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The following points highlight the three main types of cell division seen in plants and animals. The types are: 1. Amitosis 2. Mitosis 3. Meiosis.
Cell Division: Type # 1. Amitosis (= Direct Nuclear Division):
It is the most uncommon, primitive and simplest type of cell division. The nucleus starts elongating, thus a constriction appears approximately in the middle. The constriction gradually deepens and eventually gives rise to two daughter nuclei. The two daughter nuclei thus formed are not equal in size. This is said as direct cell division.
During the subsequent stages of nuclear division, a constriction appears in the cytoplasm which divides it into two parts, each with a daughter nucleus. Division of the cytoplasm is not essential. Some times it does not follow nuclear division. The result is multinucleate mass of protoplasm known as coenocyte and syncytium in case of plants and animals respectively.
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Amitosis cell division is rare and, therefore, of little genetic importance. It is visible only in some uni or acellular organisms like bacteria, yeast, amoeba, diatoms etc. It may also be seen in the higher plants but in some very old cells which are degenerating. This may be regarded as a primitive form of mitosis.
Cell Division: Type # 2. Mitosis (= Mitotic = Somatic = Homotypic = Equational division or Mitocytes):
The term ‘mitosis’ was used first by W.Flemming in 1882. It involves division of nucleus resulting in the formation of daughter nuclei which are quantitatively and qualitatively alike (with the same genetic constitution) as the parent nucleus.
It includes two parts:
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(1) Karyokinesis and
(2) Cytokinesis.
Definition:
Mitosis is characteristic cell division of somatic cells (= body cells = vegetative cells) in which each daughter cell contains the same chromosome number as present in the parental cell. In plants, mitosis is confined to the meristematic tissues of root and shoot tips, young leaves, flower buds and cambium. Healing of wounds and replacement of damaged organs in plants is based on mitotic cell division.
Stages of Mitosis:
On the basis of changes in morphology of nucleus and chromosomes, mitosis may be divided into five stages:
(a) Interphase
(b) Prophase
(c) Metaphase
(d) Anaphase
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(e) Telophase
The division of mitosis into these stages is basically for convenience of description. In real sense, cell division is a continuous process already divided into different stages.
These stages are defined in terms of chromosomal distribution in the cell, separation of two sister chromatids from a chromosome and their movement to the different poles, presence and absence of nuclear membrane and nucleolus, length and condensation of chromosomes etc. It should be well understood that all these events happen gradually so that one stage merges in to next one.
(a) Interphase:
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This is not dividing stage in which the chromosomes are invisible with the light microscope. It occurs after the telophase stage of the previous mitotic division and before the beginning of prophase of the next one. The chromatin (the nuclear material that become a part of chromosomes, readily take up basic dyes) is granular or in the form of slender or thin, randomly coiled threads.
Formerly, it was called the resting stage of the cell but later studies have shown that it is an active, vital or metabolic stage of the cell during which many physiological and biochemical functions are performed.
In other words, chromosome replication (= DNA replication), protein and RNA synthesis already takes place during this stage and all the dividing and non-dividing cells are present in this stage. In a cell having a cell cycle of 24 hours, 23 hours are spent in interphase while the mitosis may take up only one hour.
The interphase is divided in to G1, S and G2 and M phases.
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G1 phase:
This phase serves as kind of ‘gap’ in the cell cycle and is termed as first gap or G: phase. During this phase synthesis of protein and RNA takes place. The time required by this phase is 30-40%.
S-phase:
At this phase the major part of DNA synthesis and replication (or duplication) takes place. Besides it, almost all the histones of nucleus are synthesised during this period.
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The time required for this phase is 30-50%. DNA synthesis during S-phase leads to the production of two sister chromatids from the single chromatid of each of the chromosomes in the nucleus. As a result, at the end of S phase, each chromosome is composed of two morphologically and genetically identical sister chromatids.
G2 phase:
The second gap phase or G2 phase is comparatively of short duration. This follows the S-phase and shows the interval during which primary activities of calls take place. It includes RNA and protein synthesis.
Some protein synthesised during this period are essential for the entry of cells into mitosis. Some messenger RNA (mRNA) produced during G2are essential for the beginning of mitosis. Therefore, a cell entering interphase after telophase passes through G1, S and G2 phases. At the end of G2 mitosis occurs.
