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Meiosis – Definition, Types, Steps

Meiosis Definition Types of Meiosis  Meiosis occurs in the germ cells of organisms that reproduce sexually. Germ cells are localised in the gonads of plants ...

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MN Editors

Meiosis Definition

  • Meiosis is the type of cell division in which four daughter cells are produced, each with half the number of chromosomes of the parent cell.
  • J.B. Farmer created the term meiosis (Greek: meioum Meaning to reduce or diminish) in 1905. Meiosis transforms each original diploid cell into four haploid cells.
  • These haploid cells either become or give rise to gametes, which support sexual reproduction and a new generation of diploid organisms by union (fertilisation).
  • Therefore, meiosis is necessary for the reproductive cycle of eukaryotes such as Chlamydomonas, Neurospora, bryophytes, plants, and animals.
  • For instance, Chlamydomonas’ reproductive cycle consists of a protracted haploid generation and a short diploid generation involving zygote production.
  • Meiosis, the reduced division of the zygote, results in the generation of haploid spores.
  • In higher plants, however, the reproductive cycle consists of a long, multicellular, dominant diploid generation (named sporophyte) and a short, multicellular, haploid generation called gametophyte.
  • The microscopic gametophyte is nourished by specialised sporophyte tissues. Meiosis produces male and female haploid cells termed spores in the diploid (sporophyte) organism.
  • Spores develop into multicellular male and female haploid (gametophyte) structures, which create gamete-like haploid cells by meiosis.
  • In both animals and plants, male and female gametes combine during fertilisation to form a zygote with a restored number of diploid chromosomes. The zygote develops into a new diploid creature in animals and simple plants.
  • In plants that produce seeds, development is paused at an early multicellular stage as a seed, which can remain stable for an extended period of time before germination causes growth to resume.
  • Thus, the reproductive cycle alternates between haploid and diploid generations and involves meiosis.

Types of Meiosis 

Meiosis occurs in the germ cells of organisms that reproduce sexually. Germ cells are localised in the gonads of plants and animals alike. Different species undergo meiosis at different times; hence, the process can be classed as terminal, intermediate, or initial.

1. Terminal meiosis

  • This process, also known as gametic meiosis, occurs in animals and a few lower plants.
  • In terminal meiosis, the meiotic division occurs just prior to gamete production or gametogenesis.

2. Intermediary or sporic meiosis

  • It’s a trait shared only by plants with flowers. Between fertilisation and the production of gametes, this meiosis occurs.
  • It also plays a role in microsporogenesis (in the anthers) and megasporogenesis (in the ovary or pistil), the processes by which microspores and megaspores are created.

3. Initial or zygotic meiosis

  • It can be found in certain types of algae, fungus, and diatoms. Because only the egg undergoes meiosis after fertilisation, only the egg is diploid.
  • Meiocytes: Meiocytes are the cells that undergo meiosis. In males, gonocytes are meiocytes that develop into spermatocytes, and in females, oocytes. The sporangium’s meiocytes are known as sporocytes (i.e., microsporocytes and megasporocytes).

Process of Meiosis 

  • Meiosis superficially resembles two mitotic divisions with no DNA replication in between.
  • During the prophase of the first meiotic division, homologous chromosomes become tightly linked with one another and genetic material is exchanged between them.
  • In addition, during the initial meiotic division, the number of chromosomes is reduced, resulting in the formation of two haploid cells. The first division of meiosis is often referred to as the heterotypic division.
  • During the second meiotic division, the haploid cell undergoes mitosis, resulting in the formation of four haploid cells.
  • The second division of meiosis is often referred to as the homotypic division. In homotypic division pairing of chromosomes, genetic material exchange and chromosome number reduction do not occur.
  • Both meiotic divisions occur continuously and each consists of the standard meiotic phases, namely prophase, metaphase, anaphase, and telophase.
  • Most cytogenetic events, including as synapsis, crossing over, etc., occur during the prophase of the first meiotic division, which is a crucial phase.
  • The prophase is the longest phase of meiosis; hence, it is separated into six substages for convenience’s sake: proleptonema (proleptotene), leptonema (leptotene), zygonema (zygotene), pachynema (pachytene), diplonema (diplotene), and diakinesis. The successive substages of meiosis can be depicted as follows:

