The 5 differences between haploid and diploid cells

The cell is the morphological and functional unit of the living being. Every living entity, from the most basal bacteria to the human being, has at least one cell capable of self-replicating itself and exchanging substances with the environment. Prokaryotic living beings have only one cell that constitutes their entire body, but eukaryotes can integrate billions of them in our body, each one in a system much larger than the unit and with marked functionality.

As we have said, the cellular entity is equivalent to life. The only organisms that converge with this premise are viruses, viroids and prions, but they are rarely considered as living beings. Rather, they constitute a separate group of biological pathogens with infectious potential. Without the cell, the minimum requirements are not reached so that life can develop as such.

In any case, it should be noted that, for example, within humans there are 2 major cell types: haploid and diploid. In the following lines, we tell you the differences between haploid and diploid cell and its evolutionary significance.

• Related article: "Differences between mitosis and meiosis"

What are the differences between haploidy and diploidy?

In nature, no adaptation has developed by chance. Every characteristic serves (or has served) a role in the evolutionary history of the species, so the The fact that there are haploid and diploid cells within the same organism must have a reason of to be. In the following points, we explore it.

1. Haploid cells contain only one set of chromosomes, diploid cells two

This is the main difference between haploidy and diploidy. A diploid cell (2n) contains within its nucleus a set of paired chromosomes, in which all the genetic information is found of the individual, half of the father and half of the mother. In the case of humans, there are 23 pairs of chromosomes, 22 autosomal and one sexual (XX and XY), which all encompass about 25,000 different genes. Of the 46 total chromosomes that exist within the cell nucleus, 23 come from one parent and 23 from the other.

On the other hand, a haploid cell (n) is one that contains only one chromosome of each type. In the case of human gametes (eggs and sperm), the cell nucleus only contains 23 chromosomes. The explanation is simple; if each gamete were diploid, in the union to form the zygote the resulting cells would have more and more chromosomes:

• Haploid cell (n) + Haploid cell (n) = Diploid cell (2n)
• Diploid cell (2n) + Diploid cell (2n) = Tetraploid cell (4n)
• Tetraploid cell (4n) + Tetraploid cell (4n) = Cell with 8 sets of chromosomes (8n)

Thus, if haploid cells did not exist during sexual reproduction, in just 3 generations a human being would go from having 46 chromosomes (23 x 2) to 184 (23 x 8). The duplication of a single chromosome when it does not touch can already be fatal, so this mechanism of genetic accumulation would be incompatible with life.

2. Diploid cells divide by mitosis, and haploid cells by meiosis

As we have already established, a somatic diploid cell (which makes up the tissues) has a pair of each chromosome, each member of one of the two parents.

As these cells are not involved in reproduction (they are only intended to maintain and repair body structures), they have no need to divide their genetic information to half. Therefore, they divide by mitosis, a process in which a stem cell gives rise to two exactly the same daughter cells, by duplicating their DNA and the partition of the cytoplasm.

As you might suspect, the case of haploid cells is completely different. In the human body, these cell units are the eggs and sperm, those responsible for fertilization. For diploidy to remain in the zygote, each pair of chromosomes must be "split" in half and only one of the two members must be left, as we have seen in the previous section.

So that, the process of formation of a haploid cell is much more complex than that of a diploid (at least within a diploid organism). To exemplify it, we show you the synthesis process of a sperm:

• Proliferative phase: a diploid germ stem cell forms type A and B spermatogonia. The A's are divided by mitosis to increase the stock in quantity, but the B's are not.
• A spermatogonia differentiates into the primary spermatocyte, and by meiosis I this gives rise to two secondary spermatocytes. In meiosis II, each secondary spermatocyte gives rise to two haploid spermatids.
• Thus, where before there was a diploid B spermatogonia, there are now 4 haploid spermatids, with half the genetic information.
• Spermatids mature into functional sperm.

Thus, 4 haploid gametes are produced where there used to be a diploid germ stem cell. In addition, throughout this process there are crossovers and chromosomal permutations, which cause parental information not to be present in the same way in the offspring. For this reason, sexual reproduction is said to be the basis of genetic diversity in species.

• You may be interested in: "Major Cell Types of the Human Body"

3. Haploidy and diploidy are restricted to different cell groups

All the cells that make up our body are diploid, except for gametes (eggs and sperm), which are synthesized in the ovum and testis, respectively. Thus, it is generalized that human somatic cells are diploid and sexual cells haploid.

Still, this is not entirely true: for example, most hepatocytes (liver cells) are tetraploid, meaning they contain twice as much genetic information as a normal somatic cell. There are always exceptions that prove the rule.

4. Diploidy allows sex differentiation in some species

In the colonies of eusocial insects such as bees, wasps and ants (Hymenoptera) the males are haploid (X) and the females diploid (XX). This evolutionary strategy follows a clear pattern: males can be born to a fertile female without the need for it. has been fertilized previously, which greatly facilitates the reproductive period between colonies of the same population.

As you can imagine, in humans this is not the case at all, since both males (XY) and females (XX) are diploid. Anyway, it is interesting to know that haploidy codes for males in some species of the animal kingdom.

5. Each cell type has a different function

In the human body, the function of diploid cells is to keep the body's biological system afloat. For example, the somatic cells of the dermal and epidermal layers are in continuous growth, as they 40,000 keratinocytes (cells of the stratum corneum, the most superficial) shed each minute of our lifetime. Division by mitosis promotes the restoration, maintenance, and replacement of all body tissues.

On the other hand, haploid cells have an already explored functionality: sexual reproduction. Although sexual reproduction is much more expensive than simple mitosis, it makes great evolutionary sense. All descendants of a lineage divided by mitosis are genetically the same, so they have the same aptitudes in the face of environmental changes and their range of adaptive capacity is minimal.

On the other hand, the species that follow a sexual reproduction pattern present very different specimens within the same population. at the genetic level, since a child is never the same as one of her parents, but a combination of both (more mutations and crossovers). Thus, the existence of haploid cells and the formation of gametes is what generates the diversity of the planet throughout the generations, in addition to adaptive capacities.

Resume

As you have seen, the differences between a haploid cell and a diploid cell go far beyond the chromosomal endowment. It is essential to know the variations between cellular entities at the microscopic level, but also to apply it in a medical and evolutionary field.

Both cell types are two essential pieces in the same gear: diploidy maintains life, while haploidy generates it. Both processes are vital for the maintenance of species that reproduce sexually.