The 6 parts of the chromosome: characteristics and functions

These are the components of the chromosomes that we can find in human cells.

DNA is the library of life, since it contains all the information necessary for the growth, development, reproduction and death of organisms, however simple they may be at the anatomical level. Beyond allowing us to exist, this nucleic acid makes possible the inheritance of genes between generations and, therefore, the evolutionary process that has given rise to all the taxa that inhabit the Earth today.

Based on heritable point mutations and the genetic recombination that occurs during sexual reproduction, living beings "advance" genetically and natural selection favors the predominance of certain evolutionarily viable traits over others. For example, if tree bark darkens for X or Y reason, butterflies of a darker species will be more likely to survive than their lighter counterparts because they are better camouflaged in the environment due to prior adaptation.

Based on this simple premise, natural selection, genetic drift and other stochastic processes shape the gene pools of living populations over time. However, before understanding the functioning of genes and evolution on a "macro" scale, it is necessary to establish certain essential foundations. For this reason, today we go back to basics: know with us the 6 parts of the chromosome.the structure that houses the DNA of living organisms.

The basics of genetics

A gene is defined as a particle of genetic material that, together with others, is arranged in a fixed order along a chromosome. The function of the gene (genotype) is to determine the appearance of hereditary characters in living beings, including those that can be quantified with the naked eye (phenotype).

It is even more interesting to know that, classically speaking, each gene has 2 different allelesthat is, alternative forms of the same gene. Variations in these genes usually translate into phenotypic changes, such as hair color, eye color, Blood type or the appearance or absence of certain diseases. By having 2 alleles per gene, an individual can be homozygous for one trait (AA, for example, the two alleles are the same) or heterozygous for the same trait (Aa, the alleles are different).

Humans have 23 pairs of chromosomes in (almost) all our cells, a total of 46. As these are "paired", it is easy to deduce that one allele will come from the father and the other from the mother. Thus, half of the genetic information that makes us up comes from one parent and, consequently, the other half from the other.

Based on this premise, we can affirm that all somatic cells in our body (non-sexual cells) are diploid (2n), having 2 complete sets of chromosomes in the nucleus.. Diploidy has a clear evolutionary significance, because if a gene from the father mutates or is defective, it is expected that the mother's copy can fix or mask this error.

This genetics express class is based on the most basic possible, since there are many traits that are encoded by more than one gene (they are polygenic) and some alleles are dominant (A) over others (a). Sometimes, for a disease to manifest itself, only one of the 2 copies of the gene needs to be defective, but that is a topic for another time.

What are the parts of the chromosome?

Based on this premise, we have learned that all cells in our body (minus eggs and sperm) contain a nucleus with 2 complete sets of chromosomes, one from the father and one from the mother.

In biology and cytogenetics each highly organized structure, consisting of DNA and proteins, which contains almost all the genetic information of a living being, is called a chromosome.. The number of genes housed in each chromosome varies: for example, chromosome 1 (remember that there are 23 in total, 22 autosomes and a pair of sex chromosomes, multiplied by 2 because we receive a copy from both parents) contains about 2,059 genes, while the Y chromosome, which encodes the male gender, has only 108.

This coding and non-coding DNA is organized in the form of chromatin forming the chromosomes, with a typical X-shape. If we cut longitudinally (vertically) this three-dimensional shape of the chromosome, we obtain two rod-shaped structures, which are called chromatids. Thus, each chromosome is composed of each chromosome is composed of 2 sister chromatids..

Having established these premises, we can briefly describe the parts of the chromosome. Let us do so.

Film and matrix

The chromosome matrix is a compound of a chemical and organic nature, condensed and homogeneous, which covers the chromosomes.. It should be noted that its content comes from the nucleolus. On the other hand, this matrix component is surrounded by a membrane called film, which is thin in nature and consists of achromatic (colorless) substances.

2. Chromonemes and chromomers

The chromoneme is each of the filaments that make up the chromatid.. These filamentous structures are composed of DNA and proteins. When coiled together, they give rise to the chromosome, which is linked to other chromosomes in the form of rosary beads within the chromosome. Chromomeres are granules that accompany the chromoneme along its length and, therefore, each of them contains a greater or lesser number of genes.

