The 6 parts of the chromosome: characteristics and functions

DNA is the library of life, since it contains all the information necessary for the growth, development, reproduction and death of organisms, no matter how simple they are at the anatomical level. Beyond allowing ourselves to exist, this nucleic acid enables 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 genetic recombination that occurs during sexual reproduction, human beings alive we “advance” genetically and natural selection favors the predominance of certain evolutionarily viable characters over others. For example, if tree barks are darkened by X or Y motif, butterflies of a darker species will have more likely to survive than their clear companions, as they are better camouflaged in the environment due to an adaptation previous.

Based on this simple premise, natural selection, genetic drift, and other stochastic processes shape the gene pools of populations of living things over time. However, before understanding the functioning of genes and evolution on a "macro" scale, it is necessary to establish certain essential bases. For this reason, today we return to the basics:

meet with us the 6 parts of the chromosome, structure that houses the DNA of living beings.

• Related article: "Differences between DNA and RNA"

The Basics of Genetics

A gene is defined as a particle of genetic material that, along 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).

Even more interesting is knowing that, classically, each gene has 2 different alleles, that is, alternative forms of the same gene. Variations in these genes usually translate into phenotypic changes, such as hair color, eyes, blood group or the appearance and absence of certain diseases. By having 2 alleles per gene, an individual can be homozygous for one character (AA, for example, the two alleles are the same) or heterozygous for the same character (Aa, the alleles are different).

Humans have 23 pairs of chromosomes in (almost) all of 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 we make 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) are diploid (2n), having 2 complete sets of chromosomes in the nucleus. Diploidy has a clear evolutionary significance, since if a gene from the father mutates or is defective, it is expected that the mother's copy can solve or mask this error.

This express kind of genetics is based on the most basic possible, since there are many characters that are encoded by more than one gene (they are polygenic) and some alleles are dominant (A) over others (to). Sometimes, for a disease to manifest itself, it is only necessary that one of the 2 copies of the gene is 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 the cells in our body (less 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 of the highly organized structures, made up of DNA and proteins, which contains almost all the genetic information of a living being is called a chromosome. The number of genes that each chromosome houses is variable: for example, chromosome 1 (we remember that there are a total of 23, 22 autosomes and a pair of sexual ones, multiplied by 2 because we receive a copy of both parents) contains about 2,059 genes, while the Y chromosome, which codes for the male gender, has just 108.

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

After establishing these premises, we can briefly describe the parts of the chromosome. Go for it.

1. Film and matrix

The chromosomal matrix It is a compound of chemical and organic nature, condensed and homogeneous, that 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 a film, thin in nature and made up of achromatic (colorless) substances.

2. Chromonemes and chromomers

The cromonema is each of the filaments that make up the chromatid. These filamentous structures are made up of DNA and proteins. When they are rolled up, they give rise to the chromomer, which joins with others like rosary beads within the chromosome. The chromomers are granules that accompany the chromonema throughout its length and, therefore, each of them contains a more or less high 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. Said colloquially, it is the center of the X, understanding this letter as the three-dimensional shape of the chromosome.

In any case, it should be noted that, despite its name, the centromere is not located exactly in the chromosomal center. The centromere is in the primary constriction, but this can be more “above” or “below” the vertical plane.

So that, each chromatid will be divided into 2 arms, one short (p) and the other long (q). This gives the chromosome a total of 4 arms, as we remember that each one is made up of 2 sister chromatids. Based on these anatomical changes, the following types of chromosomes are conceived:

• Metacentric: the centromere is in the middle of the chromosome. It is the typical way.
• Submetacentric: the centromere is a little higher than 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 rest.
• Telocentric: the centromere is almost at the end of the chromosome. Short arms are hardly noticeable.

4. Cinetochoir

It is a protein structure located on the centromere of the higher chromosomes. Its function is essential, since the microtubules of the mitotic spindle anchor in the kinetochore., key elements for the partition of genetic information to occur during mitosis.

• You may be interested in: "Kinetochore: what is it, characteristics and functions of this part of the chromosome"

5. Secondary constrictions

They are regions of the chromosome that are found at the ends of the arms. In some cases, these places correspond to the area where the genes responsible for transcription 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 especially interesting, since they contain the foundations of the physiological senescence and old age of living beings.

With each mitosis, telomeres get a little shorter, as DNA duplication is not perfect in somatic cells. There comes a point where a last cell line will not be able to divide further due to the reduced size of the cells. telomeres and, therefore, the tissue will die with the cell bodies, unable to rebuild with new cells. This explains much of the cell death (and therefore of the organisms).

As a final curiosity, it should be noted that there is an enzyme (telomerase) that rebuilds telomeres during fetal development. At the moment of birth, somatic cells cut off their activity, so it is the body itself that programs its senescence. Ironically, many malignant tumors have cells with high telomerase activity: if a cell is capable of dividing itself in an "infinite" way and does not die, it is not difficult to imagine that it could become a Cancer.

• You may be interested in: "Telomeres: what are they, characteristics and how are they linked to age"

Resume

We have taken this opportunity to tell you, in a brief way, the bases of our own life and inheritance. To speak of chromosomes is to encompass a whole, since 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 contains the secret of life, because thanks to it we remain even after centuries of death in the genetic imprint of our relatives and relatives. Non-human living beings develop all their behaviors based on this premise: remain at the level individual is not important, because the ultimate goal is to spread genes as much as possible and leave a indelible.

Bibliographic references:

• Arvelo, F., & Morales, A. (2004). Telomere, telomerase and cancer. Venezuelan Scientific Act, 55, 288-303.
• Somatic cells and other terms, Genome.gov. Picked up on March 7 at https://www.genome.gov/es/genetics-glossary/Celulas-somaticas#:~:text=%E2%80%8BC%C3%A9lulas%20som%C3%A1ticas&text=Una%20c%C3%A9lula%20som%C3%A1tica%20es%20cualquier, one% 20 inherited% 20 from% 20 each% 20 parent.
• Moyzis, R. K. (1991). The human telomere. Research and Science, (181), 24-32.
• Multani, A. S., Pathak, S., & Amass, J. R. 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? Cephegen. Picked up on March 7 at https://cefegen.es/blog/partes-de-un-cromosoma-cuales-son