sometimes called ‘secondary constrictions’.At prophase s,tage the centromere and secondary constrictions look alike, but by metaphase, this distinction is clear because they develop entirely different relations to the spindle.
In this stage (prophase) the outlines of the chromatids present a slightly irregular woolly or hairy appearance. They do not, in general, show a series of granules (chromo-meres) as are seen at the meiotic prophase, and probably this is a real difference between the two. By the end of the prophase, the woolly appearance almost disappears and a smooth outline is visible.
2. Premetaphase:
At the end of the prophase, the nuclear membrane usually disappears and this stage may be defined as the period of spindle formation. During this stage, the chromosomes give an impression of struggling and pushing among themselves too to reach the equator of the developing spindle. The spindle is relatively a solid gelatinous body. It is composed almost entirely of protein with a very small amount of RNA.
The centrosome seems to act as an organizer of the spindle. The centrosome is the differentiated region of cytoplasm containing centriole, which is a minute granule present just outside the nuclear membrane. After the disappearance of the nuclear membrane, the centriole is doubled and the two centrioles move apart from each other to form the poles of the spindle.
The spindle fibers are probably bundles of protein molecules. A distinction may be made between continuous fibers and chromosomal fibers. The continuous fibers run from pole to pole whereas the chromosomal fibers connect the centromeres to the poles.
In the case of bipolar spindles, the equatorial plane is located perpendicular to the long axis, which is the midway between the poles. As the prometaphase proceeds, the chromosomes arrange themselves with their centromeres, on the equatorial plane of the spindle. It is the most dynamic stage which gives rise to a relatively static Metaphase.
