Stages of Meiosis: Prophase, Metaphase, Anaphase, Telophase, Cytokinesis

Meiosis is a specialised type of cell division in which the chromosomes number is reduced by half, resulting in the production of haploid daughter cells. Normally, meiosis cell division occurs in the case of sex cells, like the sperms in the case of males and eggs in the case of females. These eggs and sperms after copulation may fuse to form the zygote which then eventually forms the embryo. Meiosis is a very crucial event in eukaryotic sexually reproducing organisms as it induces the creation of an equal number of chromosomes in the daughter cells as that of the parent cells. This is because the number of alleles gets reduced during meiosis and when the two gametes fuse, they yield a zygote having the same number of alleles as that of the parents. Hence, in organisms that can't survive with polyploidy, meiosis is an essential process in their life cycle. 

Keyterms: Cell Division, Cell, Zygote, Embryo, Chromosome, Gamete, DNA, Chromatid, Allele, Meiosis, DNA Replication, Reduction process, Diploid DNA, S phase

Read More: Chromosomal Theory of Inheritance

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Stages Of Meiosis 

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Before the initiation of the process of meiosis, a very important event has to occur, which is the replication or duplication of the normal diploid DNA or chromosomes. Hence, there are now 4 copies of each of the genes, located in two full sets of DNA and each of those sets have two alleles. This is important before the starting of the reduction process. This event occurs during the S phase. After this, each chromosome has two sister chromatids which are the cloned DNA after replication. The centromere connects both of them in the chromosome at the center for storage. They may also get separated into individual chromosomes when necessary. 

After this, finally the process of meiosis, the reductional cell division occurs. Four haploid cells are formed at the end of the process of meiosis. Broadly, the meiosis cell division occurs in two steps - Meiosis I and Meiosis II, which again have their substeps. 

The stages of Meiosis | are : 

• Prophase I
• Metaphase I
• Anaphase I
• Telophase I
• Cytokinesis I

The stages of Meiosis || are : 

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

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Meiosis I - Prophase I 

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This is the first step in the process of Meiosis cell division. The Prophase of Meiosis is quite long and more complex than the Prophase of Mitosis. This single substep of Prophase is further subdivided into 5 stages, namely - Leptotene, Zygotene, Pachytene Diplotene, and Diakinesis. These are discussed briefly below.

LEPTOTENE: During this step, the chromosomes start to be seen under the light microscope. During Leptotene, the condensation of chromosomes continues throughout this stage.

ZYGOTENE: Zygotene is the second step in the Prophase of Meiosis I which follows Leptotene. During this process, the process of synapsis occurs. Synapsis is the process of association or pairing of chromosomes. These paired chromosomes are eventually called homologous chromosomes. Formation of the synaptonemal complex also accompanies the process of synapsis, which becomes evident in the electron micrographs of this stage. The complex produced by the pair of homologous chromosomes that have undergone synapsis is termed a bivalent or a tetrad. These complexes are better visible from the next steps.

PACHYTENE: In the Pachytene stage, the distinction and clear appearance of the four chromatids of each of the bivalent chromosomes as tetrads is seen. One of the characteristic features of this stage is the appearance of recombination nodules. Recombination nodules are the locations where, between the nonsister chromatids of the homologous chromosomes, crossing over takes place. In crossing over, the exchange of genetic materials between the two homologous chromosomes takes place. It is an enzyme-mediated process where the recombinase enzyme functions. The end of Pachytene terminates the process of recombination between homologous chromosomes but leaves the chromosomes linked with each other at the location of crossing over.

DIPLOTENE: Disintegration of the synaptonemal complex Marks the beginning of the Diplotene stage. Except at the crossover sites, the recombined homologous chromosomes of the bivalents tend to be separated from each other, resulting in the formation of X-shaped structures, called chiasmata. 

