Chromosome swapping: what it is and how it works

Heredity is the basis of evolution. Changes in the genes of living beings happen by random mutations, but if these are inherited from parents to children, it is possible that they end up being fixed in a population of a given species. For example, if a genetic mutation in the DNA encodes a more conspicuous coloration in the males of a specific species, they may reproduce more easily, transmitting their genes to future generations.

Some mutations are neutral, others deleterious, and a minority are positive. In the example that we have shown you, a new positive characteristic ends up "fixing" itself in the species, since that those who present it have more children and, therefore, spread their genes exponentially with each generation. In broad strokes, we have just told you about the evolutionary mechanisms by natural selection.

In any case, not everything is so simple in the world of genetics. When the sexual gametes that will give rise to a zygote are produced, half of the information comes from the mother and the other from the father, but we are not always talking about exact genetic copies. Meet with us

the mechanism of chromosome swapping, because, together with the aforementioned mutations, it represents one of the most iron bases of evolutionary processes in the natural environment.

• Related article: "Chromosomes: what they are, characteristics and functioning"

chromosomes and sex

Before diving fully into the world of chromosome permutations, it is essential that you understand certain genetic bases that are taken for granted in chromosome theory. All our somatic cells, those that give rise to our tissues (neurons, adipocytes, epithelial cells, monocytes and a very long etc.) divide by mitosis if they have the capacity, that is, they give rise to 2 exactly the same cells where before there was one parental.

In this case, genetic information is duplicated, but remains unchanged in the cell line. These cells are diploid, or what is the same, they have 23 pairs of chromosomes (22 autosomal pairs, one sexual), of which one pair comes from the mother and another from the father. Thus, each of our cells has a total of 46 chromosomes.

Sex cells (eggs and sperm) are a whole different world. These need to have half the genetic information of somatic cells, since they are going to unite with another gamete to give rise to a viable zygote. If the ovules and sperm cells had the same chromosomes as the cells of our body, when they joined together they would give rise to a fetus with 92 chromosomes (46x2), right?

To solve this problem is meiosis. In this process, unlike mitosis, 4 haploid cells (with 23 chromosomes alone) are generated from a diploid, which we remember contains a total of 46. Thus, when two haploid gametes fuse, the diploid germ line is created, which defines each and every cell in our body.

What is chromosome swapping?

You may wonder what led to such a lengthy introduction, but it was essential, since chromosome permutation, along with crossing over or crossing over, is produced within a cell during meiosis (more specifically, in prophase and metaphase), which enables sexual reproduction through the mechanism already described.

So that, Chromosomal permutation can be defined as the process by which chromosomes are randomly distributed. chromosomes between haploid daughter sex cells (n) produced by the division of a diploid cell (2n). This is produced based on the placement of homologous chromosomes, which are located at the equator of the cell before division, during metaphase I of meiosis.

Once these genetic structures have been located in the center of the cell, the mitotic spindle "pulls" them and distributes half the information at one pole of the cell and the other at the other. Thus, when cytoplasmic division occurs and two cells are formed where before there was one, both will have the same amount of genetic material, but of a different nature.

From a mathematical point of view, the possible chromosome permutations in humans can be obtained as follows:

223= 8.388.608

We explain this formula quickly and easily. As the number of chromosomes in the human genome is 23 pairs (22 autosomal + 1 sexual), the number of possible chromosome permutations during meiosis will be 2 raised to 23, with the impressive result of more than 8 million different scenarios. This random orientation of the chromosomes toward each of the poles of the cell is an important source of genetic variability.

The importance of chromosome crossing over

Chromosomal crossing over is defined as the exchange of genetic material during the process of sexual reproduction between two homologous chromosomes within the same cell, giving rise to recombinant chromosomes. At this point, it is necessary to emphasize that the term "homologous" refers to chromosomes that form a pair during meiosis, since they have the same structure, same genes but different information (each one comes from a progenitor).

We do not want to fully describe meiosis, so it will suffice for you to know that chromosome swapping occurs in metaphase I, but crossing over occurs in prophase. At this time, the homologous chromosomes form a bridge called a "chiasma", which allows the exchange of genetic information between them.

So, this exchange gives rise to two recombinant chromosomes, whose information comes from both the father and the mother, but is organized differently from the parents. We have cited this meiotic mechanism because, together with chromosome permutation, they represent the bases of the genetic variability in the inheritance mechanisms of living beings that reproduce sexually.

• You may be interested in: "The 6 parts of the chromosome: characteristics and functions"

The Biological Significance of Chromosomal Permutation

Point mutations, chromosomal permutations, and crossovers between homologous chromosomes are essential to understanding life as we perceive it today. All its functionality and biological meaning can be summarized in a single word: variability.

If all the specimens in a population are genetically the same, they will show a series of physical traits and (almost) identical behavioral behaviors, so they will be just as prepared and/or adapted to changes in the atmosphere. Evolutionary forces are not "interested" in this scenario, because if a drastic variation arrives and all the species responds in the same way, it is likely that it will become extinct over time due to lack of tools biological.

A clear example of this can be seen in some breeds of dogs and other domestic animals, which have been severely punished by the effects of inbreeding, product of genetic selection by the humans. Reproduction between relatives gives rise to homozygosity, that is, the loss of genetic variability. This phenomenon is known as “inbreeding depression” and the less availability of alleles there is in a population, the more theoretically likely it is that it is headed for extinction.

Finally, it is necessary to emphasize that we are not speaking in conjectural grounds. With these two data you will understand what we mean: 6 out of 10 dogs of the golden retriever breed die of cancer, and up to 50% of Persian cats have kidney disease polycystic It is clear that the lack of genetic variability translates into diseases in the short term, and in the long term inviability of an entire species..

Summary

In this space we have taken the opportunity to focus on chromosome permutation from an evolutionary point of view rather than physiological, because we believe that it is much easier to understand such abstract phenomena with concrete examples and the consequences causing. If we want you to stay with an idea, this is the following: DNA mutations, chromosomal permutations, and crossing over are the basis of heredity in sexually reproducing species. Without these mechanisms, we would be doomed to evolutionary failure.

Now, we close with a question that will leave more than one reader baffled: whether the mechanisms of genetic variability take place during sexual reproduction, how is it possible that there are species that have survived with asexual propagation systems throughout history? history? As you can see, there are issues that still elude us.

Bibliographic references:

• Chen, Y. M., Chen, M. C., Chang, P. C., & Chen, S. h. (2012). Extended artificial chromosomes genetic algorithm for permutation flow shop scheduling problems. Computers & Industrial Engineering, 62(2), 536-545.
• Kleckner, N. (1996). Meiosis: how could it work?. Proceedings of the National Academy of Sciences, 93(16), 8167-8174.
• Mitchell, L. A., & Boeke, J. d. (2014). Circular permutation of a synthetic eukaryotic chromosome with the telomerator. Proceedings of the National Academy of Sciences, 111(48), 17003-17010.
• Schwarzcher, T. (2003). Meiosis, recombination and chromosomes: a review of gene isolation and fluorescent in situ hybridization data in plants. Journal of Experimental Botany, 54(380), 11-23.
• Sybenga, J. (1999). What makes homologous chromosomes find each other in meiosis? A review and an hypothesis. Chromosome, 108(4), 209-219.