These two types of cell partition cause a zygote to develop into an organism.
The human body is made up of 37 trillion cells. It is surprising that this immense quantity originates from a single cell that is conceived during fertilization. This is possible due to the ability of cells to self-reproduce, a process that involves dividing in two. Little by little, it is possible to reach the aforementioned amount, forming the different organs and types of cells.
Now, there are two basic mechanisms by which cells can reproduce: mitosis and meiosis. Next we will see the differences between mitosis and meiosis and their characteristics.
Mitosis and meiosis
We have seen that little by little, a few cells can give rise to an entire organism, be it a human being or a huge whale. In the case of humans, they are diploid eukaryotic cells, that is, they have one pair per chromosome.
The structure of the chromosome is the most compact and condensed form that DNA can present together with structural proteins. The human genome is made up of 23 pairs of chromosomes (23×2). This is an important piece of information to know one of the main differences between mitosis and meiosis, the two types of cell division that exist.
The eukaryotic cell cycle
Cells follow a series of sequential patterns for their division. This sequence is called the cell cycle, and it consists of the development of four coordinated processes: cell growth, DNA replication, distribution of duplicated chromosomes, and cell division. This cycle differs in some points between prokaryotic (bacteria) or eukaryotic cells, and even within eukaryotes there are differences, for example between plant and animal cells.
The cell cycle in eukaryotes is divided into four stages: G1 phase, S phase, G2 phase (all of them are grouped at the interface), G0 phase and M phase (Mitosis or Meiosis).
This group of stages is intended to prepare the cell for its imminent partition in two, following the following phases:
- G1 phase (Gap1) : corresponds to the interval (gap) between a successful division and the beginning of the replication of the genetic content. During this phase, the cell is constantly growing.
- Phase S (Synthesis) : this is when DNA replication occurs, ending with an identical duplicate of the genetic content. In addition, the chromosomes with the best-known silhouette (X-shaped) are formed.
- G2 phase (Gap2) : cell growth continues, in addition to the synthesis of structural proteins that will be used during cell division.
Throughout the interface, there are several checkpoints to verify that the process is being performed correctly and that there are no errors (for example, that there is no bad duplication). Faced with any problem, the process stops and an attempt is made to find a solution, since cell division is a vitally important process; everything has to go well.
2. G0 phase
Cell proliferation is lost when cells are specialized so that the growth of the organism is not infinite. This is possible because cells enter a resting stage called the G0 phase, where they remain metabolically active but show neither cell growth nor replication of the genetic content, that is, they do not continue in the cell cycle.
3. Phase M
In this phase, it is properly when the cell partition occurs and mitosis or meiosis develops well.
Differences between mitosis and meiosis
The division phase is when either mitosis or meiosis occurs.
It is the typical cell division of a cell giving rise to two copies. As with the cycle, mitosis has also traditionally been divided into different stages: prophase, metaphase, anaphase, and telophase. Although for a simpler understanding, I will describe the process in a general way and not for each phase.
At the onset of mitosis, the genetic content is condensed into the 23 pairs of chromosomes that make up the human genome. At this time, the chromosomes are duplicated and form the typical X-image of chromosomes (each side is a copy), joined in half through a protein structure known as a centromere. The nuclear membrane that encloses DNA is degraded so that the genetic content is accessible.
During the G2 phase, different structural proteins have been synthesized, some of them doubly. These are called centrosomes, which are each placed at an opposite pole of the cell.
Microtubules, protein filaments that make up the mitotic spindle and that join the centromere of the chromosome, extend from the centrosomes to stretch one of the copies to one side, breaking the X structure.
Once on each side, the nuclear envelope is re-formed to enclose the genetic content, while the cell membrane is strangled to generate two cells. The result of mitosis is two sister diploid cells, since their genetic content is identical.
This type of cell division only occurs in the formation of gametes, which in the case of humans are sperm and ovules, cells that are responsible for shaping fertilization (they are the so-called germ cell line). In a simple way, it can be said that meiosis is as if two consecutive mitoses were carried out.
During the first meiosis (meiosis 1) a process similar to that explained in mitosis occurs, except that homologous chromosomes (the pair) can exchange fragments between them by recombination. This does not happen in mitosis, since in this they never come into direct contact, unlike what happens in meiosis. It is a mechanism that offers more variability to genetic inheritance. Furthermore, it is the homologous chromosomes that are separated, not the copies.
Another difference between mitosis and meiosis occurs with the second part (meiosis 2). After two diploid cells have formed, they immediately divide again. Now the copies of each chromosome are separated, so the final result of meiosis is four haploid cells, since they only present one chromosome of each (not pairs), to allow new pairings to form between the chromosomes during fertilization of parents and enrich genetic variability.
By way of compiling the differences between mitosis and meiosis in humans, we will say that the final result of mitosis is two identical cells with 46 chromosomes (pairs of 23), while in the case of meiosis there are four cells with 23 chromosomes each one (without pairs), in addition to the fact that its genetic content can vary by recombination between homologous chromosomes.