DIAKINESIS: Diakinesis is the final step in the process of Prophase I of Meiosis I. Terminalisation of the chiasmata characterises this stage. The condensation of the chromosomes gets completed in this stage. To make the homologous chromosomes ready for separation, the meiotic spindle is also assembled in this stage only. Again, in the Diakinesis stage, the nucleolus along with the nuclear envelope breaks down and disintegrates. After Diakinesis, the cell is prepared to enter into Metaphase I.

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Meiosis I - Metaphase I 

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In the step of Metaphase I of Meiosis I, the alignment of the bivalent chromosomes are seen on the equatorial plate. From the opposite poles of the spindle, the microtubules emerge to attach themselves to the kinetochore of homologous chromosomes.

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Meiosis I - Anaphase I 

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In the Anaphase I stage of Meiosis I, separation of the homologous chromosomes is seen, while the sister chromatids remain connected at their centromeres.

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Meiosis I - Telophase I

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This is the last stage of Meiosis I. Here, the reappearance of the nucleolus and the nuclear membrane is seen. It is followed by Cytokinesis and is called the dyad of cells. Interkinesis is the time between two meiotic divisions. It is usually short-lived. No DNA replication takes place during interkinesis.

Read More: Euchromatin Vs Heterochromatin 

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Meiosis II - Prophase II

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Prophase II is the first step in the process of Meiosis II. Prophase II is simpler than Prophase I to a great extent. It starts right after Cytokinesis, even before the full elongation of the chromosomes. Meiosis II is similar to a normal Mitosis. Here, by the end of Prophase II, the nucleolus and the nuclear membrane disappear, followed by the compaction of the chromosomes.

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Meiosis II - Metaphase II 

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Alignment of the chromosomes at The equator is seen in the Metaphase II of Meiosis II. Again, from the opposite poles of the spindle, the microtubules arise and get attached to the kinetochores of the sister chromatids. 

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Meiosis II - Anaphase II

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The Anaphase II of Meiosis II is characterised by the simultaneous splitting of centromeres. The centromeres were holding together the two sister chromatids. Splitting of those allows the chromatids to go to the opposite poles. This is induced by the shortening of the microtubules which are attached to the kinetochores. 
 

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Meiosis II - Telophase II

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Telophase II ends or terminates the whole process of Meiosis. Here, once again the nuclear envelope emerges which encloses the two groups of chromosomes. Eventually, Cytokinesis occurs which results in the production of a tetrad of cells or four haploid daughter cells. 

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Things To Remember

• Meiosis is responsible for the production of the sex cells or gametes , which are the essential elements of sexual reproduction.
• Activation of the genetic information and deactivation of the sporophytic information is also induced by Meiosis.
• Crossing over resulting from meiosis induces new combinations of traits and variation.
• The process of Meiosis is responsible for the maintenance of a constant number of chromosomes, by dividing them into two halves. It is very crucial in sexually reproducing organisms as after fertilisation, the chromosome number gets doubled.

Read More: Mitosis and Meiosis

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Previous Year Questions

• The largest phase of meiosis-I is? [WBJEE-2011]
• In meiosis, synapsis occurs during?
• In which stage of meiosis crossing over takes place?
• Synapsis occurs in ...... phase of meiosis? [BCECE 2004]
• Meiosis can be best studied in [CUCET 2011]
• During _________stage of meiosis I, crossing over take place. [COMEDK UGET 2007]
• Meiosis results in 
• At which stage of meiosis does the genetic constitution of gametes is finally decided?
• During anaphase I of meiosis
• During pachytene stage of meiosis, the chromosomes appear
• Identify the wrong statement about meiosis.
• The stage of meiosis in which pairing of homologous chromosomes starts, is called? [UPSEE 2016]
• The exchange of segments of non-sister chromatids between homologous pair is? [DUET 2010]
• Which of the following stages is the longest in animals? [ JIPMER 1996]
• During meiosis -I, the number of chromosomes is? [JKCET 2015]