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Meiosis is the nuclear division of diploid cells into haploid cells, which is a necessary step in sexual reproduction.
- Describe the importance of meiosis in sexual reproduction
- Sexual reproduction is the production of haploid cells and the fusion of two of those cells to form a diploid cell.
- Before sexual reproduction can occur, the number of chromosomes in a diploid cell must decrease by half.
- Meiosis produces cells with half the number of chromosomes as the original cell.
- Haploid cells used in sexual reproduction, gametes, are formed during meiosis, which consists of one round of chromosome replication and two rounds of nuclear division.
- Meiosis I is the first round of meiotic division, while meiosis II is the second round.
- haploid: of a cell having a single set of unpaired chromosomes
- gamete: a reproductive cell, male (sperm) or female (egg), that has only half the usual number of chromosomes
- diploid: of a cell, having a pair of each type of chromosome, one of the pair being derived from the ovum and the other from the spermatozoon
Introduction: Meiosis and Sexual Reproduction
The ability to reproduce in kind is a basic characteristic of all living things. In kind means that the offspring of any organism closely resemble their parent or parents. Sexual reproduction requires fertilization: the union of two cells from two individual organisms. Haploid cells contain one set of chromosomes. Cells containing two sets of chromosomes are called diploid. The number of sets of chromosomes in a cell is called its ploidy level. If the reproductive cycle is to continue, then the diploid cell must somehow reduce its number of chromosome sets before fertilization can occur again or there will be a continual doubling in the number of chromosome sets in every generation. Therefore, sexual reproduction includes a nuclear division that reduces the number of chromosome sets.
Sexual reproduction is the production of haploid cells (gametes) and the fusion (fertilization) of two gametes to form a single, unique diploid cell called a zygote. All animals and most plants produce these gametes, or eggs and sperm. In most plants and animals, through tens of rounds of mitotic cell division, this diploid cell will develop into an adult organism.
Haploid cells that are part of the sexual reproductive cycle are produced by a type of cell division called meiosis. Meiosis employs many of the same mechanisms as mitosis. However, the starting nucleus is always diploid and the nuclei that result at the end of a meiotic cell division are haploid, so the resulting cells have half the chromosomes as the original. To achieve this reduction in chromosomes, meiosis consists of one round of chromosome duplication and two rounds of nuclear division. Because the events that occur during each of the division stages are analogous to the events of mitosis, the same stage names are assigned. However, because there are two rounds of division, the major process and the stages are designated with a “I” or a “II.” Thus, meiosis I is the first round of meiotic division and consists of prophase I, prometaphase I, and so on. Meiosis II, the second round of meiotic division, includes prophase II, prometaphase II, and so on.
All the organisms in a specie have an equal number of chromosomes in their cells that are present in the form of pairs. Each pair is homologous containing two identical chromosomes except the sex chromosomes that are different in males and females.
In organisms that reproduce through sexual reproduction, the new organism is developed from a single cell called a zygote. This zygote is formed after the fusion of one cell from each parent. To ensure that the same number of chromosomes are transferred to the next generation as present in the parents, the nuclear material should be first divided into two halves before the fusions of cells.
These cells that take part in sexual reproduction are called gametes and the process that divides the chromosomes into two halves is called meiosis. It is a type of cell division in which one parent cell is divided into four daughter cells, each having half the number of chromosomes as compared to the parent cell. Meiosis is only seen in organisms that undergo sexual reproduction. It only takes place in cells that actively participate in sexual reproduction i.e. gametes.
In this article, we will study different phases of meiosis, its significance in the human body, and its difference from mitosis.
Introduction to Meiosis
Students begin by sorting cards to outline the process of meiosis, including pairing of homologous chromosomes and crossing over, followed by two divisions, resulting in four haploid cells. Online flashcards, a PowerPoint activity and a test help to consolidate learning of the stages of meiosis. This lesson concludes with a short practical activity using modeling clay to illustrate how crossing over increases the variation.
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Quick Notes on Meiosis
Every sexually reproducing organism is characterized by another type of cell division occurring in the germinal line, where the chromosome number undergoes reduction. This division occurs in the male and female organs of flowers in plants and gonads in animals.
In meiosis, single interphase is followed by two nuclear divisions — Meiosis-I and Meiosis-ll. Meiosis I is reductional division and Meiosis-II is equational division (Fig. 5.3A).
