Cell Division: 5 Significant Methods of Cell Division | Plants

The following points highlight the five significant methods of cell division.

The methods are: (1) Somatic Mitosis (2) Cytokinesis (3) Amitosis (4) Meiosis and (5) Free Cell Formation.

Growth is a fundamental feature of the living things. In unicellular organisms growth means simple cell enlargement, but in multicellular plants, which, in fact, form ever- whelming majority, growth involves much more complications.

The starting point in the life of a plant with sexual reproduction is the fertilised egg—the single-celled zygote. In course of time millions of cells are formed. Thus here growth involves formation of new cells from the pre-existing ones, instead of simple enlargement.

Cell division is the most important method of formation of new cells. There are different methods of cell division, an account of which is being given here.

Method # 1. Somatic Mitosis:

Somatic mitosis is the most common method of cell division taking place in the vege­tative parts which really constitute the body (soma) of the plants. It is a complicated pro­cess where the nucleus of the cell plays the pro­minent role.

In fact, the nucleus divides first into two identically equal daughter nuclei. This divi­sion is referred to as mitosis or karyokinesis. Nuclear division is immediately followed by the division of the cytosome or cytoplasmic division. The second division is called cytokinesis.

As already stated, the method involves a series of complicated changes in the cell. Once it starts, it continues till the division is complete. So the process is definitely continuous.

For the sake of convenience of study it is customary to divide the process into four parts or phases. Though purely man-made, the four phases have been recognised in the biological world. Somatic mitosis has been studied in different parts of the different plants.

An outline of the method as found in the root-tips (Figs. 519 & 520) of plants is being discussed now.

Prophase:

Prophase or early phase (Fig. 520 B, C & D) begins with the earliest recognisable changes in the nucleus. The nucleus before the onset of mitosis is said to be in metabolic condition (Fig. 520A).

With the commencement of mitosis the chromatin reticulum unravels itself into slender and crooked threads, the chromonemata, which could not be traced as indivi­duals in the earlier condition.

Each chromonema, on close examination, reveals a longitudinally double structure. With advancement of the process the chromonemata tend to uncoil and become thick, so that double nature is more clear. These threads are called the chromosomes.

In the mean time a second chromosomal element, matrix, appears; and the chromonemata, which again start coiling, remain embedded in the matrix. At this stage the chromosome distinctly shows a double struc­ture, consisting of two daughter chromosomes or chromatids. Each chromatid now posses­ses matrix and a chromonema, which again splits lengthwise into two, remaining twisted about each other.

So the chromosome now consists of two chromatids, each having two chromonemata or in other words, each chromosome breaks into four half-chromatids. Deeply stainable matrix renders the chromonemata rather invisible.

The chromosomes are now shorter, thicker and denser bodies. Their number in a diploid nucleus is nor­mally even and constant in a particular species of plant. The nucleolus and nuclear membrane gradually disappear in the mean time.

Metaphase:

Now die nucleus passes to the metaphase (Fig. 520E). Probably due to some complicated changes in the karyolymph some fine fibrils make their appearance and ultimately form a spindle-shaped body called bipolar spindle or achromatic figure, so-called obviously for less affinity for stains.

This appearance is probably caused by the arrangement of chemical molecules along the longitudinal axis of the spindle and a stratification of the material into firm and fluid layers.

The two ends of the spindle are referred to as the poles and the middle plane as the equator. The double chromosomes or chromatids lying intimately associated to each other arrange themselves at the equator of the spindle.

Each chromatid has a distinct region where it remains attached to the fibril. The attachment regions are called centromeres or kinetochores or primary con­striction or points of attachment. These usually appear as unstained spots on the chro­matids.

The two sides, of equal or unequal lengths, of the chromatid are referred to as arms. The centromeres face opposite poles of the spindle, the arms may lie in any position.

The fibrils directly attached to the bodies of the chromatids at the centromeres are called tractile fibres.

The nature of the tractile fibres is controversial. Suggestions have been made that it may be a local modification of spindle substance, an extension of the chromatid or something extruded from the chromatid.

