Centromere: what it is, functions and types of chromosomes to which it gives rise.

Let's see what chromosome centromeres are, their characteristics and their Biological functions.

Nucleic acids are the polymers of life. DNA is the library in which all the instructions for protein synthesis at the cellular level and thus the basis of cellular metabolism are stored. RNA is responsible for transcribing this information stored in the nucleus and transporting it to the ribosomes, where the protein materials are assembled. Because of their functionalities, these biomolecules are essential for life.

However, the work of DNA does not end with the death of the cell. Half of the genetic information that makes us us is transferred to our offspring, while the other half comes from the remaining parent. Patterns of inheritance, genetic recombination and mutations give rise to life and, therefore, to evolution itself.

Based on all these premises, today we turn our attention to the structures that condense most of the genetic information of human beings (and all other living beings): chromosomes. More specifically, we will delve extensively into the centromeres, which, as their name that, as its own name indicates, correspond to the central part of this formation.

The characteristics of chromosomes

Before we begin to talk about centromeres, we find it necessary to lay down a few simple basics about chromosomes. We begin by saying, for example, that all our body cells are typically diploid, i.e., with nuclei that possess two sets of chromosomes. If each set of human chromosomes consists of 23 units, a typical somatic (body) cell will have a total of 46. Half of them come from the mother and the other half from the father.

Diploidy is an extremely beneficial condition at the evolutionary level because, if the father's chromosome is missing in a particular region, the mother's analogous area can try to mask the deficiency. Clearly this is not so simple, as there are phenomena such as dominance and recessivity, but diploidy is associated with genetic variability, with few exceptions.

On the other hand, the cells that give rise to gametes must reproduce by meiosis, i.e. instead of giving rise to 2 identical cells from a parental cell (mitosis), 4 cells with half the genetic information are created.. Thus, when egg (n) and sperm (n) unite to give rise to a zygote, it will recover the diploidy that we require to live.

These are some of the explanations necessary to understand how chromosomes work, but we have already explored many more facts that we have left out on other occasions.

Chromosome structure

If you have ever seen a chromosome photographed (always under a microscope, since, for example, human chromosome 1 has a length of 0.001 centimeters), you will quickly associate its shape with the letter X. If we understand this structure as the previously named symbol, we can quickly imagine that each chromosome is composed of 4 arms and a center (the centromere)..

In addition, it is worth noting that, if a longitudinal cut is made in the chromosome, 2 sister chromatids are obtained that form the entire structure. These are composed of the chromoneme and chromomeres, structured on the basis of DNA and proteins.

What is the centromere?

In the lines just described, we have just introduced for the first time the term that concerns us here: centromere. This is the region of the chromosome that separates a short arm (p) from a long arm (q), i.e. each of the 2 segments that form the sister chromatid..

In addition, centromeres are responsible for the attachment of chromosomes to the mitotic spindle fibers, essential during mitosis so that the genetic information of the cell can be separated at both poles of the parental cell, for the subsequent division and creation of 2 identical daughter cells. Actually, the work per se is done by the kinetochore, but it is located above the centromere, so both structures can be encompassed in a common function.

At the molecular level, it can be noted that the centromere of human chromosomes has a total of one million "base pairs", i.e., 1,000,000 nucleotides with their corresponding nitrogenous bases (A,G,C,T). Most of these base pairs are in the form of α heterochromatin, which is an inactive, condensed form of the genetic material.

In addition, it is interesting to know that centromeric DNA is highly repetitive, as it is composed of a series of short repeated non-coding sequences, i.e., it is not transcribed.that is, they are not transcribed and translated into proteins as other parts of the genome are. This type of DNA with repeated information is known as satellite DNA, and is the main nucleic compound found at the centromeres.

The types of chromosomes according to the centromere

Beyond their properties, the physical location of the centromere provides an excellent classification criterion for naming chromosomes.. In the following lines, we will tell you briefly about these types.

1. Metacentric chromosome

They have the centromere practically in the exact middle of their structure, i.e. in the center of the chromosome.that is to say, in the center of the X. The arms formed are of very similar length, so we can hardly speak of short (p) and long (q).

2. Submetacentric chromosome

In this case the centromere is somewhat more displaced from the theoretical center of the figure.. It is not located completely at either end, but clearly there are large and small arms here.

3. Acrocentric chromosome

The centromere is far from the theoretical center. The arms are therefore enormously unequal, the arms are enormously unequal to each other (2 are very short and 2 are very long).

4. Telocentric chromosome

The centromere is so close to one end that, practically, only 2 long arms and no short arms are visible.. In other words, it is almost as if a chromosome were cut in half horizontally.

The enigma of the centromere

Determining the nature of the centromere in animals is an extraordinarily difficult task, since the highly repeated DNA that composes it is practically impossible to analyze in a laboratory. On the other hand, coding and life-essential genes are easy to "map", so sequences of the centromere are obtained.The human genome project is based on the human genome project, which is based on the human genome, which is based on the human genome project, which is based on the human genome project. The human genome project is based on that: sequencing our genes and knowing which base junction (nucleotide) gives rise to which protein.

Unfortunately, centromeres are a dead end. Current methodology does not allow sequencing the DNA of these structures, neither in humans nor in any other animal, so we are facing a real black hole of a million base pairs that is unreachable today. In any case, some researchers downplay its importance: centromeres are highly variable between species, so they do not believe that their functionality is key at the compositional level to understand life.

On the other hand, genes are universal and unambiguous.. If the triplet of AGC bases codes for a particular amino acid, it will be the same in humans, flies or whales. Therefore, it is consistent to "downplay" the importance of the centromeric dilemma, since the indispensable evolutionary mechanisms are generally conserved between taxa. However, not everything is so simple: centromere DNA evolves at a very fast rate, but still retains its functionality for cell structure and division. How is this possible?

Summary

Unfortunately, we cannot give you an answer to this question and many others that could be raised by studying the centromere. Geneticists consider this structure as a "genetic black hole", as we know that it is composed of one million repeated base pairs (satellite DNA), but little else. Its structure and viability remain unknown to this day, as the impossibility of mapping this non-coding DNA makes things very difficult.

However, we do know that centromeres give shape to the chromosome, are essential for cell division and offer a very useful classification system in certain fields. Fortunately, discovering all its enigmas and secrets will only be a matter of time (and scientific advances).

Bibliographical references:

• Avila Alvarez, S. (2009). Genetic distance to microsatellite centromere in females of red abalone haliotis rufescens swainson, 1822 from analysis of triploid progenies (Doctoral dissertation, Instituto Politécnico Nacional. Interdisciplinary Center of Marine Sciences).
• Centromere, National Genome Human Research Institute (NIH). Retrieved March 10 from https://www.genome.gov/es/genetics-glossary/Centromero#:~:text=During%20the%20division%20of%20cells%2C%20them,are%C3%A1n%20united%20by%20the%20centromere%20centromere.
• Centromere,Unioviedo. Retrieved March 10 from https://www.unioviedo.es/A.Roca/anexos/03_EL_CENTROMERO.pdf.
• Cobian, A., & Eguiarte Funs, L. E. (2002). Structure and complexity of the human genome. Ciencias, (068).
• Esteban Fernández, M. D. R. (1995). Heterochromatin removal and centromere organization in the nematode" parascaris univalens".
• Taking a closer look at genome "black holes", HHMI. Retrieved March 10 from https://www.hhmi.org/news/mirando-de-cerca-los-agujeros-negros-del-genoma