Note # 2. Divisions of Meiosis:
The first meiotic division consists of four stages-Prophase I, Metaphase I, Anaphase I and Telophase I (Fig. 5.3B).
Meiosis I has a long drawn out prophase which is divided into five sub-phases, namely, leptotene, zygotene, pachytene, diplotene and diakinesis.
In the prophase of meiosis, lep­totene is the first stage where the chromosomes appear as very long narrow thread. It is prece­ded by ‘S’ phase in interphase, where DNA replication takes place. The leptotene threads appear apparently single though being double in nature. In this stage the nucleolus is very distinct and the entire nucleus appear to be convoluted in nature.
In zygotene, two homologous chromosomes, one being derived from the female and the other from the male parent, remain paired with each other throughout the chromosome length in every gene locus. The process of synapsis where homologous chromo­somes come together, characterizes the phase.
Under electron microscope, the synaptonemal complex which is a multi-layered structure hav­ing a central axis with axial elements on both sides can be observed. The formation of synaptomal complex is an evidence of homologous pairing at homologous segments.
Homologous chromosomes which form the bivalents can be clearly observed in pachytene due to contraction of chromosome segments. The chromosome threads with the chromomeres are distinct in pachytene and each bivalent appears as four stranded structure as longitudinal split of each chromosome is distinct.
During this phase, interchange of fragments between paternal and maternal sets of chromo­somes occurs through a process known as cross­ing ever. The breakage and reunion of segments are achieved during this phase and two chromo­somes originating thereby contain interchanged segments.
Of the two chromatids present in each chromosome, one represents the original gene component (parental) and the other is recom­binant (Fig. 5.4). During the pachytene stage, because of crossing over and interchange of segments between homologous chromosomes, chiasmata or cross-shaped structure is observed which is the visible sign of cross over.
In pachytene of meiosis, the chromosome structure has been extensively studied by plant cytologists. In maize, paired pachytene chromo­somes have been utilized as indices of homo­logy between adjacent chromosomes and clari­fication of heterochromatic and euchromatic regions.
The next stage of meiosis is diplotene where bivalents are distinct and con­tracted. During this phase, the chiasmata of each bivalent undergoes terminalization, that is, the movement of two homologous chromosomes to the two ends. The number of chromosome biva­lents can be fully studied including those which are almost terminalized.
During this stage of prophase, due to chromosome contraction, they are very distinct as visible bivalent structure.
Next phase is diakinesis, where the chiasmata are almost fully terminalized and the two chromosomes remain together by their extreme terminal chiasma.
The end of diakinesis marks the end of prophase and the beginning of first meiotic metaphase. This phase is characterized by the disappearance of nuclear membrane and nucleolus. The spindle structure is formed just like mitosis and the spindle fibres are almost identical both in its structure and function with those of mitosis.
The bivalents arrange them­selves at the equator. The chromosomes point towards opposite direction of the poles and the chiasmata lie on the equatorial region. During this stage, the bivalents undergo maximum short­ening and condensation.
In the succeeding anaphase, i.e. Anaphase I, the homologous centromeres move towards opposite direction of the pole. The centromere of each chromosome remains intact. The chromosome being separated, there is no chiasmata at this stage.
Anaphase i is followed by suc­ceeding telophase I in which each set of chro­mosomes reaches to the two different poles. As two chromosomes of a bivalent go to two different poles, each daughter nucleus contains half the number of chromosomes. For example, in Oryza sativa the somatic chromosome num­ber is 2n = 24 and the daughter nucleus after first meiotic division contains 12 chromosomes.
The first division of meiosis is followed by the second division cycle of the same stages namely – Prophase-II, Metaphase-II, Anaphase-II and Telophase-II as in mitosis (Fig. 5.3C). After the first reductional meiotic division, also termed as heterotypic division, the diads are formed, containing daughter nuclei with half the number of chromosomes than the parent cell, and enter into the homotypic division.
During prophase-II, the chromosomes condense and are composed of two chromatids – one parental and the other recombinant. As such, during succee­ding metaphase, i.e., metaphase-II, each chromo­some arranges itself at the equator. The typical mitotic separation of two chromatids follows in anaphase-ll.
Thus, after telophase-II, the tetrads originating out of diads result in four cells (gametes/spores) containing haploid or half the number of chromosomes.