But no agreement could really be reached. Other fibrils running from pole to pole are known as continuous or supporting fibres. Studies by microdissection have revealed that the whole spindle with the chromosomes may be extracted from the cell, which goes to show that the spindle is a definite body and not a mere appearance.

Anaphase:

At this stage (Fig. 520F), the chromatids become separated and start moving away from the equator, the two sets towards the two poles. The distribution of original chromosomes in the two sets is exactly equal, both qualitatively and quantitatively.

It is the centromere which moves ahead along the tractile fibre, the arms trailing behind (Fig. 521). The chromosomes are quite distinct at this stage; they usually assume inverted V- or L-shaped appearance, depending on the length of the arms.

The tractile fibres gradually disappear at the close of anaphase, and also the matrix, so that the chromonemata again come to view.

The cause of anaphasic movement of chromosomes has been a subject of keen investigation and has aroused good deal of controversy. Different views have been put for­ward, but no agreement could be reached on this point. It is not, however, unlikely that some sort of tractive force is exercised by the fibres, probably due to contraction of the protein molecules, of which they are made.

Telophase:

During this phase (Fig. 520 G & H) the chromosomes reach the poles and two daughter nuclei are reconstructed. Having reached the poles the chromosomes become crowded together, their individuality being completely lost.

The matrix loses its stainability and ultimately disappears. The crooked chromonemata become closely associated and anastomose to form the new reticulum. The nuclear mem­brane and the nucleolus reappear.

The relation of the nucleolus to the chromosomes is interesting. In the diploid nucleus there is at least one pair of peculiar chromosomes, having a distinct constriction at one end.

Matrix does not collect round that point. The portion of the chromosome beyond the constriction, meaning the portion isolated from the main body due to pressure of constriction, is called sattelite; and the pair of such chromosomes arc called sat-chromosomes.

The constriction is known as the nucleolus organiser (Fig. 522), because it is connected with the formation of the nucleolus. Each sat-chromosome forms a nucleolus. The two nucleoli may fuse to form one, if the two happen to lie side by side, or they may remain separate.

Method # 2. Cytokinesis:

The division of the nucleus, though the more important part of the process, is not the whole of it. It is accompanied by cytokinesis or division of the cytoplasm, ultimately ending in laying down of the cell wall between two protoplasts.

Cytokinesis usually takes place at the telophase stage when the daughter nuclei are reconstructed. The fibrils then extend outwards and touch the lateral walls, thus assum­ing barrel-shaped appearance (Fig. 520H). Protoplasmic materials now accumulate at the equatorial region of the spindle in form of droplets which coalesce in course of time and give rise to a plate, called cell plate.

It soon undergoes physical and chemical chan­ges and is transformed into the intercellular substance—middle lamella. New cellulose wall is laid down by the protoplast on the middle lamella. In the mean time the fibrils become more and more faint and ultimately disappear.

In some cases cytokinesis also occurs by furrowing. Here instead of formation of cell plate, the plasma membrane undergoes furrowing inwards and the protoplast is ultima­tely separated into two parts.

Duration and Periodicity of Mitosis:

Mitosis is confined to the cells of specialised regions called meristems, present in root- tip, stem-tip, leaf-primordia, etc.

Once a nucleus has divided into two, the newly formed ones may have a resting stage or may again repeat the process in rapid succession. The stage between two mitosis is known as interphase.

Time required for mitotic process varies with different organisms, tissues and also with external factors like temperature. It has been found that in staminal hairs of Rhoeo discolor of family Commelinaceae mitosis takes 30 minutes at 45°C., 75 mins. at 25°C. and 135 mins. at 10°C.

In the hair cells of the stigma of some grasses (Arrhenatherum) at 19°C. prophase occupies 36 to 40 mins., the metaphase 7 to 10 mins., the anaphase 15 to 20 mins. and the telophase 20 to 35 mins.; and the total 78 to 110 mins. excluding inter­phase.

