The following points highlight the three main types of cell division in all organisms. The types are: 1. Mitosis 2. Meiosis 3. Amitosis.
Cell Division: Type # 1. Mitosis:
The word ‘mitosis’ means the division of the nucleus, but it is used to describe a process of cell division which involves both nucleus and cytoplasm. During mitosis, a cell divides into two daughter cells. The production of two cells is the attainment of a climax of extensive series of preparatory events which are blended into one another.
A. Functions of Mitosis:
1. Many single-celled organisms multiply through mitosis. This is the mode of reproduction in case of them.
2. Multicellular forms attain adult stage by this type of cell division.
3. This type of division meets normal wear and tear of the individual.
4. It occurs during wound healing and regeneration of lost parts.
B. Factors Controlling Mitosis:
It is not known what stimulates mitosis. A number of chemical and physical agents are known which either promote or inhibit mitosis, but their mode of action is not clearly understood.
The entire phenomenon depends upon a number of factors which may be grouped into two categories:
1. Condition of cellular metabolism:
Cell division involves duplication of cell substance; therefore, the event depends upon the metabolic process of the cell for raw materials and energy. Any disturbance in the metabolic pathways of cell will inhibit the metabolic activity.
2. Condition of cellular specialization:
Cells which are specialised for a particular function, tend to lose their ability to divide, whereas unspecialised cells may divide at regular intervals.
C. Events of Mitosis:
The entire process of mitosis may be separated into two broad categories, namely, chemical events and physical events. The chemical event, which are responsible for duplication, start long before the appearance of physical events which involve equal distribution of cellular materials.
1. Chemical events:
Within the nucleus of a cell, which is destined to divide, the chromosomes synthesise more DNA and it has been shown that such a cell doubles its DNA content. During this process the genes which are present on the chromosome duplicate.
The duplication of all the genes results into the doubling of the chromosome but the line of division remains invisible. The chemical events in mitosis have been extensively studied by Taylor at chromosomal level and by Messelson and Stahl at molecular level.
Their study revealed that the molecular doubling of genetic material has its parallel in the doubling of chromosomes at the cytological level. There is considerable lapse of time between the end of the chemical events and the beginning of the physical events. It is believed that some other biochemical changes occur at the stage of interphase (Fig. 4.17A) which are necessary for the physical events.
2. Physical events:
The entire visible process of mitosis may be divided into four stages. These are Prophase, Metaphase, Anaphase and Telophase (Fig. 4.17) and are followed by cytokinesis.
It is not that during division the cell stops for a while at each stage; on the contrary, the entire process is a dynamic and continuous one. Time required for the completion of a particular stage or of the entire process varies in different types of cells and also depends on different physical and chemical factors.
During the stage changes occur both within the nucleus and in cytoplasm. Within the nucleus the nucleolus disappears at the end of prophase and the chromosomes become visible.
The visibility is due to the condensation of chromatin fibres from slender thread-like appearance to stout chromosomes (Fig. 4.17B-C). In the beginning of prophase the outer surface of the chromosome remains irregular but at the end of prophase it becomes smooth.
Each chromosome, which is already duplicated, is made up of two closely set pairs, the chromatids. The two chromatids are united together at a region which does not take chromosomal stains. The region of attachment is called centromere or kineto- chore. The position of the centromere on the chromosome varies but for a particular chromosome the position is constant.
When centromere is diffused along the entire length of chromosome it is called polycentric chromosome. When the centromere is present at the centre of the chromosome, it is called a metacentric chromosome and when the centromere is placed at one end of the chromosome it is regarded as an acrocentric chromosome. In another form of chromosome the centromere is absolutely terminal, it is called telocentric chromosome.
In the cytoplasm, with the commencement of prophase stage the centrosome containing the centriole splits into two and the two parts move away from each other. When they reach two opposite poles a portion of cytoplasm forms a gel around them and encircles it as radiating fibres. This body is now called aster. Number of gelatinous fibres appear in the cytoplasm. They are called the spindle fibres.
These fibres connect the two centrioles. The spindle fibres are made up of protein and RNA and they can be isolated from the cell. The arrangement of spindle varies widely in different cells. Three sets of fibres are generally visible.
One set connects the two polar centrioles; the second set connects the centromeres with the centrioles and the third set, in between the daughter chromosomes. These inter-chromosomal fibres push the chromosomes during their journey to the poles.