Mitotic division occurs in the nuclei of the haploid spores in plants which ultimately give rise to gametes containing only one set of chro­mosomes or haploid ones. When the two male and female gametes unite, each being haploid, the zygote is formed with a diploid set of chro­mosomes.
Throughout the organization of the body, equational division is the rule followed by reductional separation during the formation of germ cells.
Significance of Meiosis I and Meiosis II:
Meiosis I is reductional division (the chromo­some number is reduced to half with segregation of alleles), whereas meiosis-II is equational division (the chromosome number remains same).
In meiosis I, separation of homologous pair of chromosomes takes place into two cells, whereas in meiosis II, the two chromatids of each chromosome separate and enter into two different cells, each with haploid set of chromosomes with single chromatid as represented in Fig. 5.5A.
Furthermore, the process of crossing over in meiosis I between the two non-sister chromatids of parental chromosomes results in new allelic combination or genetic recombination. Thus each chromosome has one parental chromatid and one recombinant chromatid.
Though the segregation of alleles occur in Meiosis I, but the complete segregation of alleles occurs only after meiosis II. In the Fig. 5.5B, segregation of Aa alle­les occurs during meiosis I, but not of Bb alleles. Bb alleles segregate only after meiosis II. There­fore meiosis I is partly reductional and partly equational, as well as meiosis II is partly equa­tional and partly reductional.
Note # 3. Importance of Meiosis:
The meiosis is a logical and necessary part in the life cycle of sexually reproducing organ­isms, since it leads to the formation of gametes or sex cells, capable of engaging in fertilization. These gametes are haploid cells having only one member of each homologous pair.
The meiosis is concomitant of doubling of chromosome number due to gametic fusion. The gametes formed as a result of meiosis are haploid and the zygote formed by their fusion is diploid. Thus it is the only means for restoring the chro­mosome number, characteristic of the species.
Meiosis provides for new combinations of genetic material. During crossing over, the hereditary factors from male and female parents get mixed due to breakage and exchange of chromatids in pachytene. Thus the gametes produced are not all alike but with variable combination of genes.
The random segregation of paternal and maternal chromosomes and the new alignments of genes in them resulting from crossing over, ensure genetic variations in the population. This inherited variability leads to the evolution of organisms.
According to Moses (1955), the synaptonemal complex is a organized structure of fila­ments between the paired chromosomes in zygotene and pachytene stages of meiosis, i.e., the morphological expression of the synapsed chromosome.
At the end of zygotene, contact between a pair of parental homologous chromosomes occur and the pairing is exact and point to point. The process is known as synapsis and this is probably due to existence of specific mutual attractive forces between the homologous chro­mosomes, known as synaptic force.
Note # 4. Ultrastructure of Meiosis:
In cross section it can be observed that the synaptonemal complex is flattened ribbon like structure (Fig. 5.6a). Under electron microscope the synaptonemal complex appears consisting of parallel dense strands lies in a single plane that are curved and are twisted along its axis. These are flanked by chromatin.
The distance between the homologous chromo­somes is considerable in molecular forms, more: than 200 nm of the three dense lines – the cen­tral element is of variable prominence, whereas the two lateral arms are very dense. The central element may also appear as a long tripartite bar with ladder like transverse connections (Fig. 5.6b).
The lateral arms vary in width in various species. They are formed of electron dense coarse granules or fibres. These arms are joined to the adjacent chromosomes by fine fibrils. The lateral elements show sub-divisions in two longi­tudinal components.
Series of lateral loops of chromatin arise from lateral elements (Fig. 5.6c). These loops fuse in the middle line to form cen­tral element. The synaptonemal complex is attached at both ends through its lateral elements to the inner surface of the nuclear membrane.
Note # 5. Function of Meiosis :
Synaptonemal complex is con­sidered as prerequisite to chiasma formation and crossing over (Meyer). Moses inferred that it may serve chiasma formation by facilitating effective synapsis by maintaining pairing in fixed state, and by providing a structural framework within which molecular recombination may occur.
King suggested that the synaptonemal complex may orient the non-sister chromatids of homologous chromosomes in a manner that facilitates enzymatically induced exchanges between their DNA molecules. Coming & Okada suggested that synaptonemal complex pulls homologous chromosomes into approximate association with each other.