Explain the Different Stages of Meiosis

Meiosis is defined as a special type of division occurring in the diploid germ cells during which the chromosomes divide once while the nucleus and cytoplasm divide twice, resulting in the formation of four haploid cells.

This type of cell division involves different stages and phases. These stages of meiosis are explained here.

In higher plants, sexual reproduction takes place by the union of specialized sex cells called sperms and ova. The fusion of these cells prevents the doubling of chromosomes in every generation. This is because of a special type of cell division called meiosis. 

It precedes the formation of gametes so that each gamete acquires only half the number of chromosomes of the somatic cells. When two gametes unite, the resulting zygote has the normal number of chromosomes. Thus, meiosis is called reduction division as the number of chromosomes is reduced to half in the daughter cells in comparison with the parent cells.

Types of Meiosis

The event of meiosis is uniform both in animals and plants. But depending on the time of occurrence, at different stages of the life cycle, the following types of meiotic divisions may be identified. 

• Sporogenic Meiosis: In certain plants, meiosis occurs in the sporophytes (2n) at the time of formation of spores. These spores develop into gametophytes. 
• Gametic Meiosis: In most plants and animals, reduction division takes place during the formation of gametes, so that haploid gametes are produced. 
• Zygotic meiosis: This is seen in lower plants, where two haploid gametes produced by meiosis fuse to form a diploid zygote, the zygote undergoes reduction division to form haploid cells which develop into the haploid plant body. 

Divisions of Meiosis

There are two successive divisions in the nucleus. The first one is called Meiosis I, and the second one is called Meiosis II. Meiosis I itself is a reductional division, as the two daughter cells produced are haploid. Meiosis II is just another division similar to that of mitosis, as there is no reduction in the number of chromosomes. During meiosis I, only the arms of the chromosomes divide, and the centromere divides only in meiosis II. 

The chromosome number in a species is maintained constant by an alternation of meiosis (haplosis) and fertilization (diplosis). In many plants, meiosis takes place in pollen mother cells of the anther and megaspore mother cells of the ovule. The cells undergoing meiosis are called meiocytes. 

Meiosis I is also called heterotypic division because here the diploid nucleus gives rise to two haploid nuclei. Meiosis II is called homotypic division because here the chromosome number is maintained. The various stages of meiosis are detailed here.

Stages of Meiosis in Order

Meiosis is a long process and it involves the following stages- meiosis I, interkinesis, and meiosis II.

Meiosis I

Before entering Meiosis I, the cell undergoes an interphase which consists of the same G₁, S, and G₂ phases, just like in mitosis. The essential events are as follows, 

• Replication of RNA and synthesis of protein during the G1 phase.
• DNA is synthesized only in the S phase.
• DNA replication takes place by a semi-conservative process. 
• During the G2 phase, the cell undergoes various preparatory processes for Meiosis I.

The various stages of Meiosis I are as follows.

• Prophase I
  • Leptotene (leptonema)
  • Zygotene (Zygonema)
  • Pachytene (Pachynema)
  • Diplotene (Diplonema)
  • Diakinesis
• Metaphase I
• Anaphase I
• Telophase I

Prophase I

The Prophase I is the longest phase in meiosis during which genetically significant changes occur in the chromosomes. It has the following stages.

• Leptotene
• Zygotene
• Pachytene
• Diplotene
• Diakinesis

Leptotene (Leptonema)

• In this stage, chromosomes are distinct, long, and less coiled.
• The chromosomes are visible on the chromosomal threads.
• The nucleus increases in volume, probably due to hydration. 
• The chromosomes are uniformly distributed within the nucleus and they are found in identical pairs, ie, for each chromosome, there is a corresponding homologous chromosome partner which will normally be identical in all respects.
• One is contributed by the male parent and the other by the female parent. 
• The duality of chromosomes is not very clear because the two threads of each chromosome are closely opposed.
• Thus, chromosomes appear as a single-stranded structure in the diploid nucleus. 

