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Meiosis Cell Division

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MVSAT 2024

What is Meiosis?

A special type of cell division that takes place in sexually-reproducing organisms is called Meiosis. It produces gametes that include sperm or egg cells. Two rounds of division take place in meiosis resulting in production of four cells having one copy of each chromosome which are haploid. 

In the process of Meiosis, the genetic material from the paternal and maternal copies of each chromosome are crossed over to create fresh combinations of code on each chromosome. After this, during fertilization, fusion of the haploid cells (produced by a male and female) takes place to create a cell with two copies of each chromosome again called zygote.

An abnormal condition in the process of meiosis may take place due to some errors  called aneuploidy and it refers to the abnormal number of chromosomes. This condition is the leading cause of miscarriage and also genetic cause of developmental disabilities. 

Meiosis in Detail

In meiosis, two rounds of cell division follows DNA replication and this time, four daughter cells with each half the number of chromosomes to that of the original parent cell takes place. 

Two rounds of meiotic divisions are known as meiosis I and meiosis II.

Meiosis I

Prior to meiosis, during the S phase of the cell cycle, each chromosome’s DNA is replicated to attain two identical sister chromatids and these are held together through cohesion of sister chromatids. This is about the premeiotic S-phase or also called meiotic S-phase.

After DNA replication, meiotic cells enter the G2-like stage called meiotic prophase where homologous chromosomes pair with each other and a genetic recombination takes place there.  A programmed process where DNA may be cut and again repaired allows them to exchange some of the genetic information. Crossing over takes place with a subset of recombination events which results in the creation of physical links called chiasmata between the homologous chromosomes.

In most of the organisms, the chiasmata help direct each pair of homologous chromosomes to separate or segregate from each other during Meiosis I. It results in creation of two haploid cells having half the number of chromosomes similar to that of the parent cell.

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Meiosis II

During this process, there releases the cohesion between sister chromatids and they segregate from one another (similar during mitosis). Sometimes, all four of the meiotic products form gametes including sperm, pollen or spores. Female animals have 3 of their 4 meiotic products eliminated by extrusion into polar bodies and at last, only one cell develops to produce an ovum. As we know the number of chromosomes are halved during meiosis, the gametes now fuse via fertilization and form a diploid zygote containing two copies of each chromosome, one from each parent. 

Therefore, alternating cycles of meiosis and fertilization enable sexual reproduction and each successive generation maintains the same number of chromosomes. For instance, diploid human cells that have 23 pairs of chromosomes include 1 pair of sex chromosomes i.e. 46 total and half of the origin comes from maternal side and the other half fro paternal side.  

Meiosis produces haploid gametes ova/sperm where each contains one set of twenty three chromosomes. When two gametes, an egg and a sperm fusion takes place, the zygote formed is diploid with the mother and father contributing 23 chromosomes each. The same pattern (excluding the same number of chromosomes) occurs in all organisms that utilize the process of meiosis. 

Meiosis occurs in all multicellular organisms and sexually-reproducing single-celled organisms and these are all eukaryotes that include plants, animals and fungi. It is an important process required for spermatogenesis and oogenesis.

How Does Meiosis Differ From Mitosis?

Although in both mitosis and meiosis, the general cell division process are related, yet they differ in two major aspects that include:

  1. Recombination: Meiosis performs shuffling of the genes between the two chromosomes in each pair which is received from each parent. It leads to creating recombinant chromosomes having unique genetic combinations in every gamete. On the other hand, mitosis takes place only if it needs to repair the DNA damage and it usually occurs between identical sister chromatids and doesn’t result in genetic changes. 

  2. Chromosome Number: Meiosis produces 4 genetically unique cells having each with half the number of chromosomes as in the parent cell. On the other hand, mitosis produces two genetically identical cells and each is with the same number of chromosomes as in the parent cell.

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FAQs on Meiosis Cell Division

1. What Happens in Meiosis I and Meiosis II?

Meiosis begins with a diploid cell containing two copies of each chromosome (termed homologs).

  • Firstly, the cell undergoes DNA replication and now each homolog consists of two identical sister chromatids.

  • After this, each set of homologous chromosomes pair with each other and perform exchanging of genetic information via homologous recombination. This leads to crossing over between homologs.

  • In the first meiotic division, the homologous chromosomes separate daughter cells by the spindle apparatus. 

  • Then, the cells proceed to a second division without the DNA replication intervening round.

  • The sister chromatids thus segregate to form daughter cells producing a total of four haploid cells.

2. Who Discovered Meiosis?

The German Biologist named Oscar Hertwig discovered Meiosis in sea urchin eggs in the year 1876.

3. What Are the Phases of Meiosis?

The phases of meiosis include Meiosis I and Meiosis II. These phases are divided into Karyokinesis I, II and Cytokinesis I, II. The preparatory steps leading Meiosis are identical in pattern and similar to interphase of the mitotic cell cycle.  Here, G1 phase or Growth 1 phase, Synthesis or S phase and Growth 2 or G2 phase take place. Thereafter, meiosis I and meiosis II follow interphase which are divided each into prophase, metaphase, anaphase and telophase stages.

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