A. Heterotypic Division or First Meiotic Division 

  • Meiosis begins following an interphase that is similar to that of intermitotic interphase.
  • During the premeiotic interphase, S phase DNA replication has happened. During the G2 phase of interphase, the cell appears to undergo a fundamental shift that drives it towards meiosis rather than mitosis.
  • In addition, at the commencement of the first meiotic division, the nucleus of the meiocyte begins to enlarge by absorbing water from the cytoplasm, resulting in a threefold increase in nuclear volume.
  • After these modifications, the cell enters prophase, the initial step of the first meiotic division.
Heterotypic Division or First Meiotic Division 
Heterotypic Division or First Meiotic Division 

Prophase I 

The longest phase of the meiotic division is the first prophase. It contains the subsequent subphases:

Stages of Meiotic Prophase I
Stages of Meiotic Prophase I

1. Proleptotene or Prolepto-nema. (Gr., pro=before; leptas= thin; nema= thread)

  • The proleptotene stage closely resembles the early prophase of mitosis.
  • At this stage, chromosomes are extraordinarily long, uncoiled, longitudinally single, and thread-like entities that are incredibly thin.

2. Leptotene or Leptonema

  • In the leptotene stage, chromosomes become less coiled and develop a form like a long thread.
  • At this stage, the chromosomes adopt a certain orientation within the nucleus; the ends of the chromosomes converge toward the side of the nucleus where the centrosome is located (the bouquet stage).
  • Each daughter centriole migrates towards the opposite cellular poles once the centriole has duplicated.
  • Each centriole replicates upon reaching the poles, such that each pole of the cell has two centrioles of a single diplosome. 

3. Zygotene or Zygonema. (Gr., zygon=adjoining)

  • In the zygotene stage, homologous chromosomes pair with one another.
  • The homologous chromosomes from the mother (through eggs) and father (by sperm) are attracted to one another, resulting in their pairing.
  • The merger of homologous chromosomes is referred to as synapsis (Greek:, synapsis).
  • The synapsis commences at one or more locations along the homologous chromosomes’ length. There are three types of synapsis recognised.

(i) Proterminal synapsis

  • In proterminal synapsis, the pairing of homologous chromosomes begins at their ends and progresses toward their centromeres.

(ii) Procentric synapsis

  • In procentric synapsis, the pairing of homologous chromosomes begins at their centromeres and continues towards the ends of the homologous chromosomes.

(iii) Localized pairing or Random synapsis

  • At various locations on homologous chromosomes, random synapsis occurs. The pairing of homologous chromosomes is extremely precise and precise (i.e., alignment of chromosomes is exactly gene-for-gene).

The paired homologous chromosomes are connected by a synaptonemal complex, a protein-containing framework approximately 0.2 m thick (SC). This complex spans the entire length of the paired chromosomes and is typically attached to the nuclear envelope at both ends. SC serves to stabilise the pairing of homologous chromosomes and allow recombination or crossing over, two forms of cytogenetic activity (occurring during pachynema). SC is absent from species when crossing over does not occur (e.g., the male fruitfly, Drosophila melanogaster).

4. Pachyteneor Pachynema. (Gr., pachus=thick) 

  • In the pachynema stage, the pair of chromosomes become spirally coiled and cannot be identified individually.
  • Each homologous chromosome divides into two chromatids in the middle of the pachynema stage.
  • In reality, the doubling of the DNA molecule strands required for the following duplication of chromosomes happens prior to the onset of meiotic prophase.
  • During the initial stages of the meiotic prophase, the DNA molecule in each chromosome behaves as a single body. During the pachynema stage, the two chromatids of each chromosome comprising half of the DNA present in the chromosome at the beginning become partially independent of one another, despite continuing to be connected by their common centromere.
  • At this stage, each synaptonemal pair is frequently referred to as bivalent or dyads because it contains two visible chromosomes, or as quadrivalent or tetrads because it contains four visible chromatids.
  • During the pachynema stage, a significant genetic phenomenon known as crossing over occurs.
  • The crossing over involves the reorganisation, redistribution, and exchange of genetic material between two homologous chromosomes from two parents.
  • One chromatid of each homologous chromosome of a bivalent may divide transversely with the aid of an enzyme called endonuclease, which Stern and Hotta indicate increases in the nucleus during this stage (1969).
  • After the division of chromatids, the nonsister chromatids of homologous chromosomes exchange chromatid segments. The broken chromatid segments are reconnected with the chromatids due to the presence of the enzyme ligase.
  • Crossing over refers to the exchange of chromatin material between one non-sister chromatid of each homologous chromosome, which is accompanied by the creation of chiasmata.
  • Stern and Hotta (1969) found that a little quantity of DNA is synthesised during the pachytene and zygotene stages.
  • During the development of chiasmata and crossing over, this quantity of DNA is utilised to repair broken DNA molecules on the chromatids.
  • It is discovered that the nucleolus is connected with the nucleolar organiser region of the chromosome at this stage.

