Introduction

All the organisms in a species 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.

Read more about the Meiotic Cell Cycle

Phases of Meiosis

Meiosis involves two rounds of cell division giving rise to four daughter cells. these two rounds of cell division are called;

• Meiosis I
• Meiosis II

The DNA replication takes place only before meiosis I. Meiosis I is immediately followed by Meiosis II without any intervening interphase. This is the reason why DNA and the number of chromosomes are halved after each meiosis. 

Meiosis I

It is the first round of division. The process of meiosis begins with the diploid cells having double the number of chromosomes. This is because the cells have undergone DNA replication before entering Meiosis I. Thus, chromosomes are present in the form of homologous pairs, each cell having 46 pairs of chromosomes at the beginning of Meiosis I. It should be kept in mind that normal cells have only 23 pairs of chromosomes as they have not undergone DNA replication. 

Meiosis I is divided into the following four phases;

1. Prophase I
2. Metaphase I
3. Anaphase I
4. Telophase I

Meiosis I is preceded by interphase during which the cell prepares itself for meiosis. The detail of all these phases is given below. 

Interphase

It is the phase during which a cell prepares itself for division. It occurs only before Meiosis I. there is no interphase between Meiosis I and Meiosis II. The interphase is divided into three phases;

1. G₁ phase, the cell grows in size and makes necessary proteins in this phase
2. S phase, the cell undergoes DNA replication
3. G₂ phase, the cell makes proteins that are needed for meiosis

After the G₂ phase is complete, the cells enter Prophase I. 

Prophase I

It is the longest phase of meiosis I during which the nuclear envelope disappears, and genetic exchange takes place. It is further divided into five phases.

Leptotene

During this phase, the chromosomes start appearing as thin threads within the nucleus. Each chromosome consists of two sister chromatids. A total of 46 chromosomes, each having 2 chromatids can be seen in the nucleus towards the end of this phase.

Zygotene

During this phase, the homologous chromosomes come close to each other to form homologous pairs. This pairing process is called synapsis.

Pachytene

The pairing of homologous chromosomes is completed during this phase. It results in the formation of tetrad chromosomes (called so because of four sister chromatids) also known as bivalent (two chromosomes). 

Once the pairing process is complete, the homologous recombination takes place. It is the process during which the non-sister chromatids can exchange their segments resulting in genetic variations. This process is called crossing-over. 

A physical link is formed between the non-sister chromatids through which the crossing-over takes place. This is known as chiasmata. 

Diplotene

During this phase, the homologous chromosomes undergo uncoiling and are visible as two threads. However, the bivalent structure is not broken as the two chromosomes remain tightly linked at chiasmata points. The chiasmata are broken only during anaphase I.

Diakinesis

This is the last stage of prophase I during which chromosomes undergo further condensation. All the four parts of the bivalent are visible at the end of diakinesis. 

During diakinesis, the nuclear envelope is disentangled, the nucleoli disappear, and the mitotic apparatus starts forming. 

This completes prophase I of Meiosis I. The cell now enters metaphase I.

Metaphase I

During this phase, the spindle fibres are formed among the centrosomes that have already migrated to the opposite poles of the cell. These centrosomes also give rise to kinetochore microtubules that attach the bivalent of homologous chromosomes at the kinetochores from each side. Tension is generated in these microtubules that arrange the bivalents along with the metaphase plate in the centre of the cell. 

The attachment of microtubules at the kinetochores is called bivalent attachment as they are attached to the entire bivalent, not the individual chromosomes. 

Anaphase I

during anaphase I, the microtubules start shortening, pulling the bivalent towards the opposite poles. As a result, the chiasma break and the bivalent structure is lost. The individual chromosome consisting of sister chromatids, having crossed segments, are pulled towards the respective pole. The sister chromatids are not separated during this process as the centromeres holding them are supported by some guarding proteins. 

The cell also elongates for division into two daughter cells. 