3. Centromere

The centromere is defined as the narrow region of a chromosome that separates a short arm from a long arm.. Put colloquially, it is the center of the X, understanding this letter as the three-dimensional shape of the chromosome.

However, it should be noted that, despite its name, the centromere is not located exactly at the chromosomal center. The centromere is at the primary constriction, but this can be located further "up" or "down" the vertical plane.

Thus, each chromatid will divide into 2 arms, one short (p) and one long (q).. This gives the chromosome a total of 4 arms, since let us remember that each one is composed of 2 sister chromatids. Based on these anatomical changes, the following types of chromosomes are conceived:

• Metacentric: the centromere is located in the middle of the chromosome. This is the typical form.
• Submetacentric: the centromere is located slightly above the theoretical center, towards one end of the chromosome.
• Acrocentric: the centromere is very close to the end of a chromosome. There are 2 arms much longer than the other arms.
• Telocentric: the centromere is almost at the end of the chromosome. Almost no short arms are visible.

4. Kinetochore

This is a protein structure located above the centromere of the upper chromosomes. Its function is essential, since the microtubules of the mitotic spindle are anchored to the kinetochore.The kinetochore is a key element for the partitioning of genetic information during mitosis.

5. Secondary constrictions

These are regions of the chromosome found at the ends of the arms.. In some cases, these places correspond to the area where the genes responsible for transcribing as RNA are located.

6. Telomeres

Telomeres are the ends of chromosomes.. They are highly repetitive and non-coding sequences (they do not contain genes that are transcribed into proteins), whose main function is to give the chromosome resistance and stability. These structures are particularly interesting because they contain the basis for the physiological senescence and aging of living beings.

With each mitosis, the telomeres shorten a little more, since DNA duplication is not perfect in somatic cells.. There comes a point at which a last cell lineage will no longer be able to divide due to the reduced size of the telomeres and, therefore, the tissue will die with the cell bodies, as it will not be able to rebuild itself with new cells. This explains a large part of cell death (and therefore of organisms).

As a final curiosity, it should be noted that there is an enzyme (telomerase) that rebuilds the telomeres during fetal development.. At birth, somatic cells stop their activity, so it is the organism itself that programs their senescence. Ironically enough, many malignant tumors present cells with high telomerase activity: if a cell is able to divide "infinitely" and does not die, it is not difficult to imagine that it could become a cancer.

Summary

We have taken this opportunity to tell you, in a brief way, the basis of life itself and of heredity. To speak of chromosomes is to encompass a whole, because DNA explains each and every one of the physical and emotional manifestations at the level of both the individual and the species.

This double helix encloses the secret of life, because thanks to it we remain even after centuries dead in the genetic imprint of our relatives and relatives. Non-human living beings develop all their behaviors based on this premise: remaining at the individual level is not important, since the ultimate goal is to extend the genes as much as possible and leave an indelible imprint.

Bibliographical references:

• Arvelo, F., & Morales, A. (2004). Telomere, telomerase and cancer. Acta Científica Venezolana, 55, 288-303.
• Somatic cells and other terms, Genome.gov. Retrieved March 7 from https://www.genome.gov/es/genetics-glossary/Celulas-somaticas#:~:text=%E2%80%8BC%C3%A9lulas%20som%C3%A1ticas&text=A%20c%C3%A9lula%20som%20C3%A1tica%20is%20any%20one%20inherited%20from%20every%20parent.
• Moyzis, R. K. (1991). The human telomere. Research and Science, (181), 24-32.
• Multani, A. S., Pathak, S., & Amass, J. R. M. M. S. (1996). Telomere, telomerase and malignant melanomas in humans and domestic animals. Archives of zootechnics, 45(170), 141-149.
• Parts of a chromosome: what are they? Cefegen. Retrieved March 7 from https://cefegen.es/blog/partes-de-un-cromosoma-cuales-son.