Cell division was first studied by Prevost and Dumas (1824) when they described cleavage in fertilized egg of Frog. Rudolf Virchow (1855, 1859) gave cell lineage theory that new cells arise by division of preexisting one-omnis cellulae cellula. Strasburger (1873) found that new nuclei develop from pre existing ones. The details were worked out by Strasburger (1875) and Flemming (1879).
The term of mistosis was coined by Flemming (1882). Meiosis was studied by Sutton (1900) and to some extent by Strasburger (1888) and Winiwater (1900). The term meiosis was coind by Farmer and Moore (1705). The agents which stimulate cell division are called mitogens e.g. cytokinins and some steroids. There are some agents which inhibits cell division. They are called mitotic poisons, e.g. azides, cyanides, colchicine etc.
(i) Cell Cycle:
Cell cycle is a series of cyclic changes through which a cell passes during its growth and divisions. Time interval between two successive divisions is called generation time. Cell cycle has two parts: long non-dividing I-phase and short dividing M-phase.
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Interphase (l-phase):
It is complex of changes that occur in newly formed cell before it is able to divide. Interphase has three stages
(a) G -Phase. In this, first growth or post mitotic gap phase, the cell and its nucleus grow in size. RNA and protein (including histone) synthesis occurs.
(b) S-Phase. Chromosomes replicate. Therefore it is called invisible stage of M-phase.
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(c) G2-Phase. It is the second growth or pre- mitotic gap phase where macro- molecules including protein tubulin are synthesised for cell division. A new centriole pair is formed.
M-Phase:
It is the phase of cell division. Cell division consists of nuclear division of karyokinesis and protoplast division or cytokinesis. Cell division or M-phase is of three types-amitosis, mitosis, meiosis.
(ii) Amitosis:
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(Direct Division, Robert Remark, 1841). During amitosis, nucleus elongates, constricts in the middle divides directly into two daughter nuclei. This is followed by division of cytoplasm through constriction, e.g. cartilage cells, degenerate cells.
(iii) Mitosis:
It is also called somatic division, because it occurs during formation of somatic or body cells. Mitosis is studied in plants in the regions of meristems, e.g. stem tip, root tip (onion 2n = 16). In animals it is studied in bone marrow, skin, base of nails etc. Mitosis is equatorial division in which a parent cell divides into two identical daughter cells; each of which contains the same number and kind of chromosomes as are present in the parent cell. Mitosis occui in two steps, karyokinesis & cytokinesis.
(A) Karyokinesis :
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It is the stage of nuclear division (indirect nuclear division) which is continuous but is divided into four stages for the sake of convenience-prophase, metaphase, anaphase and telophase.
(a) Prophase:
In early prophase, the chromatin fibres condense to form elongated chromosomes. The nucleus appears as ball of wool. Centrosome has already divided. The daughter centrosome begins to move away from each other. In mid prophase, chromosomes shorten and become distinct with each having two chromatids attached to narrow point called CENTROMERE.
The centrosomes develop astral rays and migrate farther. In late prophase, the centrosome reaches the poles, form asters and begins to develop spindle fibres. Nucleolus degenerates and nuclear envelope starts breaking. In plant cells, centrosomes are absent, spindle fibres develop without them.
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(b) Metaphase:
A bipolar spindle is produced. It is anastral (without asters) in plants and ainphiaster in animals. Spindle is observable with the help of polarising microscope. Spindle has three types of fibres (I) Continuous (pole to pole) (II) Discontinuous (come out of one pole but do not reach the other pole, (III) Chromosome Fibre (Fibres that attach chromosomes to spindle poles.
Each spindle fibre is made of 4-20 microtubules. The middle part of spindle having the maximum diameter is called equator. Chromosomes move to the equatorial plane and form equatorialor metaphasic plate with their centrosomes attached to both the poles by chromosomal fibres. Colchicine arrests cell division at metaphase stage. Metaphase is the best time to see chromosomes.
(c) Anaphase:
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The centromere of each chromosome divides. This converts the two chromatids
into daughter chromosomes each being attached to the spindle pole of its side by independent chromosomal fibre. The chromosomes move towards the spindle poles. The two pole-ward moving chromosomes of each type remain attached to each other by interzonal fibres. Ultimately, two groups of chromosomes come to lie at the spindle poles. The spindle fibres shorten one third to one fifth of the original length.
(d) Telophase:
Chromosome again unfold, elongate forming chromatin fibres. Nucleolus, nucleoplasm and nuclear envelope appear form ER so that two daughter nuclei are formed.
