The human body is made up of 37 trillion cells. It is surprising that this immense quantity has its origin in a single cell that is conceived during fertilization. This is possible because of the ability of cells to self-reproduce, a process that involves dividing in two. Little by little, it is possible to reach the aforementioned quantity, forming the different organs and types of cells.
However, 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, whether it be a human being or an immense whale. In the case of humans, 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 take along with structural proteins. The human genome consists of 23 pairs of chromosomes (23×2). This is an important fact to know one of the main differences between mitosis and meiosis, the two classes of cell division that exist.
The Eukaryotic Cell Cycle
The cells follow a series of sequential patterns for their division. This sequence is called the cell cycle, and 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 eukaryotic cells 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 in the interface), G0-phase and M-phase (Mitosis or Meiosis).
The purpose of this group of stages is to prepare the cell for its imminent splitting into two , following the following phases:
- G1 phase (Gap1) : corresponds to the interval (gap) between a successful division and the start of replication of the genetic content. During this phase, the cell is constantly growing.
- Phase S (Synthesis) : is when DNA replication occurs, ending with an identical duplicate of the genetic content. In addition, the chromosomes with the best known silhouette are formed (in the form of X).
- 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). Whenever a problem arises, the process stops and an attempt is made to find a solution, as cell division is a vitally important process; everything has to go right.
2. Phase G0
Cellular proliferation is lost when cells are specialized so that the growth of the organism is not infinite. This is possible because the cells enter a resting stage called G0 phase, where they remain metabolically active but have neither cell growth nor replication of the genetic content, i.e. they do not remain in the cell cycle.
3. Phase M
It is in this phase that cell partitioning occurs and mitosis or meiosis develops.
Differences between mitosis and meiosis
It is in the division phase that 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 by 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 point, the chromosomes are duplicated and form the typical X-image of the chromosomes (each side is a copy), linked in half through a protein structure known as the centromere. The nuclear membrane that encloses the DNA is degraded so that the genetic content is accessible.
During the G2 phase, different structural proteins have been synthesized, some of them twice. These are the so-called centrosomes , which are each placed at a pole opposite to each other in the cell.
From the centrosomes, the microtubules, protein filaments that make up the mitotic spindle and which join the centromere of the chromosome, to stretch one of the copies towards one of the sides , breaking the structure in X.
Once on each side, the nuclear envelope is formed again to enclose the genetic content, while the cell membrane is strangled to generate two cells. The result of mitosis is two diploid sister cells , as their genetic content is identical.
This type of cell division only occurs in the formation of gametes , which in the case of humans are the sperm and egg cells that are responsible for shaping fertilization (they are the so-called germ cell line). Simply put, one can say that meiosis is like two consecutive mitoses.
During the first meiosis (meiosis 1) a process similar to that explained in mitosis occurs, except that homologous chromosomes (the partner) can exchange fragments with each other by recombination. This does not occur in mitosis, since in mitosis they never come into direct contact, unlike in meiosis. It is a mechanism that offers more variability to genetic inheritance. Furthermore, what separates are the homologous chromosomes, and 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 . The copies of each chromosome are now separated, so that the final result of meiosis is four haploid cells, as they only have one chromosome from each (non-partners), to allow new pairings to form between the parents’ chromosomes during fertilisation and to enrich the genetic variability.
In order to compile 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 it is four cells with 23 chromosomes each (no pairs), in addition to which their genetic content can vary by recombination between homologous chromosomes.