Cells with the capacity to divide are said to be in interphase or ‘resting phase’. The latter is obviously a misnomer. There is no cell which is always resting. Till the period, a cell is alive, it is physiologically active. Interphase nuclei are always bigger in size than those which have lost the power of division, essentially synthesise specific proteins and nucleic acids.
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Chromosomes are not clearly visible due to uncoiled structure and are present as long and fine threads. The interphase stage generally takes two-third time of the cell cycle. In Vicia faba the cell cycle is (Cell cycle: the entire interphase- mitosis- cytokinesis sequence make up the cell cycle [Wolf 1983]) of 30 hours duration whereas mitosis is completed within one hour.
For convenience of study, mitosis may be grouped into 4 sub-stages:
(a) Prophase
(b) Metaphase
(c) Anaphase
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(d) Telophase
There is no clear demarcation between any two of these phases. Where one ends, the next begins.
(a) Prophase:
This is the first sign of the mitosis. Chromosomes appear as fine threads and the nucleus looks like a ball of wool. As the stage goes under progress, chromosome becomes thicker and thicker due to continuous dehydration or removal of water, coiling and contraction.
The chromosomes from the very beginning appear to be made up of two threads or chromatids or daughter chromosomes which are coiled like wire springs or twisted ropes around each other and are identical in all respects, both in morphology and genetic make up or constitution, attached to the single centromere (the non-stained constricted portion of chromosome is said centromere or kinetochore).
Each chromatid possesses one complete set of DNA molecules or genes, replication of which has taken place during the interphase. Nucleolus and nuclear membrane are prominently visible in the beginning but gradually disappear. Disappearance of the nucleolus and nuclear membrane indicates the end of prophase.
In early prophase, the chromosomes are clearly visible as thin and thread like structure. In mid prophase the two daughter chromosome becomes shorter, thicker and more distinct. In late prophase all the chromosomes are visible lying near the centre. The thickening and shortening of the chromosome is maximum. The tendency of chromosomes to reach in the centre of the cell is referred to as prophase-Altenburg.
(b) Metaphase:
It generally begins with the disappearance of the nuclear membrane and nucleolus. Spindle fibres start appearing and get attached to chromosomes at the point of centromere. Chromosomes are visible lying on the equatorial or metaphase plate.
Metaphase plate lies in the middle of the two poles of the spindle apparatus. It may not necessarily be the central line or plane of the cell. It is merely an imaginary plane. Chromosomes are free floating in the cytoplasm.
Fibres, that are connected to the chromosomes at the point of centromere or kinetochore are said as chromosomal fibres and those which extend from one pole to another pole without any interruption are called continuous fibres. The spindles are made up of tubulin which is mainly protein and is part of microtubules.
The main features of metaphase are:
(i) Absence of nuclear membrane and nucleolus.
(ii) Arrangement of chromosomes on the metaphase or equatorial plate.
(iii) Chromosomes appear as the shortest and thickest structure.
(iv) Appearance of spindle fibre.
(v) Absence of relational coiling between daughter chromosomes (the two sister chromatids or daughter chromosomes of each chromosome are coiled in relation to each other, are referred to as relational coiling).
When the chromosomes are visible centrally situated or lying on the equatorial or metaphase plate is referred to as metaphase-Altenburg.
(c) Anaphase:
It may also be said as the migratory phase. Each centromere divides transversely to separate the two sister chromatids, now called chromosomes move towards the different poles of the cell. Chromosomal movement during anaphase is primarily due to contraction or shrinking or shortening of the spindle fibres.
During their movement the centromeres always remain forward and appear to be dragging or pulling the chromosome arms behind them. The chromosomes thus appear characteristically V, U and J-shaped. The tendency of chromosomes to reach at the different poles is referred to as anaphase-Altenburg.
(d) Telophase:
This may also be said as the reconstruction or the re-organisation phase resulting in the formation of two identical nuclei from the division of a single nucleus.
In this process, chromosomes in each pole undergo despiralization or uncoiling and hydration and become long, thin threads which form a network, the chromatin reticulum. Nuclear membrane and nucleolus are also formed simultaneously. When the chromosomes are found situated at the different poles is referred to as telophase-Altenburg.
