MEIOSIS: FUNCTION, PHASES AND THEIR CHARACTERISTICS - BIOLOGY - 2025

The meiosis is the type of cell division that characterizes eukaryotic organisms whose life cycle is a phase of sexual reproduction. Through this process, the number of chromosomes in dividing cells is halved, which is why it is also known as "reductive division."

According to the fundamentals of the cell theory "every cell comes from another cell" and it is known that one cell gives rise to another through a division process that consists of the duplication of its internal components (DNA, proteins, etc.)) and their separation into two "daughter" cells, which are virtually identical to each other.

Summary scheme of meiosis: 1) Duplication of chromosomes 2) Pairing of homologous chromosomes 3) Crossing over 4) First meiotic division (one of each of the duplicated chromosomes per daughter cell) 5) Second meiotic division (one chromosome from each one per daughter cell) (Source: Peter coxhead via Wikimedia Commons)

This process allows the continuity of life and the "unaltered" transmission of genetic material to subsequent generations. Meiosis occurs both in the cells of multicellular organisms and in unicellular organisms (protozoa, yeast and bacteria, among many others).

For some organisms this is the main form of reproduction and is known as asexual reproduction. However, the reproduction of multicellular living beings, which have different development cycles, is a bit more complex and implies that all cells of the same organism are formed from a very special cell called a zygote.

The zygote is the result of a process called sexual reproduction, which involves the fusion of two gametic or sexual cells, produced by two different individuals (generally a "male" and a "female") and which possess half of the genetic information each.

The production process of these sex cells is what is known in multicellular organisms as meiosis and has the main function of producing cells with half the chromosomal load, that is, haploid cells.

Function of meiosis

Meiosis is the central part or "heart" of sexual reproduction, which appears to be an evolutionarily advantageous "acquisition", as it has been adopted by most animal and plant species.

This process involves the combination of two different genomes, which ends with the formation of offspring with a "new" genetic endowment, which in turn implies an increase in variability.

Through this reductive cell division, specialized cells in the body of multicellular animals and plants, known as germ line cells, produce sex or gametic cells that, when fused, give rise to a cell called a zygote..

The reduction of the chromosomal number by meiosis is an essential step for the union of the two sex cells that are produced to "regenerate" the diploid chromosomal complement in the next generation, ensuring the continuity of the species.

Chromosome number reduction is possible, since during meiosis a single round of DNA replication is followed by two successive rounds of chromosome segregation.

Competitive advantage

The fact that two individuals reproduce sexually and the fusion of two genetically different gametes occurs, whose chromosomes have also been previously “mixed” by means of random processes ”, may mean an evolutionary advantage from the point of view of competition.

Meiosis, which gives rise to cells with a new genetic combination that fuse during sexual reproduction, allows the individuals that are the product of such reproduction to adapt to survive in environments that vary in an essential way.

Elimination of "harmful" alleles

Since a population is susceptible to the appearance of new alleles by mutations (many of which can be harmful or harmful), meiosis and sexual reproduction can favor the rapid elimination of these alleles, preventing their accumulation and further spread.

Phases of meiosis

The meiotic process can be explained as the "separation" or "distribution" of the chromosomes of a cell in whose division its chromosomal load is reduced, which occurs through two divisions known as the first meiotic division and the second meiotic division, this being last quite similar to the mitotic division.

As will be seen below, each of the two meioses is composed of a prophase, a metaphase, an anaphase, and a telophase.

The phases of meiosis (Source: Boumphreyfr via Wikimedia Commons)

- First meiotic division

Meiosis I or first meiotic division begins with the union of the members of each homologous pair of chromosomes (the maternal and paternal chromosomes that diploid organisms inherit from their parents).

Interface

As in mitosis, the phase of the germline cell cycle that precedes meiosis is the interface. During this stage, the only event of cellular DNA replication occurs, which generates a maternal and a paternal chromosome (they are diploid cells) that each consist of two sister chromatids.

Prophase I

During prophase I of meiosis I, the union or physical contact between homologous chromosomes (equivalent chromosomes from two different parents, the father and the mother) occurs along their entire length.

This event is known as synapse and it is the process by which four chromatids are associated, two from each homologous chromosome, which is why the resulting structure is called a tetrad or bivalent complex (the number of tetrads in a cell during prophase is equivalent to the haploid number of chromosomes).

In each tetrad, non-sister chromatids, that is, those belonging to homologous chromosomes, recombine through a process called crossover, which results in the genetic exchange between chromosomes by “cutting and pasting” of random fragments in random positions, generating new gene combinations.

After recombination occurs, the centromeres of the homologous chromosomes separate, being joined only by regions known as chiasmas, which correspond to the crossover sites. The sister chromatids, however, remain attached via the centromere.

During this phase of meiosis I, cells grow and synthesize reserve molecules. In addition, the formation of the microtubule spindle is appreciated and, in late prophase I, the nuclear envelope disappears and the chromatid tetrads are clearly seen under the light microscope.

This phase ends when the tetrads line up in the equatorial plane of the dividing cell.

Metaphase I

During metaphase, the fibers of the microtubule spindle attach to the centromeres of homologous chromosomes and to opposite poles of the cell; This is the opposite of what occurs during mitosis, in which the centromeres of the sister chromatids are attached to microtubules at opposite poles.

Anaphase I

In this phase, the duplicated homologous chromosomes separate, as they are "pulled" towards opposite poles of the cell thanks to the microtubules of the spindle. At each pole, then, a random combination of chromosomes is found, but only one member of each homologous pair.

During anaphase I the sister chromatids remain attached to each other through their centromeres, which differs from mitosis, since during mitotic anaphase the sister chromatids are separated at opposite poles of the cell.

Telophase I

At this point, the chromatids “decondense”, that is, they become less visible under the microscope, losing their characteristic shape. The nuclear envelope is reorganized and the cytokinesis or separation of daughter cells occurs, which have a haploid number of chromosomes, but which consists of duplicated chromosomes (with their two chromatids).

Between telophase I and the next meiotic division there is a short period of time known as interkinesis, although it does not occur in all organisms.

- Second meiotic division

During the second division, the sister chromatids are separated, as occurs during mitosis, but without the DNA being replicated previously.

Prophase II

Prophase II is very similar to mitotic prophase. At this stage there is no union of homologous chromosomes and no crossover.

In prophase II the chromatids become visible again, that is, the chromatin condenses. The spindle fibers radiate from each pole, elongating toward the centromeres joining the sister chromatids.

Finally, the nuclear envelope disappears and the microtubules from opposite poles reach the centromere of each chromatid and these are aligned in the equatorial plane of the cell.

Metaphase II

Metaphase II differs from metaphase I with respect to the number of chromatids that line up in the equatorial plane. In metaphase I, tetrads are seen, while in II only the sister chromatids of the same chromosome are observed, as in mitotic metaphase.

Anaphase II

In this stage the sister chromatids separate as they are displaced towards opposite poles of the cell. From this moment on, each chromatid is considered an independent chromosome.

Telophase II

At the beginning of telophase, then, the nuclear envelope regenerates on the set of unduplicated homologous chromosomes that was distributed in each pole of the cell, after which cytokinesis or separation of daughter cells occurs.

The meiotic division of a diploid cell produces four haploid cells, each of which has a different combination of genes, as recombination took place.

References

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