Meiosis Stages: 7+ Simple Yet Detailed Steps And Facts

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Meiosis in an exclusive event of eukaryotic gametocytes to produce haploid gametes or germ cells. Meiosis stages can be broadly divided into two steps- meiosis I and meiosis II.

Meiosis is a type of cell division that produces haploid gametes in the primary reproductive organs of most eukaryotes. Meiosis stages reduce the ploidy level of a gametocyte to half. This is an indispensable step as the gametes of a progeny are contributed by the both the parents.

Unlike mitotic division, the daughter cells formed after meiotic division have only half the number of chromosomes than that of the parent cells. In meiosis stages, a single round of DNA replication is followed by two varied types of segregation processes. Prolonged meiosis is common in plants and animals.

In case the reduction of the genome fails to occur in the meiosis stages, the progeny of the parents with 2n number of chromosomes would end up having 4n number of chromosomes. This would lead to speciation of the progenies from their parents instead of propagation of the parental genome. Also, during meiosis, crossing over or recombination of genetic material between the homologous chromosomes occurs followed by independent assortment. Hence, meiosis is a necessary process to maintain species diversity.

Meiosis Stages Diagram

meiosis stages
Image credit: wikimedia commons. Meiosis Stages:- Interphase, Meiosis I (which includes Prophase I, Metaphase I, Anaphase I, Telophase I) and Meiosis II (which includes Prophase II, Metaphase II, Anaphase II, Telophase II)

How to Identify Meiosis Stages ?

Meiosis I

Meiosis I is also known as the reductional division as in this process, the diploid (2n) cells give rise to haploid (n) cells after cell division. A cell enters meiosis I after G1 and S phase. In the G1 phase the cells are actively involved in transcription and translation of protein involved in cell growth. The S phase involves replication of the chromosomes. G2 phase is usually absent in the meiosis stages.

Meiosis I consists of Prophase I, Metaphase I, Anaphase I and Telophase I. Among these steps, Prophase I can be further subdivided into five distinct steps.

Prophase I

This step is exclusive to meiosis I where, crossing over occurs between the homologous chromosomes at chiasmata by the formation of synaptonemal complex. The homologous chromosomes, also called bivalents or tetrads, are contributed by both the parents.

Leptotene
  • After duplication of sister chromatids in the interphase, a diploid cell enters the leptotene stage.
  • In the early leptotene, ends of the chromosomes begin to attach themselves to inner membrane of the nuclear envelope.
  • The chromatin network begins to condense into chromosomes but they are not distinctly visible.
  • Sister chromosomes are bound together at the centromere and as so called sister chromatids.
  • In the late leptotene, ends of the chromosomes aggregate in an area, which gives this stage the name ‘bouquet stage’.
  • This arrangement helps in formation of synaptonemal complexes between the homologous chromosomes.
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Chromosomal ends of the homologous chromosomes move closer to each other on the inner membrane of the nuclear envelope.
Zygotene
  • Initiation of the synapsis marks the beginning of this phase.
  • The homologous pair of chromosomes undergo synapses to form the synaptonemal complex.
  • Ends of the homologous chromosomes move closer in a zipper like fashion from both ends, on the inner membrane of the nuclear envelope.
  • The homologous chromosomes get bound together with the help of synaptonemal complexes.
  • The points of attachment are called chiasmata.
  • They are called bivalents (as 2 homologous chromosomes are present) or tetrads (as 4 sister chromatids are present).

Synaptonemal complexes are tripartite proteinaceous structures that are composed of a central element sandwiched in between the lateral elements. The central elements and two lateral elements, are arranged parallel to each other, such that they resemble a rail tracks because the central elements are composed of transverse filaments which interact with the lateral elements on both sides. In turn, every lateral element interacts with the sister chromatids.

Lateral elements perform important roles in different processes ranging from condensation of chromosomes, assembly of transverse filaments and preventing the progression of sister chromatids with double strand breaks (or DSBs) into the recombination stages.

Image credit: Wikimedia commons. Role of DSBs in chiasmata formation.
Pachytene
  • The non-sister chromatids of the homologous chromosomes are attached in some regions called the chiasmata where, crossing-over or recombination occurs.
  • Frequency of recombination varies throughout the length of the chromosome.
  • Those with high recombination frequency are called hot spots while those with low recombination frequency are called cold spots.
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Image credit: wikimedia commons. Crossing-over between the homologous chromosomes.