Dinomitosis is the type of nuclear division found is dinoflagellates in which the nuclear envelope persists. Microtubular spindle is not formed. Chromosome move while attached to inner membrane of nuclear envelope.
Karyochorisis : Intranuclear spindle is formed with spindle pole bodies (SPBs) developing at the two ends.
(B) Cytokinesis:
It is the process of separation of the cytoplasm. In animal cells, there is constriction at the equator that finally results in the separation of the daughter cells. It is of two types.
(a) Cleavage Cytokinesis:
The cell membrane constricts and develops a centripetal furrow or cleavage in the middle. The furrow deepens and divides the parent protoplast into two uninucleate protoplasts or cells. Cleavage is the usual method of cytokinesis in animal. It also occurs in some lower plants where wall material is deposited in the furrow between the two daughter protoplasts.
(b) Cell Plate Cytokinesis:
It occurs in plants vesicles having pectic compounds and other wall materials appear in the middle of persisting spindle called ‘Phragmoplast’. They fuse and form a film or cell plate with membrane on either side. This divides the parent binucleate cell into two daughter uninucleate cells. Cell plate grows centrifugally and functions as middle lamella. Primary wall is deposited on its either side by the two daughter protoplasts.
(iv) Meiosis:
Meiosis is a special type of cell division present in germ cells of sexually reproducing organisms. Sexual reproduction occurs by way of sexual cells or gametes (eggs & sperms), which after fertilization constitute the Zygote. Meiosis consists of single duplication divisions.
In animals and lower plants, meiosis is terminal or gametic (i.e., it occurs before the formation of the gametes). In the male, four haploid sperms are produced, in female, one ovum and three polar bodies are produced. In most plants, meiosis is intermediary or sporic i.e. it occurs sometime between fertilization and the formation of gametes). Cells in meiosis are called Meiocytes.
Meiosis is divided into division I and II. In division I, there is a long prophase of which the stages are preleptonema and leptonema, zygonema, pachynema, diplonema and diakinesis.
(A) Meiosis I:
Meiosis I is the actual reduction division which is also called heterotypic division (the two chromatids of a chromosome often become different due to crossing over.)
(a) Prophase I : It is longest phase which is complex and divisible into five stages-leptotene, zygotene,pachytene, diplotene and diakinesis.
(i) Leptotene:
(Leptonema). Chromatin fibres condense and form chromosomes. The chromosomes often show chromomeres. They may also develop baset like arrangement called bouquet stage (diverging from a common point lying near centrosome). Chromosome number is diploid where there are two chromosomes of each type called ‘homologous chromosomes’. Their chromatids are not clear because of the development of nucleoprotein core between them.
(ii) Zygotene:
(Zygonema). Homologous chromosomes join laterally in the process of synapsis (Montgomery, 1901 )lsyndesis to form bivalents. The two chromosomes of a bivalent are held together by nucleoprotein core. The whole structure is called ‘Synaptonemal Complex’ (Moses, 1956). SC is composed of two lateral arms (which appear in each homologous at the end of leptonema) and a medial element.
At the time of paring the packing of DNA is 300/1 and there is only 0.3% matching between homologous DNA. Pairing starts a random, but telomeres are generally inserted at the nuclear envelope. The 0.2 µm space between homologous chromosomes is occupied by the SC. This may have the appearance of a ladder, with bridges crossing the medial element.
The main component of this complex is protein. The macromolecular organization of meiotic chromosomes is similar to that of mitotic ones with a 20-30 nm chromatin fiber composed of nucleosomes.
In early prophase, the fibres make discrete loops that coalesce in dense foci, where the lateral arms of SC are deposited. The packing of there loops increases throughout meiotic prophase, and some RNA transcription may be seen in certain loops corresponding to active nucleolar genes.
(iii) Pachytene (Pachynema):
The nonsister chromatids of a bivalent may exchange segments in the process of crossing over. However, individual chromatids are not clear. In this pairing is complete and chromosomes become shorter & thicker. The number of chromosomes has been halved (i.e. bivalents or tetrads). Each tetrad has four kinetochores (two homologous and two sisters).
During pachynema, two homologous chromatids exchange segments at a molecular level (recombination). The SC appears to stabilise the pairing thus enabling the interchange. At pachynema the SC may show the recombination nodules or bars, which may be actual sites of crossing over.
(iv) Diplotene:
(Diplonema). During diplotene the paired chromosomes begin to separate, but they are held together at the chiasmata (points of interchange or crossing over). Chiasmata (first seen by Johanssen, 1909) or nucleoprotein attaching points occur at places between the homologous chromosomes.
