Meiosis Cell Division: Stages, Purpose, and Importance

Cell division is the process of forming new cells by the division of a parent cell. There are two types of cell division; mitosis and meiosis.

Van Beneden first described meiosis cell division in 1883. Meiosis is the process of cell division in which the original diploid cell divides twice to produce a total of four haploid cells. Thus, formed haploid cells consisting of half number of chromosome as the original diploid cell gives rise to gametes (sperm or eggs) that, on fertilization, supports sexual reproduction and a new generation of a diploid organism. 

Meiosis cell division has two subsequent division phases, unlike mitosis cell division which has only one division phase. Similarly, mitosis gives two diploid cells with an equal number of chromosomes as the parents (2n) but meiosis results in the formation of 4 haploid cells with half the number of chromosome (n).

Stages of meiosis cell division

Meiosis has broadly four stages; pre-meiotic interphase, meiosis I, intrameiotic interphase, and meiosis II.

Pre-meiotic interphase

• Before entering into meiosis I, a cell undergoes a period of growth phase called interphase
• DNA duplication occurs at the S-phase.
• The nucleus and nucleolus become visible.

Meiosis I

• Also known as Reductive Division or Heterotypic Division
• Reduction of chromosome number occurs in two haploid daughter cells produced from the original diploid cell.
• It has five stages:
  1. Prophase I
  2. Prometaphase
  3. Metaphase I
  4. Anaphase I
  5. Telophase I

Prophase I

• The longest phase of the meiotic cell division (occupies up to 90%)
• It includes six sub-stages; proleptotene or proleptonema, leptotene or leptonema, zygotene or zygonema, pachytene or Pachynema, diplotene, and diakienesis.    

1. Proleptotene or Proleptonema
   • Chromosomes are extremely thin, long, uncoiled, longitudinally single, and slender thread-like structures.
2. Leptotene or Leptonema
   • Chromosome becomes a more uncoiled and long thread-like structure with a specific orientation inside the nucleus that looks like a ball of knitting wool.
   • After duplication of centrioles, each pole of the cell possesses two centrioles.
   • The process of homology search begins to initiate the pairing of homologs.
3. Zygotene or Zygonema
   • The pairing of homologous chromosomes which come from father (sperm) and mother (ova) takes place; such pairing is known as Synapsis (or Homologous Dyads).
   • The pairing of homologous chromosomes is extremely exact and specific (gene-for-gene).
   • Paired homologous chromosomes are joined by a protein-containing framework called Synaptonemal Complex (SC) till crossing over is completed.
4. Pachytene or Pachynema:
   • Synapsed chromosomes become thick and short; each synaptonemal pair at this point is commonly referred to as bivalent or dyads because of the presence of two chromosomes and four visible chromatids, respectively.
   • The crucial genetic phenomenon “Crossing Over” takes place. Crossing over is the process of interchange of chromatin material between one non-sister chromatid of each homologous chromosome accompanied by chiasmata formation.
   • During crossing over, recombination of genetic material takes place by mutual exchange of corresponding segments by breakage and reunion with the help of enzymes recombinase and ligase, respectively.
5. Diplotene or Diplonema:
   • Unpairing or desynapsis of homologous chromosomes initiates, and chiasmata are first observed.
   • Synaptonemal complex disappears, leaving participating chromatids of the paired homologous chromosome, which physically joins at chiasmata.
   • Chromatids of the paired homologous chromosome physically join at one or more discrete points known as Chiasmata, where crossing over takes place.
6. Diakinesis:
   • The movement of chiasma from the centromere to the end terminal point of the chromosome occurs. This movement is known as terminalization.
   • Chromatid remains connected by the terminal chiasmata and exists up to the metaphase.

Pro metaphase

• The nuclear envelope disintegrates.
• The spindle assembles at the opposite pole of the cell.
• Chromosomes got coiled in a spiral manner and arranged on the spindle’s equator.

 Metaphase I

• Spindle fiber attached to the chromosome helps align the chromosome at the equator.
• This stage terminates as soon as the homologous chromosome starts to separate from each other.

 Anaphase I

• The homologous chromosome gets separated and moves towards the opposite pole.
• Actual reduction and disjunction occur at this stage.
• The number of chromosomes at each pole is precisely half (n) as each pole receives one homologous from each bivalent present in the cell.

 Telophase I:

• The arrival of a half set of chromosomes at each pole defines the initiation of telophase.
• Nucleolus reappear.
• Chromosomes uncoil, and a nuclear envelope is formed around the chromosomes.
• After Karyokinesis, cytokinesis occurs through which two haploid cells are formed.

Intrameiotic or Interkinesis

• The short resting phase between telophase- I and Prophase-I.
• No DNA replication occurs in this stage.

Meiosis II

• Also known as the Equational Division or Homotypic Division.
• This stage includes dividing each haploid meiotic cell into two haploid cells.
• It consists of four steps;

I. Prophase II

• Two pairs of centrioles are formed and move toward the opposite pole.
• Chromosomes with two chromatids become short and thick.
• The nuclear membrane and nucleolus disappear.

II. Metaphase II

• Chromosomes get organized on the equator of the spindle.
• Centromere divides into two. Thus, each chromosome produces two daughter chromosomes.

III. Anaphase II

• Daughter chromosomes move towards the opposite pole due to the shortening of chromosomal microtubules and stretching of interzonal microtubules of the spindle.

IV. Telophase II

• Chromatids migrate to the opposite poles known as chromosomes.
• A nuclear envelope is formed around the chromosome, and the nucleolus reappears.
• After Karyokinesis, cytokinesis occurs in each haploid meiotic cell, resulting in four haploid cells.
• Thus formed cells have different types of chromosomes due to the crossing over.

Purpose/Importance of Meiosis 

1. Meiosis maintains a persistent number of chromosomes in the organisms.
2. By crossing over, genetic variations among the species can lead to evolution.
3. It also facilitates segregation and an independent assortment of genes.
4. Leads to the continuity of generations by producing gametes(sperm and ova) in sexually reproducing species.

References

1. Meiosis. Scitable. Retrieved 26 July 2022, from https://www.nature.com/scitable/definition/meiosis-88/
2. What is meiosis.? yg. Retrieved 26th July 2022, from https://www.yourgenome.org/facts/what-is-meiosis/
3. Meiosis. National Human Genome Research Institute. Retrieved 27th July 2022, from https://www.genome.gov/genetics-glossary/Meiosis
4. Verma Ps and Agrawal VK (2008). cell biology, genetics, molecular biology, evolution and ecology. Ram nagar, New Delhi: S.Chand & Company Pvt.Ltd. https://books.google.com.pk/books?id=RXscEAAAQBAJ&printsec=frontcover#v=onepage&q&f=false
5. Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. Meiosis. Available from: https://www.ncbi.nlm.nih.gov/books/NBK26840