Difference Between Mitosis and Meiosis

Introduction

Cell division is a very important process in all living organisms. During the division, a parent cell divides into two daughter cells, along with replication and cell growth. The sequence of events by which this process occurs is known as the cell cycle. Now, the cell cycle comprises of two basic phases:- Interphase and M phase(mitosis phase). In some cells, instead of mitosis, meiosis takes place. Let us find out the key differences between mitosis and meiosis.

Difference Between Mitosis And Meiosis In Tabular Form

Mitosis	Meiosis
Occurs in somatic cells	Occurs in germ cells
Equational division	Equational and reductional division
Diploid cells formed	Haploid cells formed
Gives rise to 2 daughter cells	Gives rise to 4 daughter cells
Nucleus divides once	Nucleus divides twice
No. of chromosomes remains the same	No. Of chromosomes are reduced to half
crossing over of chromosomes occur	Crossing over of chromosomes takes place

What is Mitosis?

 Mitosis is a critical cellular process in which a parent cell undergoes division to produce two identical daughter cells. These cells are identical to each other and to the parent cell. Here, an already duplicated chromosome is distributed equally among two daughter cells. Since the resulting daughter cells have the same number of chromosomes as the parent cell, mitosis is often referred to as equational division.

Mitosis occurs in:

• Somatic cells(body cells) of animals
• In gonads for the multiplication of undifferentiated germ cells
• In plants, it occurs in dividing meristematic tissue and also in growing leaves, flowers, and fruits

Mitosis was first observed by Strasburger in plant cell and by Walter Flemming in animal cell. Walter Flemming gave the term mitosis in 1882.

Mechanism of Mitosis

Mitosis is an elaborate and complicated process that involves sequential changes in the nucleus as well as in the cytoplasm. The two crucial events that occur in mitosis are:-

1. Karyokinesis
2. Cytokinesis

Karyokinesis

Karyokinesis is the process of division of the nucleus. Karyon means “nucleus” and Kinesis means “movement”, hence the term Karyokinesis. It is a continuous process in which a parent nucleus divides into two daughter nuclei. There is no pause during the process, but for our convenience, mitosis has been divided into four stages. It’s important to note that clear-cut lines cannot be drawn between two stages because Karyokinesis is a progressive process.

The four stages of mitosis are:

Prophase

‘Pro’ means first and ‘phase’ means stage, therefore prophase is the first stage of Karyokinesis of mitosis. It follows the interphase stage of cell cycle. The following events occur during prophase:

• The chromatin material condenses during condensation. The DNA strands get untangled to form tightly packed mitotic chromosomes.
• Each chromosome becomes double and looks like two coiled sister chromatids joined by a centromere. At the beginning of prophase, their ends are not visible, so they appear like a ball of wool called spireme stage.
• Centrosome( in animal cell, already duplicated in interphase) starts to move toward the opposite poles. Each centrosome emits out microtubules called aster. The two asters, along with spindle fibers, constitute the mitotic apparatus.
• At the end of prophase, Golgi complex, endoplasmic reticulum, nuclear membrane and nucleus disappear.

Metaphase 

‘Meta’ means second and ‘phase’ means stage. Hence, metaphase is the second phase of mitosis. The events that occur during metaphase are:

• The beginning of the metaphase is marked by the complete disintegration of the nuclear membrane. The nuclear envelope disappears, and the chromosomes are spread throughout the cytoplasm.
• The beginning of the metaphase is marked by the complete disintegration of the nuclear membrane. The nuclear envelope disappears, and the chromosomes are spread throughout the cytoplasm.
• Each chromosome consists of 2 chromatids that are attached through the centromere. During this stage, the chromosomes are thickest and shortest. Hence, it is easy to study their morphology. The chromosomes are condensed to form short and compact rod-like structures because it is easier for short and compact structures to move through the anaphase than for long and twisted interphase chromosomes.
• The formation of the mitotic spindle is complete. The phenomenon of bringing chromosomes to the equator is called Congression.
• All the chromosomes aligned themselves at the equator into a metaphase plate.
• The surface of the centromere that holds the two sister chromatids together is surrounded by a small disc-shaped structure called Kinetochore.
• The Kinetochore present around the centromere forms the site of attachment of the microtubules. The microtubules of the spindle fiber attach to the kinetochore during the metaphase.

Anaphase

'Ana' means up, and ‘phase’ means stage. Therefore, during anaphase, the chromatids move towards the opposite poles.

The following events occur during anaphase:

• The centromere holding the two chromatids together splits and separated daughter chromatids are now referred to as chromosomes of the future daughter nuclei.
• Now, the spindle fibers attached to the kinetochore shorten, and the daughter chromosomes begin to move toward the opposite poles.
• During migration, the centromere of the daughter chromosomes moves towards the pole and the arms of the chromosomes trail behind. When the chromosomes reach the pole, it marks the end of anaphase. It is the best stage to study the shape of chromosomes(V, L, J, I).