Zygotene (Zygonema)

This is a very important stage during which the pairing of homologous chromosomes takes place. The chromosomes become shorter and thicker, and they get paired or laterally associated throughout their entire length. This pairing process is called synapsis. Each pair is termed bivalent. 

The pairing will be strictly between homologous regions. If there is a non-homologous locus, it will not take part in the pairing or synapsis, or syndesis. It may take place in one of the following ways. 

• Proterminal synopsis: IT begins at both ends and proceeds towards the centromere.
• Pro-centric synopsis: This starts at the centromere and extends to the ends.
• Random synopsis: This will occur at several points, apparently without any sequences or direction.  

Towards the end of Zygotene. The bivalents get shortened and thickened due to spiralisation and gradually become tetravalent or four-stranded.

Pachytene (Pachynema)

Due to longitudinal contraction, pachytene chromosomes appear as thick threads. The chromosomes are associated as bivalents in tetrad contraction, ie, each chromosome has two chromatids and each chromatid pair is united by a centromere which is very clear in this stage. 

• The homologous chromosomes twist and coil around one another.
• The exchange of segments between the non-sister chromatids takes place by breaking and reunion, which is called crossing over. 
• The crossing over which is an important genetic phenomenon involves reshuffling, redistribution and exchange of hereditary materials of two parents between two homologous chromosomes.
• Due to crossing over, the chromosomes produce a cross-shaped configuration called chiasmata. 
• After this interchange, the chromosomes begin to separate, repelling each other.
• However, this separation is not at all complete since the homologous chromosomes remain joined at their point of interchange.
• The nucleolus remains unattached and it may increase in size. 

Diplotene (Diplonema)

• In the diplotene stage, the homologous chromosomes repel each other because the force of attraction between the two homologous chromosomes decreases and they continue to separate; the synaptic force or coupling force is replaced by a force of repulsion. 
• The separation of bivalents cannot proceed because the chromatids remain united at the point of chiasmata. Thus, the diplotene stage is also characterized by the presence of chiasmata.
• The chromosomes become shorter and thicker.
• At the end of this stage, chiasmata begin to move along the length of the chromosome from the centromere.
• This displacement of chiasmata has been referred to as terminalisation. Now the members of each bivalent ( tetravalent) will be almost free from each other.
• As a result of crossing over, four kinds of chromatids are formed in each tetravalent.
• Two parental unbroken types and two new combination types. 

Diakinesis

This is the final stage of prophase.

• The bivalent chromosomes are now very thick and highly contracted and are distributed more towards the periphery of the nucleus.
• Terminalisation is completed in all the chromosomes.
• The nucleolus and nuclear membrane disappear.
• The spindle apparatus also makes its appearance towards this late prophase. 

Metaphase I

The nuclear membrane and nucleolus have already disappeared, and the spindle apparatus is fully developed. When the spindle apparatus is ready, the bivalents move towards the equator. Each bivalent gets itself attached to the spindle by the centromere and is directed towards the opposite poles. The repulsive force between homologous chromosomes increases greatly, and chromosomes become ready to separate. 

Anaphase I

• During anaphase, the homologous chromosomes move to the opposite poles randomly.
• The short chromosomes separate easily while the longer ones take a little more time (due to interstitial chiasmata).
• The centromere does not divide before the movement. The chromosomes assume a V shape as they are pulled towards the poles. 
• The separation of chromosomes in one bivalent is independent of the separation of chromosomes in any other bivalent or pair. E.g.. ‘A’ and ‘B’ are the two chromosomes of paternal origin, while ‘a’ and ‘b’ are of maternal origin. During anaphase, ‘A’ and ‘a’ must necessarily move to opposite poles, and so do ‘B’ and ‘b’. 

The actual reduction occurs at this stage. The chromosomes separated during anaphase are not genetically the same because they have interchanged segments during crossing over. As a result of the separation of homologous chromosomes, the number is reduced to half, and thus it is different from the anaphase of mitosis. 