Meiosis specific double stranded breaks (DSBs) are responsible for initiating recombination which are double stranded breaks in the DNA that are deliberately caused by the meiotic cells. These deliberate breaks are facilitated by the spo11 proteins which have been evolutionarily conserved and are an integral part of the recombination process in most organisms including mammals. These spo11 proteins are highly regulated by the cells for a very short time period during meiosis stages because the double stranded breaks they generate are hazardous for the cell.

These double stranded breaks are responsible for several important roles in the crossing over between the homologous chromosomes and their segregation. Double stranded breaks facilitate formation of chiasma between the homologous chromosomes when these broken strands are joined during synapsis.

Diplotene
  • In this phase, degradation of synaptonemal complexes begins but the homologous chromosomes still remain attached at the chiasmata.
Diakinesis
  • The tetrads are distinctly visible in this phase due to further condensation of the chromosomes.
  • Under the microscope, these tetrads can look like rods and ovals or have a diamond shape.
  • Such shapes are a result of chiasmata where, the homologous chromosomes are still attached.
  • Chromosomes detach from the nuclear envelope.
  • Nucleoli and the nuclear membrane disintegrate and disappear by the end of this stage.
  • Formation of spindle fibers begin from the centrosomes which had duplicated in the S phase.

Metaphase I

  • The homologous chromosomes get aligned along the equator of the cell, perpendicular to the poles.
  • The spindle fibers or the kinetochore fibers attach to the centromere of each homologous chromosome from opposite poles so that each them get pulled away in the opposite directions.
  • These homologous chromosomes get assorted independently as they undergo random separation.

Anaphase I

  • Contraction in the spindle fibers or the kinetochore fibers, pull the homologous chromosomes apart at the chiasmata.
  • Independently assorted chromosomes move towards the opposite poles leading to the reductional division of the diploid cell.
  • The sister chromatids or dyads remain attached together throughout the prophase I with the help of Rec 8 cohesin.
  • Rec 8 cohesin prevents the separation of the sister chromatids by binding to the their centromeres.
  • The cells elongate at the poles to prepare for the cytokinesis.
  • Under the microscope, the V or T shaped sister chromatids or dyads can be observed, located in the space between the equator and the periphery of the cell.

Telophase I

  • At this stage, the sister chromatids or dyads, get assembled at the poles.
  • The spindle fibers start disintegrating and the chromosomes start decondensing.
  • Nuclear envelope starts appearing around the chromatin network.
  • Cytokinesis occurs to give rise to 2 haploid daughter cells.
  • This phase is easily differentiable from Anaphase of mitotic cell division because here, the sister chromatids remain together after separation whereas, in mitotic cell division, it is the sister chromatids or dyads that get separated away.

Meiosis II

Meiosis II is comparatively similar to mitotic division where in mitotic division, a diploid undergoes chromosomes duplication and division but in the meiosis stages, a haploid cell undergoes chromosome duplication and division. As the motive of the meiosis stages is to reduce the chromosome number to half, the dyads or the sister chromatids do not undergo duplicate upon cytokinesis following the telophase of meiosis I.

Prophase II

  • The chromatin fibers begin to condense again.
  • The nuclear envelope disintegrate again.
  • Centrioles shift to the poles and spindle fiber formation begins.

Metaphase II

  • Chromosomes migrate and align themselves at the equator of the cell, perpendicular to the poles.
  • Spindle fibers from the poles bind to the common centromere of the sister chromatids and pull them apart in the opposite direction.

Anaphase II

  • Sister chromosomes which where earlier called sister chromatids, begin to move toward their destined poles.
  • Rec 8 cohesin disappears due to gene deletion.
  • This phase looks similar to the mitotic anaphase under the microscope.

Telophase II

  • Chromosomes undergo decondensing and uncoiling to form the chromatin network.
  • Spindle fibers begin to disintegrate while nuclear envelope starts to appear.
  • Nucleolus reappears in this phase along with the nucleus.
  • Cytokinesis occurs and results into 4 haploid daughter cells.
  • Such haploid daughter cells from both the parents come together during fertilization to produce a recombinant diploid zygote.