Telophase

Telo’ means end and ‘phase’ means stage, hence telophase is the end stage of mitosis. The following events occur during telophase:

• The chromosomes reached the opposite poles, followed by the disappearance of the mitotic spindle.
• The chromosomes gradually uncoil on reaching the pole and lose their identity. The chromosomes decondense, and finally, they become indistinguishable mass and collect at the poles.
• Nucleolus endoplasmic reticulum Golgi bodies and other organelles reappear.
• The nuclear membrane develops around the chromatin clusters at each pole, forming two daughter nuclei.

Cytokinesis

Cytokinesis refers to the division of cytoplasm. As the name suggests, ‘kytos’ means cell or hollow, and ‘kinesis’ means movement. Cytokinesis marks the end of cell division. Mitosis not only deals with the division of the nucleus, but it is also concerned with the division of cytoplasm. After Karyokinesis, the cytoplasm of the parent cell divides into two daughter cells so that both the daughter cells have their own nucleus and cytoplasm. The cell organelles all in the cytoplasm also distribute among the two daughter cells.

There are some organisms in which Karyokinesis is not followed by cytokinesis. In such organisms division of nuclear suckers without the division of cytoplasm. As a result, there are large number of nuclei present in the cytoplasm.

The process of cytokinesis varies in plants and animals.

In animals, the division of cytoplasm occurs centripetally. A furrow is formed in the plasma membrane, which deepens gradually and joins in the center, dividing the cytoplasm into two. The plasma membrane continues to constrict, eventually leading to the division of the animal cell into two daughter cells when it joins together.

In plant cells, the process of cytokinesis is achieved centrifugally. A cell plate called phragmoplast is formed by the Golgi complex. The cell plate grows outward and finally meets the cell wall, leading to the division of plant cell into two daughter cells.

Significance of Mitosis

1. Growth: Mitosis is the process responsible for the growth of multi-cellular organisms. Somatic cells are formed by mitosis.
2. Chromosome number maintenance : Mitosis is an equational division. The daughter cells contain the same no. of chromosomes as the parent cell.
3. Cell size: Mitosis helps in maintaining the size of a cell. An overgrown somatic cell undergoes division in order to maintain a proper surface area-to-volume ratio.
4. Repair and regeneration: Mitosis helps in replacing old and damaged cells with new cells.
5. Reproduction: Mitosis helps in the process of reproduction in unicellular organisms.

What is Meiosis?

The process in which a diploid parent cell divides to form 4 haploid daughter cells. Each daughter cell formed has half the number of chromosomes. In meiosis, the division of the nucleus occurs twice, but DNA replication occurs only once. Hence, 4 haploid daughter cells are formed.

Occurrence: Meiosis is a process exclusive to organisms that reproduce sexually. In multicellular organisms, it is limited to specific cells that the transition from mitosis to meiosis occurs in a specific stage of their life cycle.

In 1905, Farmer and Moore gave the term meiosis. The division was studied by Van Benedin Winiwater and Strasburger.

In meiosis, division occurs twice, firstly in meiosis I and secondly in meiosis II. Meiosis I is called reductional division, and Meiosis II is called equational division.

Meiosis I

It is the reductional division i.e the chromosomes number is reduced to half. It consists of 4 phases:

Prophase I

It is a more complex and lengthier stage than prophase of mitosis. The sequential events that occur in prophase are as follows:

• Leptotene: During this stage, condensation of chromatin fibers takes place.
• Zygotene: It is the second stage of prophase I after leptotene. A diploid cell possess a pair of Chromosome set. The two chromosomes which are similar in shape and size are called homologous chromosomes. The homologous chromosomes start pairing, and this process is known as synapsis. This process occurs by the formation of a synaptonemal complex. The structure now formed is known as Bivalent.
• Pachytene: This is the third stage of prophase I. In this stage, the bivalent or homologous chromosomes become distinct and appear as tetrad. The crossing over of the non-sister chromatids( chromatids of different chromosomes) of the homologous chromosomes takes place by the formation of recombination nodule. Crossing over leads to the exchange of genetic material.
• Diplotene: It is the fourth stage of prophase I. During this stage, dissolution of the synaptonemal complex occurs. The two homologous chromosomes start to separate. At the site of separation, formation of a X shaped chiasmata occurs. In oocytes, this stage can last for months or years, hence called dictyotene.
• Diakinesis: This stage is the end stage of prophase I and marks the transition to metaphase I. Spindle fibers appear and terminalisation of chiasmata takes place.

Metaphase I

Now, the chromosomes enter metaphase I. Here, the nuclear envelope degenerates and the chromosomes enter the cytoplasm.

