The 5 differences between haploid and diploid cells
The evolutionary significance of diploid and haploid cells goes beyond their genetic partitioning.
The cell is the morphological and functional unit of the living being. Every living entity, from the most basal bacterium to the human being, has at least one cell capable of self-replication and of exchanging substances with the environment. Prokaryotic living beings have only one cell that constitutes their whole body, but eukaryotes can integrate in our organism billions of them, each one in a system much larger than the unit and with a marked functionality.
As we have said, the cellular entity is equivalent to life. The only organisms that meet this premise are viruses, viroids and prions, but they are rarely considered as living beings. Rather, they constitute a separate group of pathogenic Biological agents with infectious potential. Without the cell, the minimum requirements for life to develop as such are not met.
However, it should be noted that, for example, within the human being there are 2 major cell types: haploid and diploid. In the following lines, we will tell you the differences between the haploid and diploid cell and their evolutionary significance.
What are the differences between haploidy and diploidy?
In nature, no adaptation has evolved by chance. Every characteristic serves (or has served) a role in the evolutionary history of the species, so the fact that haploid and diploid cells exist within the same organism must have a reason. In the following points, we explore it.
1. Haploid cells contain only one set of chromosomes, diploid cells contain two.
This is the main difference between haploidy and diploidy. A diploid cell (2n) contains within its nucleus a set of paired chromosomes, which contain all the genetic information of the individual, half from the father and half from the mother. information of the individual, half from the father and half from the mother. In the case of humans, there are 23 pairs of chromosomes, 22 autosomal and one sex chromosome (XX and XY), which together contain 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 contains only 23 chromosomes. The explanation is simple; if each gamete were diploid, at 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 only 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 at the wrong time can be lethal, 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 stated, a somatic diploid cell (which forms the tissues) has a pair of each chromosome, each member being from 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 split their genetic information in half.. Therefore, they divide by mitosis, a process in which a mother cell gives rise to two exactly identical daughter cells by duplicating their DNA and partitioning 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, the ones responsible for fertilization to take place. For diploidy to remain in the zygote, they must "split" each pair of chromosomes in half and keep only one of the two members, as we have seen in the previous section.
Thus, the process of formation of a haploid cell is much more complex than that of a diploid cell. (at least within a diploid organism). To exemplify this, we show you the process of synthesis of a spermatozoon:
- Proliferative phase: a diploid germ stem cell forms type A and B spermatogonia. The A spermatogonia divide by mitosis to increase the number of sperm, but the B spermatogonia do 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 there was once a diploid spermatogonia B, there are now 4 haploid spermatids, with half the genetic information.
- The spermatids mature into functional spermatozoa.
Thus, 4 haploid gametes are produced, 4 haploid gametes are produced where before there was one diploid germ cell.. In addition, throughout this process, overcrossing and chromosomal permutations occur, which cause the parental information not to be presented in the same form in the offspring. For this reason, it is said that sexual reproduction is the basis of genetic diversity in species.
3. Haploidy and diploidy are restricted to different cell groups.
All the cells that make up our body are diploid, except for the gametes (eggs and sperm), which are synthesized in the egg and testis, respectively. Thus, it is generalized that human somatic cells are diploid and sexual cells are haploid.
Yet, this is not entirely true: for example, most hepatocytes (liver cells) are tetraploid, i.e. they contain twice as much genetic information as a normal somatic cell.. There are always exceptions that confirm the rule.
4. Diploidy allows sex differentiation in some species.
In the colonies of eusocial insects such as bees, wasps and ants (Hymenoptera), males are haploid (X) and females are diploid (XX). This evolutionary strategy follows a clear pattern: males can be born from a fertile female without the need for her to have been previously fertilized, 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. However, 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 functionality of diploid cells is to keep the body's biological system afloat. For example, the somatic cells of the dermal and epidermal layers are continuously growing, as about 40,000 keratinocytes (cells of the stratum corneum, the most superficial layer) are shed every minute of our lives. Mitosis division promotes the restoration, maintenance and replacement of all body tissues.
On the other hand, haploid cells have a functionality that has already been explored: sexual reproduction.. Although sexual reproduction is much more costly than simple mitosis, it makes great evolutionary sense. All the descendants of a lineage divided by mitosis are genetically the same, so they have the same aptitudes to environmental changes and their range of adaptive capacity is minimal.
On the other hand, species that follow a sexual reproduction pattern present very different specimens within the same population at the genetic level, since an offspring is never equal to one of its parents, but a combination of both (plus mutations and overcrossing). Therefore, the existence of haploid cells the existence of haploid cells and the formation of gametes is what generates the diversity of the planet over generations, in addition to adaptive capacities.as well as adaptive capacities.
Summary
As you have seen, the differences between haploid cell and diploid cell go far beyond chromosomal endowment. It is essential to know the variations between cellular entities at a microscopic level, but also to apply it in a medical and evolutionary field.
Both cell types are two essential cogs in the same wheel: diploidy maintains life, while haploidy generates it. Both processes are vital for the maintenance of sexually reproducing species.
(Updated at Apr 12 / 2024)