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Introduction
Meiosis is also known as reduction division, where the division of a germ cell occurs and involves two nuclear fissions and produces chromosomes as can be seen in the original cell. In sexually reproducing organisms, meiosis is a process that occurs. As a result, a diploid (double) set of chromosomes, consisting of two haploid sets, is found in the nucleus of each cell in such species (one inherited from each parent). These haploid sets are homologous, meaning they have the same genes but are not always in the same order. It can be further explained by each set of homologous chromosomes in humans, for example, containing a gene for blood type, but one group may carry the gene for blood type A while the other set may have the gene for blood type B.
Crossing over and Meiosis
Each of the diploid germ cell’s chromosomes has reproduced and hence consists of a joined pair of duplicate chromatids before meiosis. Single chromosomal pair, known as tetrad or bivalent, comprises four chromatids. Crossing over occurs at this stage, and homologous chromosomes exchange genetic material. The homologous pairs are drawn to the cell’s opposing ends, pinching it in half to produce two daughter cells. After the first meiotic division, each daughter cell has a haploid pair of chromosomes. Division of haploid cells
There is no additional reduction in chromosomes since mitotic division entails the separation of each chromatid pair into two chromosomes, which are pulled to opposing ends of the daughter cells. After that, each daughter cell divides in half, yielding four distinct haploid gametes. When two gametes combine during fertilization, each contributes its haploid chromosome to the new individual, restoring the diploid number.
In comparison with mitosis, meiosis is comparable to a cell division in which a parent cell divides into two identical daughter cells.
The Process of Meiosis
After DNA replication in cells of the male or female sex organs, meiosis gets initiates. Meiosis I and meiosis II are the two divisions that contain several steps. Meiosis I is a type of cell division. Meiosis II, on the other hand, is analogous to mitosis.
MeiosisI- Prophase I It is the longest and most complex phase of meiosis I. It is further divided into 5 sub-stages as:
(i) Leptotene or leptonema (ii) Zygotene or zygonema
(iii) Pachytene or pachynema (iv) Diplotene or diplonema
(v) Diakinesis
Leptotene or leptonema: Size of nucleus increases. In this sub-stage, the chromosomes appear as thin, uncoiled thread-like structures. One half of these chromosomes are of male parent and the other half of female parent. Chromosomes having similar characters or genes (which can pair) are known as homologous chromosomes.
Zygotene or zygonema: Pairing or synapsis of homologous chromosomes takes place at this sub-stage.Synapsis or pairing leads to formation of bivalents or tetrads
Pachytene or pachynema: It is the longest substage of prophase I. Chromosomes become further shorter and thicker. Each chromosome of homologous pair is having 2 chromatids at this stage and thus a bivalent or homologous pair of chromosomes is now having four chromatids (four stranded) which is called tetrad stage or tetravalent stage.Crossing over occurs at this stage . Out of 4 chromatids of tetrad, only two are crossovers and two are non-cross overs.
Diplotene or diplonema: Chromosomes become even shorter and thicker.As forces of attraction between homologous chromosomes or bivalents reduce, so they begin to separate from each other and this separation of homologous chromosomes is called terminalization (as separation starts from centromere towards ends).The separation is not complete, but the homologous chromosomes remain attached at one or more points and these points of contact are called chiasmata.Nuclear membrane and nucleolus begin to disappear at this sub-stage.
Diakinesis: In this last sub-stage of prophase I, further contraction and shortening of chromosomes occur and chromosomes become minimum in size. Terminalization is almost complete.Both nucleolus and nuclear membrane completely disappear at this sub-stage (or in early metaphase).Characteristic difference from diplotene is that chromosomes are shorter in diakinesis).
Metaphase I: Stage of cell division in which the chromosomes align along the cell’s equator. Homologous chromosomes align parallel to each other.
Anaphase I: The Stage of cell division is the movement of homologous chromosomes to opposite ends of the cell
Telophase I:This stage is similar to mitotic telophase. Nuclear membrane appears around the chromosomes at both ends or poles. Nucleoli also appear and thus 2 daughter nuclei are produced from a single nucleus, having exactly half the number of chromosomes.
Telophase I may or may not be followed by cytokinesis or wall formation. Resting period or interphase may or may not be present between meiosis I and meiosis II
Meiosis II: Sister chromatids separate.
Prophase – II The chromosomes reappear. The nuclear membrane disappears and the formation of a spindle starts. Each prophase consists of two distinct chromatids attached to the single centromere.
Metaphase II: The Stage of cell division in which the movement of chromosomes to the cell’s equator.
Anaphase II The Stage of cell division in which the division of chromosomes and each chromatid moves to the opposite poles of the cell.
Telophase – II In telophase the chromosomes of each group uncoil and develop a nuclear membrane and are organized into a nucleus. The nuclear division is followed by the division of cytoplasm (cytokinesis).
Stages and Recombination
The homologous pairs are drawn to the cell’s opposing ends, pinching it in half to produce two daughter cells. After the first meiotic division, each daughter cell has a haploid pair of chromosomes. The chromosomes are still made up of duplicate chromatids at this time.
During the second meiotic division, each haploid daughter cell divides. However, there is no additional reduction in chromosomes since mitotic division entails the separation of each chromatid pair into two chromosomes, which are pulled to opposing ends of the daughter cells.
Each daughter cell then divides in half, resulting in four separate haploid gametes. When two gametes fertilize each other, each contributes its haploid chromosome to the new individual, restoring the diploid number.
Meiosis is a cell division process in which a parent cell divides into two identical daughter cells. However, it differs from mitosis.
Points to Remember
Bivalent: The Paired sister chromatids make a group of four chromatids, also called a tetrad.
Chiasma: The point of crossover between chromosomes.
Crossing over: Exchange of genetic material between homologous chromosomes in meiosis.
Homologous chromosome: The two copies of each chromosome contain the same sequence of genes in diploid cells, although the alleles may differ.
Conclusion
The homologous pairs are drawn to the cell’s opposing ends, pinching it in half to produce two daughter cells. After the first meiotic division, each daughter cell has a haploid pair of chromosomes. The chromosomes are still made up of duplicate chromatids at this time. In sexually reproducing organisms, meiosis is a process that occurs. A diploid (double) set of chromosomes, consisting of two haploid sets, is found in the nucleus of each cell in such species (one inherited from each parent). These haploid sets are homologous, meaning they have the same genes but are not always in the same order. It can be further explained by each set of homologous chromosomes in humans; pairs of replicated chromosomes are known as Sister chromatids that remain connected at a center called the centromere. In comparison with mitosis, meiosis is comparable to a cell division in which a parent cell divides into two identical daughter cells.
