In this article we will discuss about:- 1. Definition of Mitosis 2. Features of Mitosis 3. Genetic Control 4. Significance.
Definition of Mitosis:
The term mitosis was coined by Flemming in 1882. Mitosis refers to the spindle using nuclear division which produces two identical daughter nuclei from the parent nucleus. Since mitosis occurs in somatic cells, it is also known as somatic cell division.
Features of Mitosis:
The important features of mitosis are briefly described below:
1. Mitosis leads to production of two daughter cells from a mother cell in each cycle of cell division. In other words, nucleus divides once in each cell cycle.
2. The daughter cells are similar to the mother cell in shape, size and chromosome complement. Since the chromosome number is the same in the daughter cells as compared to that of mother cell, this is also known as homotypic or equational division.
3. In plants, mitosis takes place in somatic organs like root tip, stem tip and leaf base. It leads to growth of vegetative parts.
4. The complete process of mitosis consists of only one homotypic or equational division.
5. Segregation and recombination do not take place during mitosis.
Cell Cycle:
The period in which one cycle of cell division is completed is called cell cycle. A cell cycle consists of two phases, viz. 1. interphase and 2. mitotic phase. Interphase is generally known as DNA synthesis phase and mitotic phase refers to the period of nuclear division. The time required for completion of cell cycle differs from species to species (Table 3.1).
A. Interphase:
Interphase consists of G1, S and G2 sub phases. G1 is the resting phase, S is the period of DNA replication and G2 again is a resting stage after DNA replication.
A brief description of G1, S and G2 is given below:
G1: It is a pre-DNA replication phase. Thus, this a phase between telophase and S phase. This is the longest phase which takes 12 hours in Vicia faba (Fig. 3.1). Protein and RNA syntheses take place during this phase.
S: This phase comes after G1. The chromosome and DNA replications take place during this phase. This phase takes lesser time than G1. In Vicia faba, it takes six hours.
G2: This is the post-DNA replication phase. This is the last stage of interphase. Protein and RNA syntheses occur during this stage. This phase also takes 12 hours in Vicia faba. However, time required for completion of all these stages differs from species to species.
B. Mitotic Phase:
The M phase lead to separation of replicated DNA into two daughter nuclei without recombination. Thus, the daughter nuclei have the same chromosome combination as that of parent nucleus. The M phase consists of four stages, viz., 1. prophase, 2. metaphase, 3. anaphase and 4. telophase (Fig. 3.2).
These are briefly described below:
1. Prophase:
Prophase starts immediately after G2 stage of interphase. Chromosomes look like thin thread and uncoiled in the early prophase (Fig. 3.2B), but become shortened, coiled and more distinct during mid-prophase (Fig 3.2C).
In the late prophase, chromosomes appear more conspicuous, short and thick and longitudinally double (Fig. 3.2D). The two chromatids of each chromosome held at centromere are visible under light microscope. The nucleolus becomes smaller in size. The nuclear membrane and nucleolus disappear at the end of prophase.
2. Metaphase:
This phase begins after prophase. The spindle tubes are formed and chromosomes are oriented in the centre at equatorial plate. Chromosomes are attached to the spindle tubes at the centromere. Chromosomes are clearly visible at metaphase. Sister chromatids of each chromosome are joined together at the point of centromere, but their arms are free (Fig. 3.2E).
3. Anaphase:
This is the phase when chromatids separate at the centromere and move towards opposite sides or poles. Chromatids of each chromosome become free at the centromere, but each chromatid is attached to spindle tube. These chromatids suddenly move apart, one goes to one pole and the other towards the other pole. After separation each chromatid becomes a chromosome (Fig. 3.2F).
4. Telophase:
When chromosomes reach the pole, the last stage, telophase begins. The spindle tubes disintegrate, a new nuclear membrane is formed at each pole covering the chromosomes. The nucleoli also reappear at each pole.
Chromosomes again become thinner and longer by uncoiling and unfolding and look like a single thread under light microscope (Fig. 3.2G). Then the nucleus enters interphase. Among all the four phases of mitosis, prophase takes longest duration (Table 3.2).
Cytokinesis:
The division of nucleus is known as karyokinesis. It is followed by division of cytoplasm, which is known as cytokinesis. The division of cytoplasm into two daughter cells may take place in two ways. In plants, the division of cytoplasm takes place due to formation of cell plate.
The formation of such plate begins in the centre of cell, which moves towards periphery in both sides dividing the cytoplasm into two daughter cells. In animals, the separation of cytoplasm starts by furrowing of plasma lemma in the equatorial region. This results in division of cytoplasm into two daughter cells (Fig. 3.2H).
Genetic Control of Mitosis:
The mitotic cell division is under genetic control in all eukaryotes under normal conditions.
Important evidences in this connection are given below:
1. In eukaryotes, under similar environmental conditions, cells do not divide continuously. They sometimes divide and sometimes do not divide. This indicates that on and off mechanism must be genetically controlled.
2. Some cells divide more frequently than others. For example, somatic cell of an embryo can undergo a larger number of mitotic divisions than a somatic cell of an adult. This difference in the mitotic division is also due to genetic control at two different stages.
The genetic control is lost in somatic cells which become cancerous. Hence such cells divide mitotically continuously without any control resulting in abnormal tissue growth.
3. The position of mitotic spindle is genetically controlled. The spindles are formed in a controlled manner. For example, in water snail (Limnaea), the direction of coiling (left handed or right handed) is determined by the presence or absence of an allele.
4. The replication of each chromosome precisely into two chromatids during 3 stages of interphase again provides strong evidence that mitosis is genetically controlled.
Significance of Mitosis:
Mitosis plays an important role in the life of living organisms in various ways as given below:
1. After fusion of male and female gametes zygote is formed. Mitosis is responsible for development of a zygote into adult organism.
2. Mitosis is essential for normal growth and development of living organisms. It gives a definite shape to a specific organism.
3. In plants, mitosis leads to formation of new parts, viz., roots, leaves, stems and branches. It also helps in repairing of damaged parts.
4. In case of vegetatively propagated crops like sugarcane, sweet potato, potato, etc., mitosis helps in asexual propagation. Mitosis leads to production of identical progeny in such crops.
5. Mitosis is useful in maintaining the purity of types because it leads to production of identical daughter cells and does not allow segregation and recombination to occur.
6. In animals, it helps in continuous replacement of old tissues with new ones, such as gut epithelium and blood cells.