In this article we will discuss about Meiosis which is a type of indirect cell division:- 1. Subject-Matter of Meiosis 2. Types of Meiosis 3. Meiocytes 4. Process 5. Significance.
- Subject-Matter of Meiosis
- Types of Meiosis
- Process of Meiosis
- Significance of Meiosis
1. Subject-Matter of Meiosis:
In the sexually reproducing organism two sex cells or gametes containing haploid number of chromosomes (n) fuse and form a zygote. The zygote possesses a nucleus which contains double number of chromosomes. Such cells are commonly symbolised as diploid or 2n cells. If mitosis is the only method of nuclear division the number of chromosomes would be doubled in succeeding generations.
But, the chromosome number for a particular animal or plant species is fixed. It is, therefore, necessary that the gametes must be provided with haploid chromosome number in order to keep the number of chromosomes of a given species constant in each generation.
The number of chromosomes of a gamete is symbolised by ‘n’ (haploid or gametic chromosome number). The mechanism which reduces the double (diploid or 2n) complements of chromosomes to haploid set (n) in each sexual generation is called meiosis.
Weismann (1887) was the first to point out the reduction of chromosome number in the reproductive cells. Later Farmer and Moor (1905) proposed the term meiosis for the special type of cell division that reduces the chromosome number by half from diploid to haploid condition. The products of meiosis have half as many chromosomes as the parental cell possesses.
During meiosis, homologous chromosomes of a diploid nucleus pair, replicate only once producing 4n and undergo assortment so that each of the four daughter cells resulting from two successive divisions of the meiotic cell or meiocyte receives one representative of each chromosome set.
Thus the diploid chromosome number is meiotically reduced to haploid number (n), characteristic for the gametes or haploid phase of the life cycle.
2. Types of Meiosis:
The reduction of chromosome number takes place either before or after reproduction. In animals the reduction division occurs during germ cell formation, but in higher plants it occurs generally in the spore mother cells during sporogenesis (spore formation).
On the basis of time at which meiosis takes place, three different types of meiosis have been recognised:
a. Terminal or gametic meiosis:
It is found in animals and a few lower plants. This type of meiosis occurs just before the formation of gametes or gametogenesis.
b. Initial or zygotic meiosis:
This type of meiosis is seen in plants with haplontic life cycle and it occurs in zygote just after fertilization.
c. Intermediary meiosis or sporic meiosis:
It is a characteristic of species with diplo-haplontic life cycle. This type of meiosis takes place at sometime intermediate between fertilization and formation of gametes. It is involved in the production of spores or sporogenesis.
The cells which divide by meiosis are called meiocytes. In animals the primary oocytes and primary spermocytes are meiocytes. In most of the higher plants the meiocytes are sporocytes (spore mother cells) giving rise to microspores or pollen grains (male sex) and megaspores (female sex). In haploid plants, the zygotes act as meiocytes.
Important materials for the study of this process in plants are young anthers of Tradescantia, Datura, maize, Vicia and onion.
4. Process of Meiosis:
Meiosis completes in two successive divisions:
1. Meiosis I or reduction division or heterotypic division.
2. Meiosis II or homotypic division.
Both these divisions involve two steps:
A. Division of the nucleus or karyokinesis
B. Division of cytoplasm or cytokinesis
l. Meiosis First:
It is very important division in which the chromosome number of a cell is reduced to half in daughter cells. It is also called heterotypic division or reduction division.
The nuclear division of first meiosis includes the following four successive stages:
(i) Prophase I,
(ii) Metaphase I,
(iii) Anaphase I, and
(iv) Telophase I.
(i) Prophase I:
The prophase of heterotypic division is a long process and is further divided into five successive sub-stages for convenience in description.
These sub-stages are:
(a) Leptotene or Leptonema.
(b) Zygotene or Zygonema,
(c) Pachytene or Pachynema.
(d) Diplotene or Diplonema.