• The centromere of the two chromosomes align themselves into two metaphasic plate at the equator.
• The bivalent chromosomes are distributed randomly.
• The microtubules of the spindle fibers from the opposite poles attach to the centromere of the chromosomes facing towards it.

Anaphase I

The following series of events occurs during anaphase I :

• The two homologous chromosomes separate and starts moving towards the opposite poles.
• The intact univalent chromosome with two chromatids attached at the centromere move towards the opposite poles due to the spindle fibers attached. In this case, the division of centromere does not takes place.
• Out of the pair of chromosomes, since only one chromosome reaches the pole, the number of chromosomes is reduced to half.

Telophase I

It is the end stage of meiosis I.

• The spindle fibers disappeared completely, and the chromosomes reached the opposite poles.
• The nuclear membrane and nucleolus reappear.
• The chromosomes untangle and elongate.
• Now, two daughter cells are formed, and each daughter cell receives one of the two homologous pairs. Therefore, each one of them has half the number of chromosomes, but the amount of DNA is doubled as the chromatids didn’t separate and moved together to the single pole.

Cytokinesis

It follows the first nuclear division, and two haploid daughter cells are formed.

Interkinesis: It is gap between meiosis I and meiosis II. It is a metabolic phase where no DNA replication occurs. The RNA and protein are synthesized during this phase, which are required for meiosis II.

Meiosis II

Meiosis II is somewhat similar to mitosis as it follows equational division. But it is not completely similar to mitosis. Mitosis occurs in diploid somatic cells, but meiosis II occurs only in haploid gem cells. After mitosis, daughter cells formed are identical to each other and to the parent cell, but after meiosis II, the daughter cells are neither identical to each other nor to the parent cell.

 There are four sub-stages in meiosis II :

• Prophase II: It is not as complicated as Prophase I. During this phase, the chromatids condense, and the chromatin material becomes compact. The nuclear membrane and nucleolus disappear.

• Metaphase II: The univalent chromosomes position themselves at the equator, and the microtubules from opposite ends connect to the chromosome’s kinetochore.

• Anaphase II: The centromere holding the two chromatids splits and allows the movement of the two chromatids to the opposite poles.

• Telophase II: The chromatids reach the pole, and they start to decondense. The spindle fibers degenerate and the nucleolus and nuclear membrane reappear. Four haploid nuclei are formed.

Cytokinesis

Now, the cytoplasm division occurs by the formation of a furrow in animal cell and cell plate in plant cell. Each daughter cell from meiosis I gives rise to two daughter cells. Hence, at the end, four haploid daughter cells are formed.

Significance of Meiosis

1. Formation of gametes :  Gametes are essential for sexual reproduction. Meiosis produces gametes for sexual reproduction.
2. Maintenance of chromosome number : Meiosis reduces the chromosome number to half in gametes so that fertilization can restore the diploid number of the zygote.
3. Introduction of variations : Meiosis is responsible for variations among the progeny.

Main Difference Between Mitosis And Meiosis In Points

• The process of mitosis occurs only in somatic cells( body cells) to form two diploid daughter cells. But the process of meiosis occurs in specialized germ cells to form four haploid daughter cells.
• There is no pairing of homologous chromosomes in Mitosis, as no synaptonemal complex is formed. Crossing over is also absent in mitosis. But in meiosis, pairing of homologous chromosomes occurs as their is formation of synaptonemal complex. Crossing over occurs, which brings variations in the progeny.
• In mitosis, only one equatorial plate is formed during metaphase. But in meiosis, during metaphase I, two equatorial plates are formed and during meiosis II, one equatorial plate is formed.
• In mitosis, during anaphase, centromeres split and the separation of two chromatids occur. But in meiosis, during anaphase I the homologous chromosomes separate and there is no splitting of centromere. During anaphase II, centromeres split, and the chromatids separated.
• Mitosis basically helps in growth, repair and regeneration. But meiosis helps in maintaining chromosome numbers constant from generation to generation and produces variations.

Conclusion

Both mitosis and meiosis are very crucial events in the life of living organism, though they are distinct in their purpose and outcomes. On one hand, mitosis is responsible for the growth and repair of the body, resulting in genetically identical daughter cells, and on the other hand, meiosis is essential for sexual reproduction, generating genetically diverse offspring. Although both the processes involve similar events such as Prophase, metaphase, anaphase, and telophase, they undergo unique events during their respective divisions like crossing over, and homologous chromosome pairing. Understanding the distinction between the two processes is crucial to grasp the complexities of reproduction and genetic variations.

References

• https://biologydictionary.net/meiosis/
• https://www2.nau.edu/lrm22/lessons/mitosis_notes/meiosis.html
• https://www.britannica.com/science/mitosis
• https://www.thoughtco.com/stages-of-mitosis-373534