Cytokinesis:
Karyokinesis (nuclear division) is generally followed by cytokinesis which divides cytoplasm in to two daughter cells. At the equatorial plate the origin of phraginoplast occurs which gives rise to cell plate and subsequently to cell wall. Phragmoplast develops from small vesicles of the Golgi body and inter-zonal spindle fibers which are formed by microtubules.
In plants the division of cytoplasm begins in the centre of the cell and gradually extends outside in a plane. In animals, cytokinesis takes place by cell furrowing in which a furrow starts in the middle of cell wall and gradually deepens or extends up to the middle of the cell, thereby dividing the cell into two parts.
At certain places where the cell wall, however, remains incomplete or vesicular layer has not fused completely, tubular connections are formed between the two daughter cells. It is called plasmodesmata.
The duration of mitosis may vary from nine minutes to several hours, depending upon the nature of species of plant and kind of cell. High specialized cells such as nerve cells divide slowly while the less specialized cells like the embryonic cells, divide rapidly. Temperature is another most important factor which affect the rate of mitosis. If any tissue or organ of the plant body is damaged or injured, the cells show rapid multiplication and bring about healing.
Significance of Mitosis:
(i) Mitosis causes an increase in the number of cells thus bringing about growth or replace cells lost by injury.
(ii) Maintain the constant diploid chromosome number from one generation to next generation in the plant body.
(iii) Gives rise to daughter cells which are identical in structure and function to the parent. In other words, there is uniparental inheritance. This is of great genetic significance.
(iv) It brings about vegetative or asexual reproduction.
Cell Division: Type # 3. Meiosis (= Reduction division = Gametogenic division = Meiocytes = Heterotypic cell division = Reproductive cell division):
Meiosis is a complex and special type of cell division in which the number of the chromosome is reduced to exactly half i.e., from 2 n to n. It consists of two nuclear divisions which follow one another, usually with a short interphase or resting phase in between, followed by cytokinesis. In some cases cytokinesis may or may not be visible.
In most animals the meiosis occurs just before fertilization and haploid gametes are formed (eggs and sperms). Union of the gametes give rise to formation of a diploid zygote (2n). In other words, meiosis takes place during gamete formation.
Meiosis was discovered by Farmer and Moore (1905). Reproductive cell are the site or place of meiosis. Meiosis involves two complete cell divisions. The first division is reductional, resulting in two haploid cells, the second division is equational, dividing each haploid cell into two identical or similar haploid cells. It involves two nuclear divisions and one chromosomal division.
Definition:
Meiosis is characteristic cell division of reproductive cell in which the chromosome number is reduced to half.
Or
Two cell divisions, occurring one after the other following only one DNA duplication (=replication) showing chromosome pairing (= synapsis) in zygotene, prophase I and chromosomes separation in Anaphase I in the first division and chromatid separation in the second, leads to the formation of haploid daughter cells, said as spores or gametes in plants and animals respectively.
Types of Meiosis:
It includes three types based on the variations in times and place of meiosis in the life cycle of different eukaryotic organisms.
(a) Gametic or Terminal meiosis (Diplotonic pattern):
In this case meiosis occurs immediately before gamete formation. As a result of meiosis, cells are transformed directly in to sperms and egg cell without further cell division. This type of meiosis is found in few lower plants and animals.
(b) Zygotic or Initial meiosis (Haplotonic pattern):
As a result of fertilization gametes fuse and form zygote. This is the only diploid stage (2n) in the life cycle. No longer zygote is formed sooner it enters meiosis and form 4 haploid cells which after germination give rise to 4 haploid individuals. The best example is thallophyta. It is also called primitive meiosis.
(c) Sporic or Intermediate meiosis (Diplohaplotonic pattern):
In these individual there is alternation between haploid and diploid generations. The fertilization results the diploid sporophyte generation. At some points meiosis occurs in spores in place of gametes. The spores after germination form gametophyte.
Few cells of the gametophyte produce gametes. Fusion of gametes returns the cycle to diploid sporophytic generation. This type of meiosis is found in higher plants and in some thallophyta. It is always absent in animals.
Premeiotic Interphase:
The interphase stage just before the entry of cell in to meiosis is said as premeiotic interphase. During ‘S’ phase of premeiotic interphase, chromosome duplication takes place but nearly 0.3% of the total DNA already present in the nucleus does not duplicate during the ‘S’ phase but in the zygotene sub-stage of prophase I stage.
