Chromosomal permutation: what is it and how does it work?
Let's see what chromosomal permutation that occurs in reproduction consists of.
Inheritance is the basis of evolution. Changes in the genes of living beings occur by random mutations, but if they are inheritable from parents to offspring, it is possible that they will become fixed in a population of a given species. For example, if a genetic mutation in the DNA encodes a more striking coloration in the males of a given species, they may reproduce more easily, passing on their genes to future generations.
Some mutations are neutral, some are deleterious and a minority are positive. In the example we have shown you, a new positive characteristic ends up becoming "fixed" in the species, since those that have it have more offspring and, therefore, extend their genes exponentially with each generation. Broadly speaking, we have just told you about evolutionary mechanisms by natural selection.
However, 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. Learn with us the mechanism of chromosomal permutationThe chromosomal permutation mechanism, together with the mutations mentioned above, is one of the strongest bases of the evolutionary processes in the natural environment.
Chromosomes and sex
Before entering fully into the world of chromosomal 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 etcetera) divide by mitosis if they have the capacity, that is to say, they give rise to 2 exactly equal cells where before there was a parental one.
In this case, the genetic information is duplicated, but remains unchanged in the cell lineage.. These cells are diploid, i.e. they have 23 pairs of chromosomes (22 autosomal pairs, one sexual), of which one pair comes from the mother and the other from the father. Thus, each of our cells has a total of 46 chromosomes.
Sex cells (eggs and sperm) are a completely different world. They need to have half the genetic information of somatic cells, since they are going to join with another gamete to give rise to a viable zygote. If the eggs and sperm had the same chromosomes as the cells of our body, when they unite they would give rise to a fetus with 92 chromosomes (46x2), right?
To solve this problem there is meiosis. In this process, unlike mitosis, 4 haploid cells (with 23 chromosomes only) are generated from a diploid cell, which we remember contains a total of 46 chromosomes.. Thus, when two haploid gametes fuse, the diploid germ line that defines each and every cell in our body is created.
What is chromosomal permutation?
You may wonder why such a lengthy introduction, but it was essential, since chromosomal permutation, along with crossing-over or overcrossing, occurs within a cell during meiosis (more specifically, in prophase and metaphase), which makes sexual reproduction possible by the mechanism already described.
Thus, chromosomal permutation can be defined as the process by which chromosomes are randomly distributed among the haploid (n) daughter sex cells resulting from the division of a diploid (2n) cell.. This occurs based on the placement of homologous chromosomes, which are located at the equator of the cell prior to 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 of the information to one pole of the cell and the other half to 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 chromosomal permutations in humans can be obtained as follows:
We explain this formula in a quick and simple way. 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 chromosomes toward each pole of the cell is an important source of genetic variability.
The importance of chromosomal overcrossing
Chromosomal overcrossing 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.resulting in recombinant chromosomes. At this point, it is necessary to note 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 parent).
We do not want to describe meiosis in its entirety and, therefore, it will suffice for you to know that chromosome permutation occurs in metaphase I, but overcrossing takes place in prophase. At this time, the homologous chromosomes form a bridge called a "chiasm", which allows the exchange of genetic information between them.
Thus, this exchange gives rise to two chromosomes, this exchange gives rise to two recombinant chromosomes, whose information comes from both the father and the mother, but is organized differently from the parental chromosomes.. We have mentioned this meiotic mechanism because, together with chromosomal permutation, they are the basis of genetic variability in the inheritance mechanisms of living beings that reproduce sexually.
The Biological importance of chromosomal permutation.
Point mutations, chromosomal permutations and overcrossings between homologous chromosomes are essential for understanding life as we perceive it today. All their functionality and biological meaning can be summed up in a single word: variability.
If all specimens in a population are genetically the same, they will show a series of (almost) identical physical and behavioral traits, so they will be equally prepared and/or adapted to changes in the environment.. Evolutionary forces are not "interested" in this scenario, because if a drastic variation occurs and the whole species responds in the same way, it is likely to become extinct over time for lack of biological tools.
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, a product of genetic selection by humans. Breeding between relatives results in homozygosis, i.e., loss of genetic variability. This phenomenon is known as "inbreeding depression" and the less alleles available in a population, the more theoretically likely it is that the population will become extinct.
Finally, it is necessary to emphasize that we are not talking on conjectural grounds. With these two facts you will understand what we mean: 6 out of 10 golden retrievers die of cancer, and up to 50% of Persian cats have polycystic kidney disease. It is clear that the lack of genetic variability translates into disease in the short term, and in the long term into the non-viability of an entire species..
In this space we have taken the opportunity to focus on chromosomal permutation from an evolutionary rather than a physiological point of view, as we believe that it is much easier to understand such abstract phenomena with concrete examples and the consequences they cause. If we want you to keep one idea in mind, it is the following: DNA mutations, chromosomal permutations and overcrossing are the basis of inheritance 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 puzzled: if 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? As you will see, there are questions that still elude us.
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(Updated at Mar 28 / 2023)