5. Diplotene or Diplonema

  • Unpairing or desynapsis of homologous chromosomes begins in diplonema, and chiasmata appear for the first time.
  • During this phase, the chromatids of each tetrad are typically clearly visible, but the synaptonemal complex appears to be dissolved, leaving participating chromatids of the paired homologous chromosomes physically joined at one or more discrete points known as chiasmata (Greek:, chiasma = cross piece).
  • These are the locations where crossings occurred. During this stage, chromatids frequently unfold, allowing for RNA synthesis and cellular development.

6. Diakinesis

  • During the diakinesis phase, the bivalent chromosomes become more compact and uniformly dispersed within the nucleus.
  • The nucleolus separates from the nucleolar organiser and eventually vanishes. The nuclear envelope disintegrates.
  • During diakinesis, the chiasma proceeds from the centromere to the chromosomal ends, while the intermediate chiasmata shrink.
  • This type of chiasma movement is known as terminalization. Terminal chiasmata continue to join the chromatids throughout metaphase. 

Prometaphase

During the prometaphase, the nuclear envelope disassembles and the microtubules form a spindle between the two centrioles, which occupy the positions of the cell’s two opposite poles. The chromosomes become tightly wound in a spiral fashion and are positioned on the spindle’s equator.

Metaphase I 

  • Metaphase I involves the attachment of spindle fibres to chromosomes and the alignment of chromosomes at the equator.
  • During metaphase I, the microtubules of the spindle bind to the centromeres of each tetrad’s homologous chromosomes.
  • Each chromosome’s centromere is oriented toward the opposite pole.
  • The forces of repulsion between homologous chromosomes intensify, as the chromosomes prepare to separate.

Anaphase I 

  • At anaphase I, homologues are separated and, as a result of the shortening of chromosomal fibres or microtubules, each homologous chromosome with its two chromatids and undivided centromere moves to the opposite poles of the cell.
  • Typically, chromosomes with a single or few terminal chiasmata separate more frequently than those with several chiasmata.
  • At this stage, the actual reduction and disjunction take place.
  • Here, it is important to notice that the homologous chromosomes that migrate towards the opposite poles are either of paternal or maternal origin.
  • In addition, because one of the two chromatids of a chromosome changed its counterpart during chiasma formation, the two chromatids of a chromosome do not genetically resemble one another.

Telophase I 

  • The arrival of a set of haploid chromosomes at each pole marks the beginning of telophase I, during which the nucleus is reconstructed.
  • The endoplasmic reticulum produces the nuclear membrane surrounding the chromosomes, causing them to uncoil.
  • The nucleolus returns, resulting in the formation of two daughter chromosomes. After karyokinesis, cytokinesis occurs, resulting in the formation of two haploid cells. Both cells undergo a brief interphase resting phase.
  • There is no DNA replication during interphase, therefore chromosomes in the second prophase are the same double-stranded structures that disappeared in the first telophase.
  • In Trillium, neither telophase I nor interphase I occur, and anaphase I is immediately followed by prophase II.

Homotypic or Second Meiotic Division 

Each haploid meiotic cell divides into two haploid cells during the homotypic or second meiotic division, which is actually the mitotic division. The second meiotic division consists of the four stages listed below.