The ‘S’ phase of a premeiotic stage is longer in duration in comparison of mitosis interphase to the same species. It is only because of synthesis of some specific type of histone. This histone is found in cells undergoing the process of mitosis.
During meiosis yet the nucleus divides twice ‘S’ phase occurs once only during the premeiotic interphase. In other words, clearly speaking, chromosome replicate only once, while the division of nucleus occurs twice during meiosis.
(The reduction in chromosome number of the daughter cell is mainly due to a single ‘S’ phase during the premeiotic interphase, followed by two successive nuclear divisions. Meiosis of one diploid cell (2n) gives rise to four haploid (n) daughter cells.)
1. Meiotic Division or First Nuclear Division:
Prophase I:
The prophase of the first meiotic division is usually a stage of very long duration. Its duration varies from a few hours to a few days.
On the basis of crucial events like:
(i) Pairing between homologous chromosomes
(ii) Condensation of chromosomes
(iii) Crossing over between them
(iv) Terminalization i.e., movement of chiasma to the outside or distal end etc. that occur during this stage has been subdivided in to 5 sub-stages-
(i) Leptotene
(ii) Zygotene
(iii) Pachytene
(iv) Diplotene and
(v) Diakinesis.
Leptotene or Leptonema (leptos-thin; nema = thread)
(i) This is also said as condensation stage. Chromosomes appear as a single, long, fine, thin, and thread like structure. Yet the chromosomes appear to be single but actually they are double due to DNA replication in interphase preceding leptotene.
(ii) Volume of the nucleus is markedly increased due to RNA synthesis.
(iii) Granular chromomeres on all chromosomes are visible at this stage. The number, shape and size and location of chromomeresis major characteristic of individual chromosomes and, therefore, it helps in their identification.
(iv) In microsporocytes of Lilium the chromosomes are visible in form of bunches on one side of nucleus or generally associated with the nuclear membrane and the remaining part of nucleus remain vacant. This peculiar arrangement is called ‘Bouquet’ and stage is called synizesis or synizetic knot.
(v) Half of the chromosomes are maternal and other half are paternal.
(vi) For every maternal chromosome, there is a corresponding paternal chromosome, identical in shape, size and nature of inherited characters.
(vii) It is of very short duration,
(viii) Nuclear membrane is very distinct.
(ix) Nucleolus is increased in size due to synthesis of RNA and protein.
(x) Chromosome number is haploid (n).
(ii) Zygotene or Zygonema (Zygo= mating or pair, nema=thread):
(i) Chromosomes become shorter and thicker due to continuous dehydration or removal of water.
(ii) Longitudinally or lengthwise the pairing of homologous chromosome begins called as synapsis or syndesis or synizesis. It should be well understood that the synapsis or lateral association of homologous chromosomes take place at the point of centromere only.
(iii) Due to synapsis chromosome now are seen in bivalent condition (2n) and become approximated throughout their length.
(iv) Synaptonemal complexes appear for the first time in zygotene. A protein made structure formed between homologous chromosomes during synapsis about 1000 Å wide is considered to make crossing over easier (Meyer).
(v) Pairing between the homologous chromosome begins at the point of centromere and proceed towards the end (pro-centric pairing) or may begin at the ends and proceed towards the centromere (pro-terminal pairing. Occasionally it may occur at any place simultaneously or at the same time.
(vi) Completion of replication of the remaining DNA (approximately 0.3%) of each nucleus takes place at this stage. This DNA does not replicate during the ‘S’ phase of premeiotic interphase and referred to as zygotene-DNA or Z-DNA. This is essential for synapsis or pairing of homologous chromosomes.
(iii) Pachytene or Pachynema (Pachy = thick, nema = thread):
(i) At this stage, chromosomes contract longitudinally so that they become thicker, shorter and coiled.
(ii) During this stage each bivalent become divided longitudinally in two sister chromatids. Thus, each bivalent contains 4-chromatids and are said to be in 4 strand or tetrad stage (4n). Electron micrographic studies have shown that each chromatid has its own centromeres. Thus in a tetrad, there are 4 centromeres.
(iii) The chromomeres, nucleoli and nuclear membrane are very distinct.