At the end of prophase, the nuclear membrane disappears and the chromosomes are shifted mechanically and come towards the equator of the spindle. The phase of spindle formation and movement of chromosomes to the centre is also termed as pro-metaphase (Fig. 4.17D).
It is a brief phase during which very little visible changes occur. Arrangements along the equator depend upon the nature of chromosomes, i.e., metacentric chromosomes become V-shaped and acrocentric chromosomes remain straight or L-shaped (Fig. 4.17E).
This phase involves the journey of chromatids to the opposite poles. It starts with the movement of daughter centromeres. Each daughter centromere while moving to the pole drags one chromatid from the other. At the same time the central spindle between the two chromatids elongates to form a pack of filiform structures called stem body.
Thus anaphase stage is the result of two processes, movement of centromeres and elongation of spindle, both of which vary in different organisms. The indication of cytoplasmic separation begins from this stage (Fig. 4.17F-G).
The daughter chromosomes after reaching the poles, lose their smooth texture and start to de-condense. The chromosomes form a loose network around which a new nuclear membrane is formed. It is believed that the nuclear membrane arises from the endoplasmic reticulum. A nucleolus reappears in each of the daughter nuclei (Fig. 4.17H-I).
During the end of telophase a furrow is formed in the cell membrane along the equator. This furrow deepens and considerable movement of cytoplasm takes place. Then all on a sudden the cell is pinched into two along the furrow in the equator and the cytoplasmic turbulence ceases (Fig. 4.17 I).
Cell Division: Type # 2. Meiosis:
Meiosis is a special type of cell division which occurs in sexually reproducing organisms. In all organisms the chromosomes remain in pairs. The organisms reproducing asexually multiply by mitosis. Thus, there exists no chance of alteration of chromosome number. On the contrary, sexual reproduction demands contribution from two individuals. Thus there lies a risk of chromosomal dis-balance.
The process of meiosis helps to avert this probability by reducing the number of chromosomes to half. It may happen after gametic union (as in sporozoa) or before fertilisation (in all higher organisms). In higher organisms, therefore, mitosis occurs in both somatic and germ cells but meiosis takes place in the germ cells alone and only during the formation of gametes.
A. Functions of Meiosis:
1. It checks the dis-balance of chromosome number (by reducing the chromosome number to half in the gametes, which after union restores the specific number).
2. It produces random assortment of chromosomes, which results into the production of a large number of variations.
B. Steps of Meiosis:
Meiosis involves two divisions of the cell but one division of the chromosome.
Thus the entire process of meiosis may be splitted into:
(1) First meiotic division and
(2) Second meiotic division.
1. First meiotic division:
This involves all the phases of mitosis:
Cytokinesis (Fig. 4.18).
But the incidents during the events are different.
It is the longest phase in the first meiotic division and much longer than mitotic prophase.
It can be subdivided into the following stages:
(i) Leptotene or Leptonema
(ii) Zygotene or Zygonema
(iii) Pachytene or Pachynema
(iv) Diplotene or Diplonema, and
It is a short stage during which chromosomes become elongated and slender (Fig. 4.18A). Whether the leptotene chromosomes are split to form chromatids or not, is still a matter of dispute.
At this stage an important event called synapsis or pairing occurs. During this event, the homologous chromosomes pair and their homologous regions come in close approximation with each other throughout their length.
The pairing begins from the end away from the centromere and gradually extends along the entire length of the chromosome (Fig. 4.18B) and it ends with the pairing of the centromeres. The pairing of homologous chromosomes results into the formation of bivalents.
At this stage (Fig. 4.18C) the bivalents condense and each chromosome of the bivalent divides into two strands. Thus at the end of pachytene, the bivalent looks like a four-stranded structure, all the strands being closely set together.
(iv) Diplotene (Fig. 4.18d):
The force that kept the bivalents together ceases to act at this stage and the members of the bivalent separate except at certain points where two strands, one from each homologous chromosome, unite together to form Xs (Fig. 4.20). These points are called chiasmata. The number of chiasma in a bivalent varies from 1-12. Two hypotheses are well known to explain the formation of chiasma.
According to the Classical hypothesis, on one side two paternal and two maternal chromatids are paired and on the other side a paternal chromatid pairs with a maternal and a maternal chromatid pairs with a paternal one. It means that formation of a chiasma may or may not give rise to a crossing-over.
According to Chiasma type hypothesis, two strands in a four-stranded bivalent, break and unite diagonally in a X-shaped fashion to form chiasma. It means that crossing-over precedes the formation of chiasma.
