WHAT ARE HAPLOID CELLS? - BIOLOGY - 2025

A haploid cell is a cell that has a genome made up of a single basic set of chromosomes. Haploid cells therefore have a genomic content that we call the base charge 'n'. This basic set of chromosomes is typical of each species.

The haploid condition is not related to the number of chromosomes, but to the number of the set of chromosomes that represents the genome of the species. That is, its basic load or number.

In other words, if the number of chromosomes that make up the genome of a species is twelve, this is its basic number. If the cells of that hypothetical organism have twelve chromosomes (that is, with a base number of one), that cell is haploid.

If it has two complete sets (that is, 2 X 12), it is diploid. If you have three, it is a triploid cell that should contain about 36 chromosomes in total derived from 3 complete sets of these.

In most, if not all, prokaryotic cells, the genome is represented by a single DNA molecule. Although replication with delayed division can lead to partial diploidy, prokaryotes are unicellular and haploid.

Generally, they are also unimolecular genome. That is, with a genome represented by a single DNA molecule. Some eukaryotic organisms are also single-molecule genomes, although they can also be diploid.

Most, however, have a genome divided into different DNA molecules (chromosomes). The complete set of your chromosomes contains the entirety of your particular genome.

Haploidy in eukaryotes

In eukaryotic organisms we can find more diverse and complex situations in terms of their ploidy. Depending on the life cycle of the organism, we come across cases, for example, where multicellular eukaryotes can be diploid at one point in their lives, and haploid at another.

Within the same species, it may also be that some individuals are diploid while others are haploid. Finally, the most common case is that the same organism produces both diploid cells and haploid cells.

Haploid cells arise by mitosis or by meiosis, but can only undergo mitosis. That is, one haploid 'n' cell can divide to give rise to two haploid 'n' cells (mitosis).

On the other hand, diploid '2n' cells can also give rise to four haploid 'n' cells (meiosis). But it will never be possible for a haploid cell to divide by meiosis since, by biological definition, a meiosis implies division with reduction of the basic number of chromosomes.

Obviously, a cell with a base number of one (i.e., haploid) cannot undergo reductive divisions, as there is no such thing as cells with partial genome fractions.

The case of many plants

Most plants have a life cycle characterized by what is called alternation of generations. These generations that alternate in the life of a plant are the sporophyte generation ('2n') and the gametophyte generation ('n').

When fusion of 'n' gametes occurs to give rise to a diploid '2n' zygote, the first sporophyte cell is produced. This will be divided successively by mitosis until the plant reaches the reproductive stage.

Here, the meiotic division of a particular group of '2n' cells will give rise to a set of haploid 'n' cells that will form the so-called gametophyte, male or female.

The haploid cells of gametophytes are not gametes. On the contrary, later, they will divide to give rise to the respective male or female gametes, but by mitosis.

The case of many animals

In animals the rule is that meiosis is gametic. That is, that gametes are produced by meiosis. The organism, generally diploid, will generate a set of specialized cells that instead of dividing by mitosis will do so by meiosis, and in a terminal manner.

That is, the resulting gametes constitute the ultimate destination of that cell lineage. There are exceptions, of course.

In many insects, for example, males of the species are haploid because they are the product of development by mitotic growth of unfertilized eggs. Upon reaching adulthood, they will also produce gametes, but by mitosis.

Is it advantageous to be haploid?

Haploid cells that function as gametes are the material foundation for the generation of variability by segregation and recombination.

But if it were not because the fusion of two haploid cells makes possible the existence of those that do not (diploids), we would believe that gametes are only an instrument and not an end in themselves.

However, there are many organisms that are haploid and are not unaware of evolutionary or ecological success.

Bacteria and archaea

Bacteria and archaea, for example, have been here for a long time, long before diploid organisms, including multicellular ones.

They certainly rely much more on mutation than other processes to generate variability. But that variability is basically metabolic.

Mutations

In a haploid cell the result of the impact of any mutation will be observed in a single generation. Therefore, any mutation for or against can be selected very quickly.

This contributes greatly to the efficient adaptability of these organisms. Thus, what is not beneficial for the organism, may turn out to be beneficial for the researcher, since it is much easier to do genetics with haploid organisms.

In fact, in haploids, the phenotype can be directly related to the genotype, it is easier to generate pure lines and it is easier to identify the effect of spontaneous and induced mutations.

Eukaryotes and diploids

On the other hand, in organisms that are eukaryotic and diploid, haploidy constitutes a perfect weapon to test for less useful mutations. By generating a gametophyte that is haploid, these cells will express only the equivalent of a single genomic content.

That is, the cells will be hemizygous for all genes. If cell death derives from this condition, that lineage will not contribute gametes due to mitosis, thus acting as a filter for undesirable mutations.

Similar reasoning can be applied to males that they are haploid in some species of animals. They are also hemizygous for all the genes they carry.

If they do not survive and do not reach reproductive age, they will not have the possibility of passing that genetic information to future generations. In other words, it becomes easier to eliminate less functional genomes.

References

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5. Griffiths, AJF, Wessler, R., Carroll, SB, Doebley, J. (2015). An Introduction to Genetic Analysis (11 ^th ed.). New York: WH Freeman, New York, NY, USA.
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