Meiosis

We explain what meiosis is and what each of its phases consists of. Also, what is mitosis and its differences from meiosis.

What is meiosis?

Meiosis is called one of the ways cells divide which is characterized by giving rise to daughter cells that are genetically different from the cell that originated them. This type of cell division is key for sexual reproduction, since through meiosis organisms produce their gametes or sexual cells. The new individual resulting from the union of two gametes (one male and one female) will have genetic material different from that of the parents, which arises from the combination of these.

Meiosis (from the Greek meioumdecrease) consists of the division of a diploid cell (2n), that is, provided with two sets of chromosomes to give rise to four haploid cells (n), provided with a single set of chromosomes, that is, half the genetic load of the initial cell.

In animals (including humans) most of the cells in the body are diploid and are called somatic cells. Only in the germinal tissue are special cells found that give rise, through meiosis, to haploid cells. These haploid cells are the gametes or reproductive cells that are involved in sexual reproduction, that is, they are the sperm (male gametes) and the eggs (female gametes).

When a sperm and an egg fuse together during fertilization, each of them contributes half of the genetic load of the new individual that is formed as a result of this union. Thus, both haploid sets of each gamete combine to form a complete diploid set, which is the genome of the newly formed new individual.

Meiosis is a essential process prior to sexual reproduction since during this process gametes are formed. However, meiosis is also part of complex life cycles in algae, fungi and other simple eukaryotes, to achieve a certain generational alternation, reproducing their cells sexually and asexually in different stages.

meiosis It was discovered in the 19th century by the German biologist Oscar Hertwig (1849-1922), based on his studies with sea urchin eggs. Since then, successive research has contributed to understanding this process in greater depth and to understand its vital importance in the evolution of higher forms of life.

See also: Eukaryotic cell

Phases of meiosis

Meiosis is a complex process that involves two distinct phases: meiosis I and meiosis II. Each of them is made up of various stages: prophase, metaphase, anaphase and telophase. This warrants a more detailed study:

• Meiosis I. The first cell division of the diploid (2n) occurs, known as reductive, as it results in cells with half the genetic load (n). Meiosis I is distinguished from meiosis II (and mitosis) because its prophase is very long and during its course the homologous chromosomes (identical because one comes from each parent) pair and recombine to exchange genetic material.
• Prophase I. It is divided into several steps. In the first step, the DNA is prepared by condensing into chromosomes and becoming visible. Then, the homologous chromosomes join together in pairs to form a complex in which they exchange genetic material. This process is known as gene recombination. Finally, the chromosomes separate, although at some points they remain united: these are the points where gene recombination has taken place. Furthermore, the envelope of the nucleus is broken and a kind of dividing line appears in the cell.
• Metaphase I. The bivalent chromosomes (composed of two chromatids each, which is why it is also called a tetrad) are arranged in the equatorial plane of the cell and join a structure made up of microtubules called the achromatic spindle.
• Anaphase I. The homologous chromosomes of each bivalent (each formed by two sister chromatids) separate from each other, tend to one pole of the cell and form two haploid poles (n). The random genetic allocation has already been carried out.
• Telophase I. The haploid chromosome groups reach the poles of the cell. The nuclear envelope forms again. The plasma membrane separates and gives rise to two haploid daughter cells.

• Meiosis II. Known as the duplicative phase, it is similar to mitosis: two entire individuals are formed by duplicating DNA.
• Prophase II. Haploid cells created in meiosis I condense their chromosomes and break down the nuclear envelope. The achromatic spindle appears again.
• Metaphase II. As before, the chromosomes tend toward the equatorial plane of the cell, preparing for a new division.
• Anaphase II. The sister chromatids of each chromosome separate and are pulled to opposite poles of the cell.
• Telophase II. Each pole of the cell receives a haploid set of chromatids that are called chromosomes. The nuclear envelope forms again, followed by the partition of the cytoplasm and the formation of cell membranes resulting in four haploid cells (n), each with a different distribution of the individual's complete genetic code.

Meiosis and mitosis

The differences between mitosis and meiosis are several:

• Mitosis is associated with asexual reproduction. Mitosis consists of the division of a parent cell to form two genetically identical daughter cells. Mitosis is used as a mechanism in different types of asexual reproduction, in which an organism produces cellular “clones”, without adding variety to the genetic pool. Meiosis, on the other hand, is a process required in preparation for sexual reproduction, and unlike mitosis, it allows for high genetic recombination.
• Mitosis is associated with development and growth processes. Multicellular organisms use the mechanism of mitosis to maintain and renew their structures. This type of cell division allows new cells to be added during the development and growth of the individual and to replace old and worn out cells throughout the life of the organism.
• Mitosis creates two daughter cells. Both diploid and identical. Meiosis, on the other hand, produces four descendant cells, but all of them are haploid and different from each other and from the cell that originated them.
• Mitosis preserves DNA. Mitosis is a mechanism for preserving genetic material intact (although random mutations can occur during the process), while meiosis subjects it to a recombination process in which errors can occur, but which also enriches the genome and allows creation of particularly successful chains. Meiosis is at some point largely responsible for genetic variation between individuals.

References

• Cellular and Molecular Biology. From Robertis Edward, Hib José. (2012) 16th ed. Promed Editorial.
• Molecular Biology of the Cell. Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, and Peter Walter. (2015). Sixth edition. Published by Garland Science