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:

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.
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: Stages, Process & Importance

1. What is meiosis cell division, and why is it called reductional division?

Meiosis is a specialised type of cell division that occurs in sexually reproducing organisms to produce gametes (sperm and egg cells). It is called reductional division because it reduces the chromosome number in the parent cell by half. A diploid parent cell (2n), which has two sets of chromosomes, divides to produce four genetically unique haploid (n) daughter cells, each with a single set of chromosomes.

2. What are the main differences between Meiosis and Mitosis?

While both are forms of cell division, Meiosis and Mitosis have distinct purposes and outcomes:

Purpose: Meiosis produces genetically unique gametes for sexual reproduction. Mitosis produces identical daughter cells for growth, repair, and asexual reproduction.
Number of Divisions: Meiosis involves two successive cell divisions (Meiosis I and Meiosis II), whereas Mitosis involves only one.
Chromosome Number: Meiosis reduces the chromosome number from diploid (2n) to haploid (n). Mitosis maintains the diploid (2n) number in its daughter cells.
Genetic Variation: Meiosis introduces significant genetic variation through processes like crossing over and independent assortment. Mitosis results in genetically identical cells (clones).

3. What are the key events that occur during the stages of Meiosis I?

Meiosis I is the first and most complex phase, where homologous chromosomes are separated. The key stages are:

Prophase I: This is the longest phase, where chromosomes condense and homologous chromosomes pair up (a process called synapsis). Critically, crossing over occurs, where genetic material is exchanged between non-sister chromatids, creating new gene combinations.
Metaphase I: The paired homologous chromosomes (bivalents) align along the cell's equatorial plate.
Anaphase I: The homologous chromosomes are pulled to opposite poles of the cell. Importantly, the sister chromatids remain attached to each other.
Telophase I: The chromosomes arrive at the poles, the cytoplasm divides (cytokinesis), and two haploid daughter cells are formed.

4. What is the overall importance of meiosis in living organisms?

The significance of meiosis is fundamental for life on Earth. Its primary importance includes:

Formation of Gametes: It is the essential process for creating sex cells (sperm and eggs) necessary for sexual reproduction.
Maintenance of Chromosome Number: By halving the chromosome number in gametes, meiosis ensures that when fertilisation occurs, the normal diploid chromosome count of the species is restored and maintained across generations.
Inducing Genetic Variation: Meiosis is a major source of genetic diversity in a population. The shuffling of genes through crossing over and the random alignment of chromosomes during Metaphase I create unique genetic combinations in the gametes.

5. Why is 'crossing over' during Prophase I so crucial for evolution?

Crossing over is the physical exchange of genetic segments between homologous chromosomes. This process is crucial for evolution because it creates new combinations of alleles on the chromatids, a phenomenon known as genetic recombination. This shuffling of genetic information ensures that the gametes produced are genetically distinct from each other and from the parent cells. The resulting genetic variation is the raw material upon which natural selection acts, allowing populations to adapt to changing environmental conditions and driving the evolutionary process.

6. How does Meiosis II differ from Meiosis I?

Meiosis II is often compared to a mitotic division, but it acts on haploid cells. The key difference lies in what separates: in Meiosis I, homologous chromosomes are separated, reducing the chromosome set from diploid to haploid. In contrast, Meiosis II separates the sister chromatids from each other. Meiosis I is a reductional division, while Meiosis II is an equational division because the chromosome number of the cells entering and leaving the phase remains the same (haploid).

7. What are the consequences if an error, such as nondisjunction, occurs during meiosis?

An error during meiosis, such as the failure of chromosomes to separate properly (a process called nondisjunction), can have severe consequences. If nondisjunction occurs, gametes may be produced with an incorrect number of chromosomes (either one extra or one missing). If such a gamete participates in fertilisation, the resulting zygote will have an abnormal chromosome number, a condition known as aneuploidy. This is the cause of several genetic disorders in humans, such as Down Syndrome (Trisomy 21), Turner Syndrome (Monosomy X), and Klinefelter Syndrome (XXY).

8. Who is credited with the discovery of Meiosis?

The process of meiosis was first discovered and described by the German biologist Oscar Hertwig in 1876 while he was studying sea urchin eggs. His work laid the foundation for understanding how chromosome numbers are managed during sexual reproduction.