The relative duration of the phase may be deduced from the data available. The pro­phase is the longest phase occupying something like 40% of the duration, next comes telophase with 32%, anaphase comes next with nearly 20%, and, lastly, the shortest phase, metaphase, with less than 10% of the total duration.

It has been stated that mitosis is the most common method of division in all vege­tative or somatic cells. During this process, each chromosome, which is the bearer of hereditary units, is divided longitudinally into two exactly equal and identical halves; and the two sets consisting of the halved chromosomes, are distributed to the daughter nuclei.

So the daughter nuclei formed are similar to each other, both qualitatively and quantitatively, and also to the mother nucleus. Somatic mitosis is thus equational.

Since the nuclei are formed from pre-existing ones by mitosis and they have same chro­mosome complements, the somatic nuclei are all similar. The number of chromosomes thus remains constant in a given species of plant.

Method # 3. Amitosis:

Amitosis or direct nuclear division is very simple, and at the same time, a rare pro­cess found in some lower organisms like bacteria, in the internodal cells of a green alga, Chara.

Here the nucleus undergoes constriction and ultimately divides into two parts, which are in all probability unequal (Fig. 523). There is nothing like chromosomes, their characteristic manoeuvres nor the spindle.

Method # 4. Meiosis:

In all organisms with sexual method of reproduction the union of the two gametes results in the formation of the zygote.

The zygote, which is the starting point of the new organism, has thus ‘diploid’ or ‘2n’ chromosomes, the two sets being contributed by the two gametes, male and female. The two sets of chromosomes do not unite in the zygote. In fact, for each individual in the set of the male gamete, there is a corresponding one in the female gamete. The two matching chromosomes are usually not only alike in external characters, but also carry similar or matching hereditary units or genes.

Such chromosomes are said to be homologous. The somatic cells develop from the zygote and thus possess ‘2n’ or diploid chromosomes. But haplosis or reduction of the number of chromosomes does take place somewhere in the life cycle when ‘2n’ or diploid number is reduced to or haploid number.

In plants reduction or haplosis occurs before the formation of spores or gametes, so that the ‘2n’ number may be restored during union of the gametes.

Thus two cardinal points in the life cycle of plants with sexual method of reproduction are (i) gametic union, i.e., fertilisation or conjugation, when the chromosome number becomes diploid (2n); and (ii) haplosis or reduction of the diploid number into haploid (n) before formation of the spores.

Meiosis is the process of cell division in which the diploid number of chromosomes is reduced to haploid. It is a very complicated process restricted only to the reproductive cells. The cells in which meiosis is initiated are called meiocytes. In plants usually the spore mother cells or sporocytes are the meiocytes. In heterosporous plants the meiocytes are the microspore mother cells and megaspore mother cells.

Meiosis involves two divisions, of which the first division is reductional, whereas the second one is equational. The two divisions were formerly called heterotypic and homotypic respectively; but now they are referred to as first meiotic division and second meiotic division.

Thus after meiosis four new cells are formed, each with reduced or haploid number of chromosomes (Fig. 524), in contrast to two cells with diploid chromosomes in mitosis. Like mitosis it also passes through four phases—prophase, metaphase, anaphase and, finally, telophase of the two divisions I and II.

Prophase I:

The prophase as usual begins with the earliest recognisable changes in the nucleus. It is rather a lengthy phase involving a series of complicated changes. So it warrants further subdivisions into a number of stages. The changes occurring in the nucleus and the chromosomes are being discussed with reference to those stages.

Leptotene:

The reticulum of the meiocytic nucleus opens out and the chromonemata, which represent the chromosomes at this stage, become more distinct and appear as long and slender threads (Fig. 525A). The chromosomes are obviously in diploid number.

Zygotene:

Now a pair of chromosomes become intimately associated, but does not actually fuse. The two threads forming the pair are called homologues and the chromo­somes are said to be in bivalent condition.

This selective pairing or synapsis of homolo­gous chromosomes seems to begin at one or more points, often at the centromeres and proceed side by side as if drawn by a zipper till it is complete, when they twist or coil round each other. Now the threads become gradually shorter and thicker (Fig. 525B).