Telophase I

This is the final stage of Meiosis I. During this phase, the mitotic apparatus disappears while the new nuclear membrane is formed around the daughter chromosomes present at each pole of the cell. 

Each daughter nucleus carries half the number of chromosomes (23 chromosomes, each having two sister chromatids) as compared to the diploid parent nucleus. The resulting daughter nuclei contain only one copy of each chromosome and are haploid. The two sister chromatids are not copies of the chromosome as they are only formed as a result of DNA replication.

Cytokinesis

Meiosis I is followed by cytokinesis in which a cleavage furrow is formed dividing the cell into two daughter cells. 

Meiosis II

The two daughter cells formed in Meiosis I immediately undergo the second round of division, Meiosis II. There is no interphase or resting phase between the two cell divisions. Meiosis II resembles mitosis in a way that it does not further decrease the number of chromosomes. Each haploid cell having 23 chromosomes is divided into two haploid daughter cells. It only involves the separation of sister chromatids. It is divided into the following four phases. 

Prophase II

During this phase, the nucleoli and nuclear envelope disappears, centrosomes duplicate and start migrating towards opposite poles with the mitotic apparatus being formed between them 

Metaphase II

During this phase, the kinetochore microtubules centrosomes attach to the chromosomes and align them along with the metaphase plate in the centre of the cell. 

Anaphase II

The sister chromatids are separated and are pulled towards the respective poles. The cell also elongates to be divided into two daughter cells. 

Telophase II

It results in the formation of nuclear envelopes around the segregated chromosomes., two daughter nuclei are formed at each pole of the cell, containing the same number of chromosomes as in the parent cell. 

Cytokinesis

It divides each cell into two daughter cells. 

The process of meiosis is thus completed with four daughter cells formed from a single parent cell. Each daughter cell is haploid carrying only 23 chromosomes, as compared to the diploid parent cell having 46 chromosomes. The chromosomes in the daughter cells have only one chromatid. 

Meiosis vs Mitosis

Meiosis II is similar to mitosis with no key differences. However, there are several differences between Meiosis I and mitosis. Some of the significant differences are mentioned below.

• Mitosis involves only one division while Meiosis involves two round of cell division after one interphase
• Mitosis produces two daughter cells while meiosis produces four daughter cells
• The daughter cells in mitosis have same number of chromosomes as the parent cell while the chromosome number is reduced to half in case of  daughter cells originating from meiosis
• Meiosis I involves the crossing-over of chromosomes while it is not seen in mitosis
• Meiosis is seen only in organisms that undergo sexual reproduction while mitosis is present in all organisms
• Meiosis takes place only in gametes while mitosis takes place in all cells except gametes. 
• The purpose of mitosis is cell proliferation while that of meiosis is sexual reproduction
• Meiosis results in genetic variations among the daughter cells while mitosis produces genetically identical cells

Significance

Meiosis has a profound importance for the continuity of life in eukaryotes. Here are some key points regarding its significance. 

Sexual reproduction

Meiosis is necessary for the synthesis and proliferation of gametes. Gametes are the cells that take part in sexual reproduction. The sperm and ova in the case of humans are produced as a result of meiosis. 

Chromosome Number Maintenance

As mentioned earlier, meiosis is needed for the maintenance of an equal number of chromosomes in the offspring. It does so by reducing the number of chromosomes to half in each gamete so that when the male and female gametes fuse to form a zygote, it carries the same number of chromosomes as is present in the cells of both parents. 

If meiosis is not present, the number of chromosomes will double in each next generation causing serious consequences. However, this does not occur in nature. Sexual reproduction and formation of the zygote are only possible if the two gametes, male and female, having half the number of chromosomes, fuse. 

Genetic Variations

The crossing over of chromosomes during the prophase I of meiosis is responsible for the genetic variations in the next generation. These combinations increase the chance of the appearance of some new genes in the individuals. 