There is at least one chiasma per bivalent chromosome. At diplonema, the SC is shed from the bivalents. At each chiasma, there is a piece of SC that ultimately disappears and is replaced by a chromatin bridge. Diplonema may last for months or years. In this the chromatids become clear. Each bivalent now appear as tetrad.
(v) Diakinesis:
Chiasmata shift towards the chromosome ends (terminalisation). Nucleolus degenerates. Nuclear envelope breaks at places. A spindle begins to develop with centrioles in animals & without centrioles. There is reduction in the number of chiasmata and further contraction. This is followed by prometaphae, metaphase I, anaphase I, telophase I.
(b) Metaphase I:
A bipolar fibrous spindle appears in the area of nucleus. It has asters at the two poles in animal cells (amphiaster) while the same are absent in plant cells (anastral). The chromosome pairs shift to the equator of spindle forming two or double metaphasic plate with the help of their centromeres. Each chromosome gets attached to spindle pole of its side by a chromosome fibre.
(c) Anaphase I:
Chiasmata disappear completely and the homologous chromosomes separate. The process in called disjunction. The separated chromosomes (univalents) show divergent chromatids and are called dyads. They move towards the spindle poles and ultimately form two groups of haploid chromosomes.
(d) Telophase I:
Chromosomes elongate. Nucleolus, nucleoplasm and nuclear envelope appear over each chromosome group forming nuclei.
During the interphase between the two meiotic divisions there is no replication of the chromosomes, and division II is very similar to mitosis, by the end of which each nucleus has a haploid number of chromosomes composed of single chromatids.
The fact that in meiotic division I there is separation of homologous kinetochores (and chromatids) and in division II the sister kinetochores (and chromatids) are separated, can be explained by the orientation of kinetochores, which determines nucleation and preferential polarization of microtubules.
(B) Meiosis II:
It is homotypic or equational division which is meant for maintaining the haploid number and separating the two chromatids of a chromosome which have become “different due to crossing over. DNA replication is absent.
(a) Prophase II:
The chromatin fibres shorten to form chromosomes. Nucleolus and nuclear envelop break down. Spindle in formed in the area of each nucleus. Both telophase I and prophase II are omitted in some organisms where anaphase I directly leads to metaphase II; e.g. Trillium.
(b) Metaphase II:
Chromosomes come to lie at the equator of the spindle forming a single metaphasic plate. The centromere of each chromosome gets attached by both its surfaces to the spindle poles of their sides by distinct chromosome fibres.
(c) Anaphase II:
Centromere of each chromosome divides into two. This separates the two chromatids of a chromosome into two independent daughter chromosomes. Each daughter chromosome is attached to spindle pole of its side by a chromosome fibre. Chromosomes move towards the spindle poles forming two groups. Since there were two spindle, a total of four groups are formed.
(d) Telophase II:
The four groups of chromosomes organise themselves into four haploid nuclei.
(C) Cytokinesis:
Cytokinesis may occur after each division (successive type) or simultaneously at the end of meiosis. It is generally through cleavage. In case of plants, wall material is deposited in the furrows. Cytokinesis gives rise to four haploid cells.
(D) Significance of Meiosis:
In all organisms ‘Meiosis’ takes place at the time of gamete formation. 4 haploid cells are produced as a result of meiosis in a diploid cell. The product of meiosis gives rise to gametes after metamorphosis some of them may be viable or nonviable.
The object of reduction division is as follows:
(a) To keep the chromosome number same in daughter cells.
(b) To keep the parental characters in daughter cells.
(c) This division enables the cells to pass from one generation to another.
(d) In the sporophytic generation, the number of chromosome is 2x i.e. diploid and passes into gametophytic generation, reduction in number of chromosomes take place so that each of the gametophytic generation has got only x number of chromosomes. So this phenomenon is most essential for completing a life history of a plant.
(e) Fertilization duplicates the chromosome number but meiosis halves it to maintain the chromosomal balance in all organisms.
(f) At the time of chiasmata formation, the exchange of chromatid material causes a combination of new characters. Thus meiosis helps in bringing about variation in population.
Meiosis may last for 50 years in human female as the number of oocyte in a new born female is about one million. By the age of 07 years, there are some 300,000 oocytes left while only 400 reach maturity between 12 & 50 years. Thus meiosis may last upto 50 years. This may explain the increase in incidence of chromosonal aberrations with the increasing age of mother. While in human male, meiosis starts after puberty.