Meiosis Stages Under Microscope

Meiosis I

Meiosis I consists of Prophase I, Metaphase I, Anaphase I and Telophase I. Among these steps, Prophase I can be further subdivided into five distinct steps. Each stage can be identified under the microscope as they have some distinct features.

Prophase I

In this phase, crossing over between the homologous chromosomes occur, followed by independent assortment of the homologous chromosomes in the late prophase I.

Leptotene
  • Ends of the chromosomes begin to attach themselves to inner membrane of the nuclear envelope.
  • The chromatin network start condensing but the chromosomes are not distinctly visible yet.
  • By the end of leptotene, the chromosomes assume a peculiar arrangement called ‘bouquet stage’.
Zygotene
  • Chromosomes are still attached to the inner nuclear membrane
  • The homologous chromosomes start pairing up.
  • Synaptonemal complexes form between these homologous chromosomes.
  • Chiasmata forms at the point of attachment.
  • Now these pair of homologous chromosomes are called tetrads or bivalents.
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Image credit: wikimedia commons. Zygotene and Pachytene stages in grasshopper testes.
Pachytene
  • Crossing over occurs in this phase.
Diplotene
  • Crossing over ceases in this stage and the synaptonemal complexes degrade the but the pairs of homologous chromosomes remain attached to each other at the chiasmata.
Diplotene
Image credit: wikimedia commons. Structure of chromosomes during Diplotene and Diakinesis in grasshopper testes.
Diakinesis
  • The bivalents or tetrads or the pair of homologous chromosomes are well visible in this phase because ample condensation of the chromosomes has occurred.
  • They take the shape of rods, ovals and diamonds under the microscope.
  • The tetrads take up these peculiar shapes due to their attachment at the chiasmata.
  • For example, if the homologous chromosomes have chiasmata at both the ends, they can take up a diamond shape.

Metaphase I

  • The tetrads or bivalents get aligned at the equator of the cell.
  • In early Metaphase I, homologous pairs begin to move towards the equator and so are seen scattered close to the equator.
  • Under the microscope, in the late metaphase I, all the homologous pairs will be congregated and arranged in a line at the equatorial plane of the cell.

Anaphase I

  • The tetrads can be seen to be moving away from the equator of the cell towards the opposite poles.
  • The homologous pairs of chromosomes assume V or T shapes when being pulled away by the spindle fibers.
  • In this phase, dyads or the sister chromosomes are also distinctly visible.
  • Only half the number of chromosomes can be observed to be present between the equator and the poles on both the sides of the cell.
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Image credit: wikimedia commons. Anaphase I.

Telophase I

  • Half the number chromosomes (dyads or the sister chromatids) aggregate at each of the opposite poles.
  • The chromosomes decondense and gives a woolly appearance in the late telophase I.

Meiosis II

In the second meiotic division, the process is a lot similar to that of the mitotic divisions.

Prophase II

  • As the chromosomes begin to condense in this phase, can be observed in the cell.

Metaphase II

  • Chromosomes are seen aligned on the equatorial plane of the cell.
  • In some slides, chromosomes can be seen arranged in a circular manner within the cell that occurs due to cross sectional cutting of the cell during slide preparation.

Anapahse II

  • Sister chromosomes are seen to be present between the equator and the pole region on both extremities of the cell as they are being pulled away from each other at their centromeres.
  • As this phase looks very similar to the anaphase of mitotic division, differentiating the two can pose a challenge.

Telophase II

  • Nucleolus reappears and is visible in this phase within the developing nuclear membrane at the two poles.
  • Chromosomes are not distinctly visible as they are under the process of decondensing.
  • The sister chromosomes congregated the opposite poles are equal in number to the total chromosome number of the cell.

Conclusion

Meiosis not only ensures propagation of the right number of the chromosomes from one generation to another but also allows recombination of the homologous pair of chromosomes in the parent cells to occur. Recombination by the means of crossing over and independent assortment ensures that the progeny is a hybrid of both the parents and has equal opportunity to get genes from both the parents and each of their homologous chromosomes as well.

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