Evolution

The genetic variations produced as a result of meiosis can also give rise to some new species, a process called evolution. Biologists believe that meiosis played a significant role in the evolution of eukaryotic cells. 

Errors in Meiosis

The errors in meiosis occur during the process of crossing-over and separation of homologous chromosomes. The normal separation of homologous chromosomes is called the disjunction of chromosomes. If they fail to separate normally, it is called the non-disjunction of chromosomes. 

The non-disjunction of chromosomes can produce gametes that have more or less than a normal number of chromosomes. When such gametes fuse to form a zygote, it also carries an abnormal number of chromosomes. The individual who develops from this zygote can suffer from several chromosomal syndromes. Some of these syndromes are as follows.

• Down Syndrome, trisomy of chromosome 21
• Patau Syndrome, trisomy of chromosome 13
• Edward Syndrome, trisomy of chromosome 18

Summary

Meiosis is a type of cell division in which the daughter cells have half the number of chromosomes as compared to the parent cells. 

It includes two rounds of cell division after one phase of DNA replication. These are;

• Meiosis I
• Meiosis II

Meiosis I has the following stages;

• Prophase I: It is further divided into five substages and involves pairing of homologous chromosomes called synapsis, crossing-over of sister chromatids, formation of chiasma, and disintegration of nuclear membrane
• Metaphase I: It involves the arrangement of homologous chromosomes along the metaphase plate.
• Anaphase I: It involves the separation of homologous pairs and the chromosomes are pulled apart. 
• Telophase I: In this phase, a new nuclear membrane is formed around the chromosomes at each pole resulting in the division of a parent cell into two haploid daughter cells. 

Meiosis I is followed by Meiosis II is similar to mitosis and results in the formation of four daughter cells from a single parent cell. 

There are several differences between Meiosis I and mitosis that have been discussed in this article.

Meiosis is necessary for;

• Sexual reproduction
• Genetic variation 
• Maintenance of chromosome number
• Evolution

The error can occur during meiosis, causing the non-disjunction of chromosomes. These errors result in several syndromes in the offspring. 

Frequently Asked Questions

What is meiosis?

Meiosis is a type of cell division in which a parent cell divides into four daughter cells, each having half the number of chromosomes as compared to the parent cell. 

Which type of cells undergo meiosis?

All the cells do not divide by the process of meiosis. This type of cell division is only seen in cells that actively participate in the process of sexual reproduction. In other words, germ-line cells undergo meiosis. The somatic cells divide through the process of mitosis. 

What is the difference between meiosis and mitosis?

Mitosis involves only one set of cell division followed by duplication of DNA whereas meiosis involves two divisions after single duplication of DNA. Mitosis happens in somatic cells while meiosis takes place in germ-line cells. The number of chromosomes is preserved in mitosis while they are halved in meiosis. Meiosis forms the basis of genetic variations in the offspring of sexually reproducing organisms.

Which cells undergo meiosis in females?

The primary oocytes present in the ovaries of females undergo meiosis. 

References

1. Freeman, Scott (2011). Biological Science (6th ed.). Hoboken, NY: Pearson. p. 210.
2. Hassold T, Hunt P (April 2001). “To err (meiotically) is human: the genesis of human aneuploidy”. Nature Reviews Genetics. 2 (4): 280–91. doi:10.1038/35066065. PMID 11283700.
3. Letunic I, Bork P (2006). “Interactive Tree of Life”. Archived from the original on 29 January 2018. Retrieved 23 July 2011.
4. Bernstein H, Bernstein C (2010). “Evolutionary origin of recombination during meiosis”. BioScience. 60 (7): 498–505. doi:10.1525/bio.2010.60.7.5.
5. Lodé T (June 2011). “Sex is not a solution for reproduction: the libertine bubble theory”. BioEssays. 33 (6): 419–22. doi:10.1002/bies.201000125. PMID 21472739.
6. Battaglia E. (1985). Meiosis and mitosis: a terminological criticism. Ann Bot (Rome) 43: 101–140.