In this article we will discuss about the meiotic division of a cell.
The meiotic division includes two complete divisions of a diploid cell resulting into four haploid nuclei. The first meiotic division includes a long prophase in which the homologous chromosomes become closely associated to each other and interchange of hereditary material takes place between them.
Further, in the first meiotic division the reduction of chromosome number takes place and, thus, two haploid cells are resulted by this division.
The first meiotic division is also known as the heterotypic division. In the second meiotic division, the haploid cell divides mitotically and results into four haploid cells. The second meiotic division is also known as the homotypic division. In the homotypic division pairing of chromosomes, exchange of the genetic material and reduction of the chromosome number do not occur.
Both the meiotic divisions occur continuously and each includes the usual stages of the mitosis, viz., prophase, metaphase, anaphase and telophase. The prophase of first meiotic division is very significant phase because the most cytogenetical events such as synapsis, crossing over, etc., occur during this phase.
The prophase is the longest meiotic phase, therefore, for the sake of convenience, it is divided into six sub stages, viz., preleptonema (proleptotene), leptonema (leptotene), zygonema (zygotene), pachynema (pachytene), diplonema (diplotene) and diakinesis.
The successive meiotic sub stages can be represented as follows:
Heterotypic Division or First Meiotic Division:
In the beginning of the first meiotic division, the nucleus of the meiocyte starts to swell up by absorbing the water from the cytoplasm and the nuclear volume increases about three folds. This increase in the volume of the nucleus causes modification of nuclear components. After these changes, the cell passes to the first stage of first meiotic division which is known as prophase.
The first prophase is the longest stage of the meiotic division. During this stage, the amount of DNA becomes double. Most of the synthesis of DNA occurs at the beginning of this phase.
It includes following sub stages:
1. Preleptotene or Preleptonema:
The preleptotene stage closely resembles with the early mitotic prophase. In this stage, the chromosomes are extremely thin, long, uncoiled, longitudinally single and slender thread-like structures.
2. Leptotene or Leptonema:
In the leptotene stage, the chromosomes become more uncoiled and assume a long thread-like shape. The chromosomes at this stage take up a specific orientation inside the nucleus; the ends of the chromosomes converge toward one side of the nucleus, that side where the centrosome lies (the bouquet stage).
The centriole duplicates and each daughter centriole migrates towards the opposite pole of the cell. On reaching at the poles, each centriole duplicates and, thus, each pole of the cell possesses two centrioles or a single diplosome.
3. Zygotene or Zygonema:
In the zygotene stage, the pairing of homologous chromosomes takes place. The homologous chromosomes which come from the mother (by ova) and father (by sperm) are attracted towards each other and their pairing takes place. The pairing of the homologous chromosomes is known as synapsis (Gr., synapsis = union). The synapsis begins at one or more points along the length of the homologous chromosomes.
Three types of synapsis have been recognized:
(i) Pro-terminal synapsis:
In pro-terminal type of synapsis, the pairing in homologous chromosomes starts from the end and continues towards their centromeres.
(ii) Pro-centric synapsis:
In pre-centric synapsis, the homologous chromosomes start pairing from their centromeres and the pairing progresses towards the ends of the homologous chromosomes.
(iii) Localised pairing or Random synapsis:
The random type of synapsis occurs at various points of the homologous chromosomes. The pairing of the homologous chromosomes is very exact and specific. The bouquet is supposed to maintain a regularity in the synapsis mechanism.
4. Pachytene or Pachynema:
In the pachytene or pachynema stage, the pair of chromosomes become twisted spirally around each other and cannot be distinguished separately. In the middle of the pachynema stage, each homologous chromosome splits lengthwise to form two chromatids.
Actually, the doubling of the DNA molecule strands, which is necessary for the subsequent duplication of the chromosomes, occurs earlier, before the beginning of meiotic prophase.
Through the earlier part of the meiotic prophase, however, the DNA molecule in each chromosome behaves as a single body. In the pachynema stage, this is now changed, the two chromatids of each chromosome containing half of the DNA present in the chromosome at start, become partially independent of one another although they still continue to be linked together by their common centromere.
The pachynema chromosome, thus, consists of four chromatids closely joined together in one complex unit called a bivalent, because it actually contains a pair of chromosomes.
During pachynema stage, an important genetic phenomenon called “crossing over” takes place. The crossing over involves reshuffling, redistribution and mutual exchange of hereditary material of two parents between two homologous chromosomes.