Why do the chromatids break and then re-join? Wherefrom comes the motive force of chiasma formation? Why both the chromatids from maternal and paternal chromosome split at a particular region? These are a few of the many unanswered questions about the chiasma formation.
But this is known that occurrence of one chiasma prevents the formation of another chiasma at the nearby region. This phenomenon is called chiasma interference. At the end of diplotene, the chromosomes thicken and become short. In some forms, the chiasma slips and comes to the terminal end of chromosomes. This is called terminalisation.
Bivalents become short, thick and darkly stained. These bivalents move towards the inner side of the nucleus (Fig. 4.18E).
At the end of prophase, nuclear membrane disappears and the spindle is formed. The bivalents remain attached to the spindle fibres by its two centromeres. During their arrangement in the equator, one centromere remains above and the other remains below the equator.
It may be mentioned here that the centromere during mitosis remains perfectly on the equator. The chromosomes become much condensed and gain a smooth appearance. The gap between two centromeres is dependent upon the position of chiasma (Fig. 4.18F).
Each member of the bivalent chromosomes begins to move towards the pole (Fig. 4.18G). It is dragged by the centromere with which the fibres of the spindle are connected. The behaviour of chromosomes is the same as in mitosis. Only difference is that during mitosis, a half centromere and one chromatid migrates while here an entire chromosome having two chromatids and an intact centromere does the same behaviour.
The movement ends the pairing of the bivalent which causes the chiasmata to slip off from the terminal end. It must be remembered that chromosomes which separate during anaphase are not the same which appeared during zygotene to form the bivalent. Due to chiasma formation and crossing- over, many parts of it are reorganised.
This phase resembles that of mitosis. Only difference is the orientation of chromatids. Two chromatids of each chromosome are arranged either like L or V. A narrow stem body persists between the nuclei at the two poles.
The occurrence of cytoplasmic division may or may not follow the nuclear separation. In some cases a resting stage or interphase or interkinesis appears, while in many instances the telophase nuclei pass directly into the prophase stage of second meiotic division.
2. Second meiotic division (Fig. 4.19):
The process is almost similar to the normal mitosis:
It is a brief stage and is similar to the mitotic prophase. No complication occurs as in the first meiotic division. (Fig. 4.19A1, A2).
(b) Pro-metaphase and metaphase:
Spindles are organised very quickly and the chromosomes which reduced to half are seen to possess widely separated chromatids with attachment only with the centromere.
Centromere of the chromosome divides and two halves are drawn towards the opposite poles (Fig. 4.19B1B2). Each half of centromere carries with it the already separated chromatid.
The process is same as that of mitotic telophase, with the only difference that the telophase nuclei here contain only half the number of chromosomes.
It is same as in mitosis and results into two daughter cells. Thus four cells are produced, each with half the number of chromosomes of the mother cell (Fig. 4.19 C1, C2, D1, D2).
Significance of meiosis:
1. In higher organisms meiosis occurs in the cell which forms the gametes. In the formation of both male and female gametes from one gametocyte, four daughter cells are produced with haploid number of chromosomes.
In the males, all the daughter cells become functional gametes or sperm cells. But in females, the unequal cytokinesis results into the formation of one large cell and three small cells. Each of them contains haploid number of chromosomes, but only the large cell becomes the functional gamete and the other three, called the polar bodies or polocytes, become abortive.
2. Another fascinating aspect of meiosis is that it begins at the very early life in the individual but remains arrested for a considerably long time in the prophase state. In males the completion depends upon the attainment of sexual maturity. In the female, the completion of the division comes only shortly before or after fertilisation.
3. The process of meiosis not only reduces the chromosome number to half for the purpose of reproduction but also by random distribution of paternal and maternal chromosomes and by crossing-over through chiasma (Fig. 4.20), it produces gametes, none of which are exactly alike. Thus, a large number of variations result, which have got great significance in evolution.
Cell Division: Type # 3. Amitosis:
For a long time it was known that in several instances the nucleus divides without the disappearance of nuclear membrane and the formation of spindle apparatus (Fig. 4.21). Such direct division of nucleus is known as amitosis.
The application of improved techniques has shown that in cells dividing amitotically, probably there exists some intra-nuclear mechanism for equal distribution of chromosomes. The division in amoeba, which was regarded to be amitotic in nature, is now established to be mitotic division.