Prior to onset of prophase I, the nucleus is said to be in metabolic stage. The metabolic nucleus is bounded with a definite nuclear membrane. The nuclear substance is very dense, and contains one or more nucleoli and inconspicuous chromatic reticulum. DNA synthesis takes place during the S-phase of premeiotic interphase, but the linkers remain in single state (Uhl, 1965).
It is a short process. At this stage, the nucleus size increases considerably. The chromosomes appear as long slender threads bearing numerous chromomeres in a bead like fashion (Fig. 11.7 A and II. 10 A).
The chromosomes are evenly dispersed in the nuclear field, but sometimes they are present only dispersed in the nuclear field, but sometimes they are present only in one part of the nucleus leaving the other side completely empty. The latter condition is called “synizesis”. Sharp termed it, “bouquet formation”.
The chromosomes are widely despiralized showing only relic or minor coils. They appear as optically single chromonemal threads and the differentiation into chromatids is not yet recognizable although DNA is doubled during S phase. They are so fine that microscopic observations are difficult and even if they are double stranded, the two strands are entirely beyond the resolution limits of the light microscopes.
In some cases, the nucleolus produces by budding process subsidiary nucleoli. There is a marked increase in RNA and protein synthesis during leptotene which accounts for the increase in size and budding of nucleolus.
At the beginning of this stage, the homologous chromosomes start pairing (Figs. 11.7 B and 11.10 B). Here morphologically and genetically similar chromosomes are generally referred to as homologues, but some cytologists hold the view that only those chromosomes should be called homologues which pair at zygotene stage.
One chromosome in each pair of homologues is maternal, i.e., it comes from the mother and the other is paternal, i.e., from the father side. The phenomenon of chromosome pairing is known as synapsis. The pairing of homologous chromosomes is initiated by some sort of attraction force. The early suggestion was that the electrostatic forces were involved in the synapsis.
The second idea based on physical observations holds that hydrodynamic force which develops in the nucleoplasm causes vibration in the chromosomes. Due to this force the homologous chromosomes move slowly but surely to form the synaptic pairs.
Three types of synapsis have been recognised which are as follows:
(i) Precentric pairing:
C.D. Darlington (1935) demonstrated that the pairing starts at the centromeres. This is procentric synapsis.
(ii) Proterminal synapsis:
Nathani, Nandi and many others have observed that the pairing of homologous chromosomes starts from the end and proceeds towards their centromeres. This is proterminal synapsis.
(iii) Localised or Random pairing:
In this type of synapsis the pairing of homologues occurs at several points at random. Synapsis continues along the length of chromosomes until it is complete and no unpaired region is left. Why do the homologous chromosomes pair in meiosis only? There are two theories to explain the phenomenon of pairing:
(a) Precocity Theory:
According to Darlington, the chromosomes in early meiotic division are in single state while those in the mitosis are double from the very beginning. The argument then is that ‘pairing force’ is satisfied by similar chromatids of the mitosis chromosomes and thus no residual force is left which can bring the homologous chromosomes together.
In meiosis during the leptotene stage the chromosomes are in singlet state, thus the pairing force would exist between the homologues. In light of the fact that DNA synthesis and chromosome replication occur during S phase of pre-meiotic interphase this theory is not substantiated.
(b) Retardation Theory:
According to Sax and others, the pairing of homologues occurs on account of retardation of cellular metabolism during early prophase I. Since the prophase I is of long duration, the reduced metabolism results in uncoiling of relic spirals of chromosomes and subsequent point to point pairing of homologous chromosomes in Zygotene. This theory is also not very satisfactory but may have some support.
Sometimes, pairing may also take place between two partially homologous or dissimilar chromosomes, although synapsis may involve considerable distortion of the longer chromosome. Partially homologous chromosomes are referred to as homoelogous chromosomes. Homoeology designates the residual homology of originally completely homologous chromosomes.
In case of diploid organisms, the number of pairing configurations, the bivalents, is equal to half their somatic chromosome number. If there are more than two sets of homologous chromosomes in a cell as for example, in autotetraploids, the pairing takes place between two chromosomes only in one region.