Telophase I

• In telophase, the chromosomes reach the respective poles.
• The spindle persists for some time.
• The chromosomes undergo despiralisation, the nucleolus reappears and so does the nuclear membrane.
• In some instances, the nucleolus may not appear at this stage.
• The cell wall is laid down in the form of a cell plate. In some cases, cytokinesis does not take place at this stage.
• It takes place when both meiotic divisions are complete.
• In pollen mother cells of flowering plants, cell walls are laid down after both meiosis I and II are called successive, and laid down only after meiosis is called simultaneous.
• So after telophase I, two cells or two nuclei are formed.

Interkinesis

The two daughter nuclei or daughter cells undergo a resting period, which is called interkinesis. It may be of long duration, in which cytokinesis will take place. There is no S phase during interkinesis.

On the other hand, it may be of short duration, or sometimes interkinesis may be absent. In this case, a cell wall is seldom formed, and the chromosomes go through a second division. Thus, depending on various external as well as internal conditions and also the ploidy nature of species, the interkinesis may be comparatively long, short, or even absent. If there is no interkinesis, the incompletely formed nuclei of telophase I may enter prophase II. 

Meiosis II

Meiosis II is similar to ordinary mitotic division but with a reduced number of chromosomes. This is also divided into four stages such as prophase II, metaphase II, anaphase II, and telophase II. 

• Prophase II
• Metaphase II
• Anaphase II
• Telophase II

Prophase II

The nucleus in prophase II resembles the prophase of mitosis.

• The arms of chromosomes are widely separated.
• The chromosomes are still coiled, with short and thick structures.
• If there is no interkinesis, there is no further coiling of chromosomes.
• The nucleolus and nuclear envelope disappear with the formation of the achromatic figure- the spindle.
• The microtubules get themselves arranged to form the spindle at right angles to the previous spindle apparatus. 

Metaphase II

It is of a very short duration like that of mitosis.

• The chromosomes get arranged on the equatorial plate of the spindle as the centromere divides.
• It separates the two chromatids which eventually become separate chromosomes.
• They can be called daughter chromosomes.
• The daughter chromosome consists of only one strand resembling the leptoten chromosome. 

Anaphase II

The daughter chromosomes move towards opposite poles due to the contraction of chromosomal fibres. Then, at each pole, there will be a haploid set of chromosomes. 

Telophase II

The chromosomes having reached the opposite poles recoils and become less distinct. Nuclear membrane and nucleolus reappear resulting in the formation of daughter nuclei. The nuclei are haploid, they have half the number of chromosomes in comparison with the parental cells. 

Cytokinesis

Cell wall formation began as a cell plate and thus totally at the end of meiosis four daughter cells were formed. In animal cells, the wall is formed by furrowing. 

Significance of Meiosis

• Helps maintain constant chromosome numbers in organisms.
• It is responsible for the formation of gametes.
• Crossing over which is the central focus in meiosis brings about the recombination of genetic material leading to variations amongst individuals. 
• Meiosis provides the basic material (variations) for evolution. 
• The random recombination among homologous chromosomes gives rise to a variety of mixes of parental characters and new characters. 
• This genetic recombination increases genetic diversity, which is essential for both adaptation and survival of species in a changing environment.

References

• Agarwal, P. V. |. V. (2004). Cell biology, Genetics, Molecular Biology, Evolution, and Ecology: Evolution and Ecology. S. Chand Publishing.
• https://bio.libretexts.org/Courses/Lumen_Learning/Biology_for_Non-Majors_I_(Lumen)/07%3A_Cell_Division/7.06%3A_Meiosis
• https://www.longdom.org/open-access-pdfs/brief-note-on-stages-of-cell-meiosis-and-its-significance.pdf

Additional Reading

• Write a Short Note on Cell Division
• Explain The Mitotic Cell Cycle
• Different Stages of Mitosis