In this essay we will discuss about the three main methods of cell division in animals. After reading this essay you will learn about:- 1. Mitosis 2. Meiosis 3. Amitosis.
1. Essay on Mitosis:
In the year 1882 Walther Fleming gave the term mitosis to describe the division of cell. It is a complicated process of cell division, in which the patent cell divides into two similar daughter cells. These daughter cells have the same number of chromosomes that are present in the parent cell.
Mitosis includes two distinct phases:
(1) Division of Nucleus,
(2) Division of cytoplasm, Division of Nucleus is important in cell division.
Cell division includes the following stages:
(a) Prophase
(b) Metaphase
(c) Anaphase
(d) Telophase
(e) Interphase.
a. Prophase:
This period is the beginning of mitosis. It is the longest phase which may last from one to several hours.
Prophase is marked by the following events:
1. The cell assumes a rounded shape
2. Chromosomes reach the opposite ends of the cell.
3. Nuclear membrane breaks off and dissolves.
4. The centrioles separate and move towards the opposite poles of the cell.
5. Nuclecic spindle is formed between the two poles of the dividing cell. The fibers which connect the two centrosomes are called continuous spindle fibers. The two chromatids and chromosomes become clearly visible.
6. The nucleus diminishes in size and gradually disappears. Due to these changes prophase is called the phase of disorganization.
b. Metaphase:
This period is shorter than prophase. It lasts for about 20 to 30 minutes.
The following events are happening during this phase:
1. The nuclear membrane disappears completely.
2. The chromosomes become shorter and thicker.
3. Chromosomes become displaced in the middle line of the cell. They move towards the equator of the spindle, a place half way between the two poles.
4. The chromosomes are arranged at the equator in such a manner that their centromeres lie in one plane with their arms towards the pole.
5. The centromeres are attached to the poles by means of spindle fibers.
c. Anaphase:
This is the shortest phase which lasts for about 6 to 12 minutes. But it is the most active stage of mitosis. In this stage repulsion in chromosomes takes place.
The events of this phase are:
1. The beginning of anaphase is marked by the division of centrosome and separation of sister chromatids of each chromosome.
2. Spindle fibers move towards their respective poles along with the chromatids attached to them.
3. Chromatids reach their respective places and form two groups around the centrioles. The number of chromatids of these groups are same as in parentcell.
4. The centromeres are pulled towards the poles with the arms of the chromosomes facing the equatorial plane.
5. Descending upon the position of the centromere the chromosomes appear ‘J’ or ‘U’ or ‘V’ shaped.
6. Newly separated chromatids now are new chromosomes.
d. Telophase:
This phase is known as the phase of reconstruction and reorganization.
In this phase the following events are accruing:
1. Nuclear membrane form around the group of chromosomes at each pole.
2. The chromosomes gradually uncoil and unfold to form thin and thread like structure called chromatin.
3. Spindle fibers disappear.
4. A small daughter nucleus is formed at each pole.
5. Nucleus and centrosome appear.
6. A new unclear membrane is formed.
7. The daughter cells are thus restored to the interphase condition.
e. Interphase:
This is a period between the end of the Telophase of one cell division and beginning of the prophase of next division.
It is marked by the following events:
1. The cell and its nucleus reach maximum growth. The nucleus is much more distinct in comparison to cells undergoing division.
2. Required energy for cell division is stored in the form of protein.
3. Duplication of chromosomes and synthesis of DNA is completed.
4. Nucleus becomes more distinct.
5. Chromosomes are not clearly visible.
6. The Chromosomes remain inactive.
7. Two newly formed cells grow.
Mitosis plays an important role in reproduction. It is essential for embryonic development. It ensures genetic stability and genetic continuity.
2. Essay on Meiosis:
It is a special kind of cell division described by J.B. Farmer and J.E. more in the year 1905. This cell division is occurring in sexually reproducing organisms. In sexual reproduction the sperm of the male and the ovum of the female fuse to form the ‘Zygote’ which develops into an individual by repeated mitotic cell division.
In the sex organs the ovary and the testis, the occurring of the cell division is called, meiosis. During the formation of sex-cells, meiosis provides an opportunity by which the number of chromosomes is reduced to half, so that the spermatozoa contains only twenty three chromosomes and the egg-cell or the spermatozoon and ovum are united to form the cell (zygote) which develops in a new individual, the number of chromosomes become forty-six. Meiosis ensures genetic continuity and constant chromosome number in a species.
3. Essay on Amitosis:
It is the most simple and direct method of cell division.
It involves the following events:
1. The nucleus elongates and constricts in the middle.
2. The nucleus becomes dumbbell shaped in appearance.
3. Finally the nucleus separates into two parts and two daughter cells are formed.
4. The new cells are almost equal in size but the genetic material is not equally distributed between the two daughter cells.
Amitosis is rarely occurred. It takes place only under certain specific conditions.