In autotetraploids in which there are four homologues, two chromosomes pair in one region and the other two will pair with unpaired parts of the partly paired homologues. The pairing configurations consisting of more than two homologues are called multivalents like trivalents, quadrivalents and so on.
Stem has recently provided evidence for the synthesis of a small amount of DNA during zygotene and pachytene in trillillium. This is one of the basic differences between the prophase of mitosis and meiosis because DNA synthesis is completely suppressed in mitotic cells.
Along with DNA synthesis there also goes on the synthesis of nuclear protein, possibly the histone that complexes with DNA. It has been suggested that for normal pairing the DNA and histone must be in a definite proportion.
The pairing of homologous chromosomes continues throughout the zygotene in a manner resembling a zipper.
The physical basis of chromosomal pairing has been somewhat elucidated by the observation of synaptinemal or synaptonemal complex. In electron micrograph the synaptonemal complex (SC) appears as a linear complex of three parallel proteinaceous strands situated between the pairing of homologous chromosomes.
It consists of a dense central element flanked by two lateral elements (Fig. 11.9). The dimensions and spacing of the various components may vary in different species. The lateral elements vary in diameter from 30 to 65 nm, the central element between 12 and 50 nm and the central space between 65 and 120 nm.
The total width of synaptonemal complex ranges from 160 to 240 nm, i.e., it is less variable than its component parts. The central element of the synaptonemal complex is joined to the lateral elements by delicate transverse filaments or L.C. fibres that may serve to align and join two homologous chromosomes via lateral elements and to stabilize the pairing homologues at a fixed distance.
The transverse filaments are fine (1.5 to 2.0 nm), straight, non-looping protein fibres that originate in the lateral elements, have a remarkable fixed length and in central region, together with additional protein material, they form central element. The synaptonemal complexes of most organisms terminate at the nuclear envelope.
The attachment point may be polarized or may show no definite pattern of arrangement.
The current ideas on structure and function of the synaptonemal complex propose that synaptonemal complex forms a structural framework around which chromatin is affanged in such a way that the mass of chromatin which is not directly involved in genetic recombination, is sequestered on the outer surface of the lateral elements and only small stretches of chromatin penetrate the lateral element and enter the central space where they engage in molecular pairing and genetic exchange.
Usually the synaptonemal complex is dissociated from the chromosomes at diplotene stage and then dispersed or aggregated into polycomplexes.
The pachytene starts when the pairing is complete. It is usually of long duration. The chromosomes show thickening and shortening. Two components of each bivalent coil around each other by relational coiling (Figs. 11.8 B and 11.10). The coiling is supposed to develop due to some internal forces in the chromosomal threads.
Now the homologous chromosomes of all the bivalents split longitudinally and thus in late pachytene stage the bivalents appear four stranded. The chromatids of each homologue remain paranemically coiled (Fig. 11.5). The polar arrangement of chromosomes, which is so characteristic of leptotene and zygotene, usually disappears to a considerable extent in the late pachytene stage.
The nucleolus still remains attached to nucleolar organising region of a particular tetrad.
This stage is marked by cessation of attraction force between two homologous chromosomes. Now uncoiling of the homologues takes place and a repulsion force develops which separates the two partners of the bivalents apart (Figs. 11.7, 11.8 C and 11.10 D). The homologous chromosomes although tending to separate from one another remain still attached at certain points.
These points are called chiasmata (singular-chiasma). Number of chiasmata determines the shape of chromosomes. If there are two chiasmata, the chromosomes appear ‘O’ shaped and if there are many such points then they may be loop-like.
At chiasmata, the sister chromatids of bivalent are broken and rejoined crosswise, i.e., reunion takes place between segments of non-sister chromatids as is shown in Fig. 11.11. This phenomenon is known as ‘crossing-over’.
