Mitosis and Meiosis

Meiosis occurs in germs cells in gonads and results in gamete formation, and Mitosis repeatedly takes place in the somatic cells. It is the process in which chromosomes of a nucleus split longitudinally into two halves and migrate to the two poles, each pole with one-half of the chromosomes. These organise into two daughter nuclei, one at each pole. The daughter nuclei are identical to each other and the parent nucleus qualitatively and quantitatively. Mitosis and meiosis are the two phases that are used for cell division.

Mitosis

Mitotic cell division involves two processes : (i) karyokinesis or nuclear division and (ii) cytokinesis or division of cytoplasm. Before cell division, the nucleus is in interphase. 

The four phases of mitosis are:

1. Prophase (chromatid coiling, nucleolar disintegration, and spindle formation). Prophase is the longest of the four mitotic phases, lasting several minutes to more than an hour. It begins with the appearance of chromosomes as thin threads inside the nucleus. 
2. Metaphase (chromosomal orientation at the equatorial plate). The disappearance of the nuclear membrane and nucleolus marks the beginning of metaphase. At this stage, the spindle is fully developed, and in an equatorial plane, chromosomes are arranged.
3. Anaphase (movement of daughter chromosomes towards the poles). Division of centromere denotes the start of anaphase, and it is the shortest phase in the mitotic cycle. In this phase, the sister chromatids or daughter chromosomes shift towards either side of the poles of the spindle.
4. Telophase: It is the process of uncoiling chromosomes, disappearing the spindle, and reappearing the nucleolus and nuclear membrane. This is the last phase of mitotic division in which daughter cells are formed. The chromosomes with their centromeres at poles begin to uncoil and lengthen. A nuclear membrane is rebuilt around every group of chromosomes; thus, one nucleus is established at each pole. 

Cytokinesis – In this process, there is segmentation and separation of cytoplasm into two daughter cells.

Meiosis

The process of meiosis occurs in reproductive cells. The daughter nuclei formed by this process contain only half of the parent nucleus’ chromosomes. The chromosome number gets reduced in this process, also called reduction division. J.B. Farmer and J.E. Moore termed the process of meiosis.

• Meiosis is a complicated process and comprises two consecutive divisions: meiosis I and meiosis II. 
• First Meiotic Division or Heterotypic Division (Meiosis I) is heterotypic and is also known as reduction division because the chromosome number is halved. The first meiotic division is categorised as prophase I, metaphase I, anaphase I, and telophase I.

• The primary phase of meiosis is Prophase I and differs considerably from the prophase of mitosis. Based on chromosomal behaviour, it can be further subdivided into five phases:
  1. leptotene,
  2. zygotene,
  3. pachytene,
  4. diplotene, and
  5. diakinesis. 

• Chiasmata may form between any two of the four chromatids, and there may be up to eight chiasmata in a bivalent. It performs the following two functions :
  • To hold homologous chromosomes together while they move into position on the spindle before segregation.
  • Crossing over (or exchanging genetic material) occurs at the chiasmata leading to increased variation.

Meiosis II (Second Meiotic Division or Homotypic Division) 

• Meiosis II is equational (mitotic) in character, also known as meiotic mitosis. 
• Mitosis and meiosis differ in that the DNA does not duplicate, but centromeres do in meiosis. The two haploid cells formed due to heterotypic division divide mitotically into two cells each. Meiosis II comprises four stages,
  1. prophase II,
  2. metaphase II,
  3. anaphase II, and
  4. telophase II.
• Telophase II. At this stage, chromosomes become thin and begin to uncoil. Nuclear membrane and nucleoli are reconstituted.
• Two nuclei are separated by cytokinesis. Till the end of meiosis II, each daughter cell of meiosis I produce two haploid daughter cells.
• Thus a diploid cell produces four haploid daughter cells by meiosis. 

Mitosis vs Meiosis

• The major difference between mitosis and meiosis is that it occurs in body cells, and meiosis occurs in sex cells.
• Two identical daughter cells are formed after mitosis, whereas meiosis results in the formation of four sex cells.
• Only one cell division occurs in mitosis, whereas two cell divisions(Meiosis I and Meiosis II) occur in meiosis. 
• The daughter cells formed after mitosis are diploid where the chromosome number remains the same as the parent, whereas, in meiosis, the daughter cells formed are haploid where the chromosome number gets halved from the parent cell.
• In prophase I, no recombination/crossing-over takes place in mitosis, whereas it occurs in meiosis.

Conclusion

Mitosis takes place repeatedly in body/somatic cells. Meiosis occurs in germs cells in gonads and results in gamete formation. It is the process in which chromosomes of a nucleus split longitudinally into two halves and migrate to the two poles, each pole with one-half of the chromosomes. Mitotic cell division involves two processes : (i) karyokinesis or nuclear division and (ii) cytokinesis or division of cytoplasm. Before cell division, the nucleus is in interphase.