This process is the one that explains how new organisms originate through the union of gametes.
Something wonderful about life is how a single cell can give rise to an entire organism. I am talking about the birth of a new living being through sexual reproduction. This is possible due to the union of two specialized cells, called gametes (eg ovum), in fertilization. The surprising thing is that it allows the transmission of information from the two parents, so the new cell has different genetic material. To achieve this, a different system of proliferation to mitosis is necessary, to remember that its result was identical cells. For this case, the method used is meiosis.
In this article we will see what are the phases of meiosis and what this process consists of.
Forming haploid cells
In the case of humans, cells are diploid, which means that they each have two copies per different chromosome. It is easy; humans have 23 different chromosomes, but being diploid, we actually have 46 (one more copy for each). During the phases of meiosis, what is achieved are haploid cells, that is, they only have one chromosome per type (23 in total).
As in mitosis, the interface is present to prepare the cell for its imminent cell division, increasing its size, replicating the genetic content and manufacturing the necessary tools. This is the only similarity of the two processes, since from here everything changes.
Two consecutive divisions: phases of meiosis
Meiosis has the same four phases as mitosis: prophase, metaphase, anaphase, and telophase; but they don’t happen in the same way. In addition, meiosis performs two cell divisions in a row, which explains that its result is four haploid cells. For this reason we speak of meiosis I and meiosis II, depending on which partition one speaks of; and in reality there are 8 phases of meiosis, 4 for each division.
Before continuing, there are two key concepts to understand. The first is that of homologous chromosomes, and refers to the pair of chromosomes per hole. The second is sister chromatids, which is the result of the duplication of a chromosome during interphase.
During prophase I, the homologous chromosomes are very close together, allowing parts to be “swapped” with each other, as if they were swapping chromos. This mechanism serves to generate more genetic diversity in the offspring. Meanwhile, the nucleus is degraded and the chromosome transport pathway is generated: the mitotic spindle.
Metaphase I occurs when the chromosomes are attached to the mitotic spindle. Then it enters anaphase I, which is when they are transported to opposite poles. But this time, what is separated are the homologous chromosomes and not the sister chromatids, which occurs in mitosis. Once separated, a rapid telophase I begins, where only cytokinesis occurs, that is, the separation into two cells. With no more time, these new cells enter a second cell division.
At this time of the meiosis phases we have two diploid cells, but the chromosome pairs are the replicas (except for the parts exchanged during prophase I) and not the original pair, since what has been separated are the homologous chromosomes .
As it is a new cell division, the cycle is the same with some difference, and this phase is more like what happens in a mitosis. During prophase II , the mitotic spindle re-forms so that in metaphase II it joins the chromosomes at its center and, now, during anaphase II, the sister chromatids are separated towards opposite poles. During telophase II, the nucleus is formed to contain the genetic content and the two cells separate.
The end result is four haploid cells, each having only one copy per chromosome. In the case of humans, this mechanism generates sperm or ovum, depending on the gender, and these cells contain 23 chromosomes, unlike the 46 chromosomes of the rest of the cells (23×2).
The objective that has been achieved throughout the phases of meiosis is to generate haploid cells, called gametes, which can originate a new organism. This is the foundation of sexual reproduction, the ability for two individuals of the same species to have offspring by matching their genetic content.
For this reason, it is logical that these cells are haploid, so that at the time of fertilization, which is the union of the two types of gametes (in the human case of the sperm and the ovum), a new diploid cell is generated whose genetic material It is formed by the pairing of chromosomes from each gamete.