Crossing-over may take place in pachytene stage after the duplication of chromosomes. Crossing-over involves the exchange of the chromatid segments and in this way it results in new types of chromatids.
If there is no exchange of segments in the reunion, then the original type of chromatids will be formed. Stern and Hotta (1969) have reported that the breakage of chromatid is facilitated by an enzyme the endonuclease which is reported to increase in the nucleus during the pachytene stage.
After the division of chromatids, the segments of non-sister chromatids are united and an enzyme ligase is known to help in this process.
In the late stage, the loops between the chiasmata become wider and tend to move away from centromere towards one or both the ends of the chromosomes. This process is known as terminalisation. Nucleolus becomes disorganised and simultaneously the matrix sheaths appear around the chromosomes. The chromosomes become very much coiled and shortened. The nuclear membrane remains visible.
The last sub-stage of prophase I is diakinesis. In this stage, chromosomes appear highly contracted due to formation of major coils. The separation of homologous chromosomes is complete. The nuclear membrane begins to disappear gradually (Figs. 11.7 E, 11.8 D and 11.10 E). Exchange of parts between the chromatids of homologous chromosomes may also take place at this stage.
The matrix sheath obscures the internal structure of the chromosomes. The surfaces of the chromosomes appear rough. The sheath is visible only under the light microscopes. De Robertis (1954), on the basis of observations made with electron microscope, denied the existence of the matrix sheaths around the chromosomes.
The metaphase stage of first meiosis is marked by complete disappearance of the nuclear membrane. Spindle fibres develop from the nuclear wall material and ground substance. Four stranded chromosome bivalents are arranged at the equator of the spindle in such a way that their individual undivided centromeres lie in the middle of spindle equidistant from the poles (Figs. 11.7F, 11.8E and ll.10F).
Parts of both the homologues of a bivalent on one side of the centromeres show coiling in one direction, but those on the other side of the centromeres show coiling in reverse direction. The spindle fibres become attached with the centromeres of bivalents; one centromere of each bivalent becomes attached with the fibre of one pole and the other centromere attaches with the fibre coming from the opposite pole.
The metaphase I is terminated by anaphase I. In the anaphase I, the centromeres of homologous chromosomes of bivalents repel each other (Figs.) 1.7 G, 11.8 F, G and 11.10 G). After the separation of centromeres, the homologous chromosomes begin to move apart towards the two opposite poles of spindle.
The chromatids of chromosome are not separated at the spindle attachment region as the centromere remains undivided in the heterotypic division. At late anaphase, one homologue of each bivalent reaches to one pole and the other to the opposite pole. Though there are chances that the maternal and paternal chromosomes may go to different poles yet their movements is independent and at random.
The future haploid cells would, therefore, contain a mixture of paternal and maternal chromosomes. If there are two bivalents in nucleus four different combinations are possible from these two pairs. If there are ‘n’ pairs of chromosomes, the possible number of chromosome combinations is calculated by a formula (2)n.
In human, for example, there are 23 pairs of chromosomes, so the possible number of chromosome combinations is (2)23 or 83, 88, 608 even after neglecting the additional possibilities provided by crossing over.
When the chromosomes reach to the poles, they become shortened in length as the spirals are brought I closer. Each chromosome is surrounded by thick matrix sheath. The major coils disappear, and minor coils then become very clear. The chromosomes assume regular zig-zag shapes. Two daughter nuclei with nuclear membranes and nucleoli are formed from two groups of chromosomes.
The two newly formed nuclei with reduced chromosome number are sometimes separated by a cross I wall. Thus a dyad is formed. In some cases, however, cytokinesis is postponed till the end of the second division.
This stage may be very short or long or in some cases absent. In the cases, where interphase is absent, the doubled chromosomes directly pass to the prophase of homotypic division or second meiosisi without undergoing any remarkable change. However, in the cases where interphase exists, the daughter nuclei increase in volume and become ovoid.
The major spirals are further despiralized and minor spirals also uncoil to a certain extent. The matrix seems to be lost and the threads appear more clear and delicate.