(iv) Two homologous chromosomes usually twist around each other, thus forming a relational coiling between them.
(iv) Diplotene or Diplonema (Diplo= two; nema = thread):
(i) In this stage, the homologous chromosomes of each bivalent begin to separate or repel from each other.
(ii) The two chromosomes constituting each bivalents become clearly visible, thicker and shorter due to longitudinal contraction.
(iii) Nucleolus, Nuclear membrane begins to disappear.
(iv) The chromosome number is maintained as 2n.
(v) The homologous chromosomes do not separate from other completely but still remain intact or jointed at one or more points. These attachments are called as chiasmata (plural of chiasma). The non-sister chromatids of inner side of the homologous chromosomes make the segmental interchange at certain points, thus the exchange of genetic material between the non-sister chromatids takes place. This phenomenon is called as crossing over. It is fundamental cause of variation. The chiasma is the result of crossing over, not the cause.
(vi) Crossing over was discovered first by Stern in Drosophila. The number of chiasma ranges from one to many depending on the length of chromosomes and nature of species. In long and short chromosomes more and less chiasma are formed respectively.
It has usually been found that the number of chiama is not more than 4 although the maximum number described is 13 or 14. Usually the number of crossing overs ranges from 1-3 and are said as single, double, triple crossing over respectively.
(vii) In this stage, synaptonemal complex is disappeared.
(viii) This stage is now said as transcription stage where the chromosomes more or less become unfolded known as de-condensation and become activated for the synthesis of RNA, proteins, lipids & carbohydrate molecules.
(ix) Matrix deposition already begins around the chromosome. Chiasmata are found in the meiosis of almost all eukaryote organisms. However, achiasmatic meiosis (meiosis without chiasma) has been reported in some organisms like males of higher diptera (including Drosophila), scorpion fly, many mantids, some scorpions and grass hoppers.
(A chiasma formed at the end of chromosome is said as terminal chiasma and formed along the lengths of chromosomes are called interstitial chiasmata).
(v) Diakinesis (Di= across or moving between):
(i) Nucleolus and nuclear membrane almost disappear at this stage.
(ii) Chromosomes become shorter and thicker due to further condensation.
(iii) Chromosomes continue to undergo further contraction, the only difference between diakinesis and diplotene the more contracted state of bivalents at diakinesis.
(iv) Chiasmata move towards the ends of chromosome or move from centre to peripheral ends. This is called as terminalization.
(v) Due toterminalization chiasmata are mainly terminal and chromosome counting is easy at this stage.
(vi) After the terminalization is completed in this stage, the different bivalents appear as in to different forms which depends upon three factors:
(i) Position of centromere in chromosomes
(ii) Number of chiasmata
(iii) Extent of terminalization on the basis of above mentioned factors.
(vii) Bivalents or tetravalent are visible in form of X, O, V or loop like in structure. According to Swanson (1942, 1957), terminalization is the result of despiralization of chromosomes.
(viii) The bivalents are now seen scattered in whole of the cytoplasm.
Metaphase I:
(i) Disappearance of nuclear membrane, nucleolus.
(ii) The chromosomes reach at their maximum contraction or shortening or shrinking.
(iii) The chromosomes now become arranged on the equatorial plate (= metaphase plate) so that the centromeres of homologous chromosome point towards opposite poles and lie on either sides of the equatorial plate where as the chiasmata lie on the equatorial plate. The arrangement of each bivalent is independent. On other hand maternal and paternal chromosomes point towards two opposite poles. In a polar view the bivalents appear to be arranged in a ring.
(iv) The spindle fibres now become visible and get attached to the centromere of the two homologous chromosomes.
(v) The bivalents are shortest at this stage and the centromeres are a little stretched.
(vi) Each bivalent consists of two centromeres.
(vii) The chromosome number is 2n.
(Spindle fibers: 3 types of spindle fibers have been distinguished, (i) chromosomal (ii) continuous and (iii) inter-zonal fibers. Chromosomal fibres extend from the centromere (=kinetochore) to the pole of the spindle. During cell division they become shortened and pull the daughter chromosomes apart. The microtubules (tubulin) of which the fibres are made, exhibit growth continuously from the centromeres to the poles.
