A wonderful thing in life is how a single cell can give birth to an entire organism. I am talking about the birth of a new living being by sexual reproduction. This is possible by the union of two specialized cells, called gametes (eg, egg), in fertilization. What is amazing is that it allows the transmission of information from both parents, so that the new cell has different genetic material. To achieve this it is necessary a different system of proliferation in mitosis, remember that their result was identical cells. In 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.
Formation of haploid cells
In the case of humans, cells are diploid, which means that they each have two copies per different chromosome. It’s easy; humans have 23 different chromosomes, but being diploid, we actually have 46 (one more copy for each). During the meiosis phases, haploid cells are obtainedIn other words, they only have one chromosome per type (23 in total).
As in mitosis, the interphase is present to prepare the cell for its impending cell division, Increase its size, reproduce the genetic content and manufacture the necessary tools. This is the only similarity between the two processes, because from there everything changes.
Two consecutive divisions: meiosis phases
Meiosis has the same four phases as mitosis: prophase, metaphase, anaphase, and telophase; but they don’t happen the same way. In addition, meiosis performs two cell divisions in a row, which is why the result is four haploid cells. For this reason, we speak of meiosis I and meiosis II, depending on the partition in question; and there are actually 8 phases of meiosis, 4 for each division.
Before continuing, there are two key concepts you need to understand. The first is that of homologous chromosomes, And refers to the pair of chromosomes by the void. The second is sister chromatids, which are the result of the duplication of a chromosome during interphase.
During prophase I, homologous chromosomes are very close to each other, allowing the parts to “swap” with each other, as if they were changing chromosomes. this mechanism it serves to generate more genetic diversity in the offspring. During this time, the nucleus is degraded and the chromosomal transport pathway is generated: the mitotic spindle.
Metaphase I occurs when chromosomes are attached to the mitotic spindle. It then enters anaphase I, that is, when these are transported to opposite poles. But on this occasion, what separates are the homologous chromosomes and not the sister chromatids, which occurs during mitosis. Once separated, start a rapid telophase I, Where only cytokinesis occurs, that is, separation into two cells. Without further ado, these new cells enter a second cell division.
At this stage 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, as what was separated are homologous chromosomes.
Since this is a new cell division, the cycle is the same with a few differences, and this phase is more like what happens during mitosis. During prophase II the mitotic spindle is reformed so that in metaphase II it binds to chromosomes through its center and now yes during anaphase II the sister chromatids separate to opposite poles. During telophase II, the nucleus is formed to contain the genetic content and the two cells are separated.
The end result is four haploid cells, because each has only one copy per chromosome. In the case of humans, by this mechanism, sperm or ova are generated, Depending on the sex, and these cells contain 23 chromosomes, unlike the 46 chromosomes of the rest of the cells (23×2).
The goal that has been achieved throughout the phases of meiosis is to generate haploid cells, called gametes, which can give rise to a new organism. It is the foundation of sexual reproduction, the ability for two individuals of the same species to have offspring by matching their genetic content.
So it makes sense 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 sperm and ovum), a new cell is generated. Diploid cell genetic material is formed by mating chromosomes from each gamete.