II. Homotypic Division or Second Meiosis:
It is a simple mitotic division.
The nuclear division includes the following four successive stages:
(i) Prophase II,
(ii) Metaphase II,
(iii) Anaphase II, and
(iv) Telophase II.
This stage is of very short duration. The chromosomes do not undergo any appreciable change. They are already split into two sister chromatids during pachytene of first prophase. The two chromatids of each chromosome remain loose except at the centromere. At late prophase, the chromosomes are much thicker and shorter than at early stage (Figs. 11.7 I,11.8 I and 11.12 A1, and A2).
The centromeres of the chromosomes remain still undivided. Disappearance of the nuclear membrane and simultaneous formation of the spindle terminate the prophase II. In some cases the prophase II is absent and the chromosomes after anaphase I directly from metaphase II plate.
The doubled chromosomes in each daughter cell move in middle region of the cell and are finally arranged on the equator of the newly formed spindle (Fig. 11.7 I and 11.8 J). The centromere of each chromosome now divides in the longitudinal plane to form two daughter centromeres; each daughter centromere thus becomes the centomere of the chromatid.
Finally, the centromeres of chromatids become connected with attachment fibres from their respective spindle poles.
The anaphase II starts with the repulsion of centromeres of sister chromatids (Figs. 11.7, 11.8 K and 11.12BI, B,). Now the daughter chromatids of chromosomes start moving apart towards the opposite poles of spindle. At the end of this process one chromatid of each chromosome goes to one pole and the other to opposite pole.
When the daughter chromosomes have reached to the spindle poles, the chromonemata anastomose to form the nuclear reticulum. The nucleolus begins to organise on one chromosome and the nuclear membranes appear around the chromosomal groups (Fig. 11.7 L and 11.8 L).
Intermediate walls develop between two daughter nuclei of each cell after the completion of telophase n. Division of the cytoplasm is accomplished either by furrowing or by cell plate formation. Finally, four cells, each with single haploid nucleus, are formed in meiosis (Figs. 11.12 C1, C2, D1, and D2).
From the above analysis of meiosis, it is concluded that the chromosomes in the leptotene stage are found in singlet state. At zygotene stage homologous singlet chromosomes pair and then the reproduction (duplication) of each chromosome occurs during pachytene. After duplication, the homologous chromosomes show a tendency to repel each other during diplotene and diakinesis stages.
The duplication of chromosomes is followed by two divisions. First division is heterotypic or reductional which separates the homologous chromosomes into two equal sets. Thus the original number of chromosomes is reduced by half the second division is a simple mitosis which takes place in the two nuclei resulted from heterotypic division and separates the daughter chromatids of the chromosomes.
The two successive divisions of diploid nucleus thus result into four nuclei having haploid set of chromosomes.
The discussion ends with one question—what is the need of second meiosis or homotypic division if the reduction of the chromosome number has already taken place in the first meiosis? No proper explanation is given for this question, but during the gametogenesis, second division is of much importance in the organisms which have 2n chromosomes.
5. Significance of Meiosis:
Meosis is a complex process and is of great significance in the life cycle of plants and animals which reproduce sexually.
Significance of meiosis is as follows:
1. If the mitosis is the only method of cell division, after every act of fertilization the chromosome number of individual will become doubled in subsequent generations. The increase in nun of chromosomes is harmful and creates imbalance between cytoplasm and the nucleus which may ultimately cause several variations and malformations in the organisms. Meiosis helps keeping the number of chromosomes constant in the species.
2. During the diplotene stage of meiosis 1, crossing-over provides an opportunity for the exchange of genes between homologous chromosomes. Thus chromosomes with changed genetic constitution are formed which may cause mutations in the species. Variations are root causes of evolution.
3. Failure of meiosis leads to the formation of diploid gametes which after fertilization form polyploid forms.
4. Meiosis which results in the formation of haploid gametes also provides a way for the segregation and independent assortment of genes.