According to recent views, one chromatid of each homologous chromosome of a bivalent may divide transversely by the help of an enzyme, the endonuclease which is reported to increase in the nucleus during this stage by Stern and Hotta (1969).
After the division of chromatids, the interchange of chromatid segments takes place between the non-sister chromatids of homologous chromosomes. The broken chromatid segments are united with the chromatids due to the presence of an enzyme, the ligase (Stern and Hotta, 1969).
This process of interchange of chromatin material between non-sister chromatids of each homologous chromosome is known as the crossing over which is accompanied by the chiasmata formation.
Stern and Hotta (1969) have reported that during the pachytene and zygotene stages, synthesis of small amount of DNA takes place. This DNA amount is utilised in the repairing of broken DNA molecules of the chromatids during the chiasmata formation and crossing over. The nucleolus remains prominent up to this stage and it is found to be associated with the nucleolar organizer region of the chromosome.
5. Diplotene or Diplonema:
In diplotene or diplonema stage, the homologous chromosomes repel each other because the force of attraction between the two homologous chromosomes decreases. The two homologous chromosomes, thus, separate from each other, however, not completely because both remain united at the point of interchange or chiasmata.
In the diakinesis stage, the bivalent chromosomes become more condensed and evenly distributed in the nucleus. The nucleolus detaches from the nucleolar organiser portion of the chromosome and ultimately disappears. During diakinesis, the chiasma moves from the centromere towards the ends of the chromosomes and the intermediate chiasmatas diminish.
This type of movement of the chiasmata is known as terminalisation. The chromatids still remain connected by the terminal chiasma and these exist up to the metaphase.
In the prometaphase, the nuclear envelope disintegrates and the microtubules get arranged in the form of spindle in between the two centrioles which occupy the position of two opposite poles of the cell. The chromosomes become greatly coiled in the spiral manner and get arranged on the equator of the spindle.
In the metaphase I, the microtubules of the spindle are attached with the centromeres of the homologous chromosomes of each tetrad. The centromere of each chromosome is directed towards the opposite poles. The repulsive forces between the homologous chromosomes increase greatly and the chromosomes become ready to separate.
Due to the contraction of chromosomal fibres of microtubules, each homologous chromosome with its two chromatids and undivided centromere moves towards the opposite poles of the cell. The chromosomes with single or few terminal chiasma usually separate more frequently than the longer chromosomes containing many chiasma.
The actual reduction occurs at this stage. Here it should be carefully noted that the homologous chromosomes which move towards the opposite poles are the chromosomes of either paternal or maternal origin.
Moreover, because during the chiasma formation out of two chromatids of a chromosome, one has changed its counterpart, therefore, the two chromatids of a chromosome do not resemble with each other in the genetical terms.
In telophase I, the endoplasmic reticulum forms the nuclear envelope around the chromosomes and the chromosomes uncoil. The nucleolus reappears and, thus, two daughter chromosomes are formed. After the karyokinesis, cytokinesis occurs and two haploid cells are formed.
Both cells pass through a short resting phase or interphase. In case of Trillium, telophase I and interphase do not occur and the anaphase I is followed by prophase II directly.
Homotypic or Second Meiotic Division:
The homotypic or second meiotic division is actually the mitotic division which divides each haploid meiotic cell into two haploid cells.
The second meiotic division includes following four stages:
In the prophase second, each centriole divides into two and, thus, two pairs of centrioles are formed. Each pair of centrioles migrate to the opposite pole. The microtubules of fibres get arranged in the form of spindle at the right angle of the spindle of first meiosis. The nuclear membrane and the nucleolus disappear. The chromosomes with two chromatids become short and thick.
During the metaphase II, the chromosomes get arranged on the equator of the spindle. The centromere divides into two and, thus, each chromosome produces two monads or daughter chromosomes. The microtubules of the spindle are attached with the centromere of the chromosomes.
The daughter chromosomes move towards the opposite poles due to the contraction of chromosomal microtubules and stretching of inter-zonal microtubules of the spindle.
The chromatids migrate to the opposite poles and now known as chromosomes. The endoplasmic reticulum forms the nuclear envelope around the chromosomes and the nucleolus reappears due to synthesis of ribosomal RNA (rRNA) by ribosomal DNA (rDNA) and also due to accumulation of ribosomal proteins.
After the karyokinesis in each haploid meiotic cell, the cytokinesis occurs and, thus, four haploid cells are resulted. These cells have different types of chromosomes due to the crossing over in the prophase I.