Pachytene:

The synaptic chromosomes forming the pair become more thick and short. Now each of the two chromosomes splits longitudinally into two halves. So they are now quadruple, each being a tetrad of threads (Fig. 525C). These half-chromosomes are called chromatids.

Diplotene:

At this stage a tendency of separation is noticed in the four chromatids, one pair from the other pair, as if the synaptic force which brought them near is being replaced by a sort of repulsive force.

During this separation there are one or more points where they tend to remain together. Due to presence of these points, known as chiasmata, where the two chromosomes exchange bits of chromatids, a disturbance is noticed in the dissociation of the chromosomes.

This exchange of bits, referred to as crossing-over, un­doubtedly makes the process more complicated; none-the-less it has significant bearing on genetical phenomena which will be discussed later.

It is often observed that the paired chromatids continue shortening and thickening. The matrix becomes evident now with coiled chromonemata embedded in the same (Fig. 525D).

Diakinesis:

Now the compact chromatid tetrads are well-distributed in the nucleus, their individuality becoming more clear due to abundance of matrix.

This is an ideal stage for counting the chromosome number when they assume different shapes depend­ing on the chiasmata (Fig. 525E) There prophase comes to an end.

Metaphase I:

In metaphase the nuclear membrane and nucleolus disappear and the achromatic figure (spindle) is formed. The chromatids in tetrads arrange themselves at the equator, the centromeres facing the poles (Fig. 525F).

Anaphase I:

Anaphasic movements begin now. Two chromatids of each tetrad, forming a diad, are now separated from the other two and move towards opposite poles. This separation is referred to as disjunction. The two chromatids forming pairs are call­ed diads. Some resistance is offered at the chiasmata, but ultimately the diads become free (Fig. 525G). This stage is significant, in view of the fact that the two homologues brought together in synapsis are separated now.

Telophase I:

Now the chromosomes undergo telophasic changes resembling the same in mitosis (Fig. 525H). Cytokinesis does not always occur immediately. The two newly reorganised nuclei now pass through division II.

The interval between the two divisions is called interkinesis, the duration of which is quite variable. The two nuclei are fairly large at this stage and the chromatids occur as slender threads.

Meiotic Division II:

The first division is followed by second meiotic division which is equational and not reductional.

Prophase II:

Here the chromosomes still remain associated in diads (Fig. 525 I), in contrast to the closely parallel split chromosomes of the somatic prophase. Matrix appears now.

It is followed by metaphase II when achromatic figure is formed and the chro­mosomes arrange themselves at the equator (Fig 525J). Anaphase II involves moving apart of the two chromatids of the diad towards two poles along the fibres (Fig. 525K).

The centromeres, as usual, move ahead and chromatids assume different shapes. During telophase II the chromatids reach the poles and are reorganised into daughter nuclei (Fig. 525L).

It is to be noted that four groups of chromatids, now independent chromo­somes, reconstruct four new nuclei. Each nucleus possesses a set of haploid chromosomes; each set having one chromatid of each of the tetrads of prophase I.

Cytokinesis follows by furrowing, forming four cells, each with haploid or reduced number of chromosomes in the nucleus.

Method # 5. Free Cell Formation:

This process is really a modification of mitosis. Here the nucleus of the mother cell divides into two by mitosis, those two again divide, and so on, till a good number of nuclei are formed in the cell In some algae and fungi each nucleus thus formed is surrounded by cytoplasm and ultimately wall is secreted.

Thus a few cells are formed within the mother cell. The formation of ascospores within the ascus in the group Ascomycetes of fungi may be cited as an example (Fig. 526B). These cells are set free in course of time by rupture of the original cell wall.

Cytoplasm may also undergo cleavage forming a few naked cells within the original one. During formation of endosperm of many higher plants and embryo cells of Gymnosperms like cycad and Ephedra multinucleate mass (coenocytic stage) undergoes cytokinesis by cell plate formation into uninucleate cells (Fig. 526A).