Homotypic or Second Meiotic Division 
Homotypic or Second Meiotic Division 

Prophase II 

  • In the second stage of prophase, each centriole divides into two, resulting in the formation of two pairs of centrioles.
  • Every pair of centrioles migrates toward the opposing polarity.
  • The microtubules construct a spindle at the correct angle relative to the spindle of the first meiosis.
  • The nuclear membrane and nucleolus both vanish.
  • Two-chromatid chromosomes get shorter and thicker.

Metaphase II 

  • During metaphase II, the chromosomes align along the spindle’s equator.
  • Each chromosome generates two monads or daughter chromosomes as a result of the centromere’s bifurcation.
  • The spindle’s microtubules are connected to the centromeres of the chromosomes. 

Anaphase II 

  • Due to the shortening of chromosomal microtubules and stretching of interzonal microtubules of the spindle, the daughter chromosomes move toward the opposite poles.

Telophase II 

  • Chromosomes are chromatids that move to opposite poles and are now known as such. The nucleolus returns due to the production of ribosomal RNA (rRNA) by rDNA and accumulation of ribosomal proteins.
  • In each haploid meiotic cell, cytokinesis occurs following karyokinesis, resulting in four haploid cells.
  • Due to the crossing over that occurred in prophase I, these cells had different types of chromosomes.

Significance of Meiosis 

Meiosis is the most important process in the biological world due to the following functions:

  • Meiosis keeps the number of chromosomes in organisms fixed and constant.
  • Meiosis provides a chance for the exchange of genes and, as a result, is responsible for the genetic differences across species. The variances are the evolutionary process’s basic ingredients. Meiosis is thus a unique taxonomic, genetic, and evolutionary process.

Function of Meiosis

  • Numerous sexually reproducing species require meiosis to ensure that their progeny inherit the same number of chromosomes as their parents.
  • In the process of fertilisation, two cells fuse together to form a zygote. If the number of alleles of each gene is not reduced to a single copy in the gametes that make the zygote, the progeny will include four copies of each gene. This would lead to several developmental abnormalities in many species.
  • Polyploidy is widespread in other organisms, and they can exist with many copies of the same gene.
  • Meiosis must take place prior to reproduction if the organism cannot live if it is polyploid. Meiosis occurs in two distinct divisions, each with its own specific phases.
Meiosis - Sperm
Meiosis – Sperm
Meiosis - Oocyte
Meiosis – Oocyte

Comparison/Difference Between Mitosis And Meiosis

Comparison/Difference Between Mitosis And Meiosis
Comparison/Difference Between Mitosis And Meiosis

Can a haploid cell undergo meiosis?

Haplodiploid cells are incapable of meiosis. By mitotic division, a haploid organism (n) generates gametes (n). A zygote that is diploid is created after the fertilisation of these gametes (n). After meiosis, this zygote or diploid cell divides once more to create a haploid organism.

Can meiosis occur in plants?

Meiosis occurs in both plants and animals. The end result, the production of gametes with half as many chromosomes as the parent cell, is identical, but the process is distinct.

Is meiosis an animal or plant?

Animals create gametes directly through the process of meiosis, whereas plants make spores. The spores generate a gametophyte, which then splits into gametes.

can meiosis occur without crossing over?

If crossing over does not occur, the resulting gametes are parental. The products of crossing over are recombinant gametes. The allelic mix of parental and recombinant gametes is determined by whether the initial cross involved genes in the coupling or repulsion phase.
Without crossing over, the alleles of two genes on each chromosome travel together and remain linked during meiosis. Therefore, we obtain gametes that are 100 percent “parental” and are classified into two categories based on allele separation.

can meiosis occur in all organisms?

Meiosis occurs in both plants and animals. The ultimate outcome, the creation of gametes with half as many chromosomes as the parent cell, is identical, but the mechanism is distinct. Meiosis creates gametes directly in mammals.
Haploid creatures conduct meiosis only during the zygote phase. The diploid zygote undergoes meiosis to generate the haploid thallus.

Citation

APA

MN Editors. (December 6, 2022).Meiosis – Definition, Types, Steps. Retrieved from https://microbiologynote.com/meiosis/

MLA

MN Editors. "Meiosis – Definition, Types, Steps." Microbiology Note, Microbiologynote.com, December 6, 2022.

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