Continuous fibres extend from pole to pole without attaching to the chromosomes. They elongate during cell division and push the spindle poles apart; thus separating the chromosomes. The inter-zonal fibres are formed between the poles, regardless of whether chromosomes are present or not. They are formed even after the chromosomes are removed by microsurgery.)
Anaphase I:
(i) The characteristic of this stage is the movement of one half of the chromosome to one pole and other half to the other pole. As a result, the number of chromosomes at each of the two poles of a cell is exactly half (n). Thus the chromosome number is reduced.
(ii) At this stage each chromosome is made up of two chromatids united by a centromere.
(iii)The movement of chromosomes to the poles is at random.
(iv) In each homologous chromosome one chromatid situated outwardly remain unchanged while the other situated inwardly has undergone mixing of maternal and paternal segments.
(v) Centromeres remain undivided and chiasmata disappear.
(vi) Reduction in number of chromosome is not only important from quantitative point of view but also from qualitative point. The chromosome complement of an individual is maintained always constant as well as some new characters also are produced due to re-assortment of chromosomes.
(vii) The chromosomes do not separate at a time. The short chromosomes separate quickly while the separation of long chromosomes is delayed because they have interstitial chiasmata (chiasma pertaining to intervening spaces).
Telophase I:
(i) Arrival of chromosome sets to opposite poles marks the beginning of telophase.
(ii) Only one partner of homologous pair (half the somatic number of chromosome reaches to the different poles, e.g., in Pisum sativum (2n= 14), 7 chromosomes go to one pole and rest 7 to other pole.
(iii) Chromosomes begin uncoiling.
(iv) Nuclear membrane and Nucleolus reappears.
(v) Daughter cells are produced by the formation of cell plate between the two group of chromosome. In other words, equal division of nucleus giving rise to two daughter nuclei is followed by equal division of cytoplasm bringing about formation of two daughter cells by act of cytokinesis said to as dyad.
Cytokinesis:
After nuclear division in Meiosis I, the cytoplasm -of every cell is divided equally in two parts thus each cell now containing haploid (n) nuclei. In some species, it has been found that division of cytoplasm only occurs at Meiosis II, never at the end of Meiosis I. The example is Trillium or several dicot plants. However, it has been observed in majority of monocots.
[In some species like Trillium, cells enter directly in to Prophase II from the Anaphase I. Cells do not enter into Telophase I and Interphase.
Interphase:
Normally Meiosis I is immediately followed by Meiosis II and interphase is absent but in some species, where it comes, it is of very short duration.
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It is most important and notable that there is no DNA replication during this interphase stage.
2. Meiosis II or Second Nuclear Division:
Prophase II:
This stage is like Prophase I but short duration. There is coiling, contraction of chromosomes and disappearance of nucleoli and nuclear membrane.
Metaphase II:
This is very much similar to metaphase of mitosis. Chromosomes are arranged on the equatorial or metaphase plate. Spindle fibres appear and remain attached at the centromere.
Anaphase II:
Two centromeres of chromosomes divide longitudinally and the sister chromatids of each chromosome begin to separate from each other and move away from the middle to the opposite poles by their centromeres.
Telophase II:
During this stage, sister chromatids reach to the different or opposite poles. Nucleolus reappears. Nuclear membrane is recognized from the elements of endoplasmic reticulum and chromosomes lose their individuality and elongate.
Cytokinesis:
Cytokinesis follows the Telophase II, New cell wall divides the two cells in to four, each of which has a haploid number of chromosomes. In plant cells, a set of such four cells is called Tetrad.
Significance of Meiosis:
(i) In sexual reproduction, zygote is formed by the union of male and female gametes. These male and female gametes known as haploid containing ‘n’ number of chromosome are formed as a result of meiosis and have received the half number of chromosomes. This diploid zygote (2n) develops into a new diploid organism. Thus meiosis helps to keep the chromosome number constant in the organism and in such a way the original number in an organism is maintained.
(ii) The haploid germ or sex cells, formed as a result of meiotic division provide a physical basis for the segregation and independent assortment of genes.
(iii) The phenomenon of meiosis is most essential for the completion of life cycle of a plant as it brings about a change from diploid to haploid generation.
(iv) The crossing over of genes between homologous chromosomes in the diploid stage is very useful as the exchange of genes leads to the formation or production of new recombinants.
