Chromosomes: what they are, characteristics and function.

A summary of the characteristics of chromosomes and their functions in cells.

Our DNA, found in the nucleus of our cells, is organized in the form of chromosomes, structures visible during cell division that are inherited from both fathers and mothers.chromosomes, structures visible during cell division that are inherited from both fathers and mothers.

They contain the genes that code for our anatomical and personality characteristics. They are not unique to humans, since every organism has chromosomes, albeit in different forms and quantities.

Let's take a closer look at what they are, what their parts are, what they contain, and what the difference is between eukaryotic organisms and prokaryotic organisms.

What are chromosomes?

Chromosomes (from the Greek "chroma", "color, staining" and "soma", "body or element"). are each of the highly organized structures made up of DNA and proteins, in which most of the genetic information is contained.in which most of the genetic information is contained. The reason for their name is due to the fact that they were discovered because they are structures that stain darkly in microscope preparations.

Although chromosomes are inside the cell nucleus in eukaryotic cells, it is during mitosis and meiosis, when the cell divides, that the chromosomes present their characteristic X (or Y) shape.

The number of chromosomes of individuals of the same species is constant.This is a criterion widely used in the Biological sciences to determine where a species begins and where it ends. The number of chromosomes of a species is specified by a number, it is called Ploidy and is symbolized by 1n, 2n, 4n... depending on the type of cell and the characteristics of the organism. Humans possess 23 pairs of chromosomes, one pair of which determines our sex.

Structure and chemical composition of chromatin

The chromosomes of eukaryotic cells are long double helix DNA molecules with long, double-helical DNA molecules that are closely that are closely related to proteins of two types, histone and non-histone.

The form in which chromosomes can be found depends on the phase of the cell.. They can be found loosely packed and lax, as in the nuclei of cells in interphase or in the normal state, or highly packed and separately visible, as happens during mitotic metaphase, one of the phases of cell division.

Chromatin is the form in which the DNA is manifested in the cell nucleus.and could be said to be what chromosomes are made of. This component is composed of DNA, histone and non-histone proteins, as well as RNA.

Histones

Histones are proteins rich in lysine and arginine, which interact with DNA to form a subunit, called a nucleosome, that repeats along the chromatin. The main histones found in eukaryotic organisms are: H1, H2A, H2B, H3 and H4.

The genes encoding histones are grouped in niches or "clusters", which are repeated tens to hundreds of times. Each cluster contains genes rich in G-C (guanine-cytosine) pairs, coding for histones in the following order H1-H2A-H3-H2B-H4.

Nucleosome

Chromatin, during interphase, can be observed by electron microscopy, presenting a shape similar to that of a necklace or a rosary. Each bead of the necklace is a spherical subunit, called a nucleosome, bound together with DNA fibers, and is the basic unit of chromatin.

A nucleosome is normally associated with 200 base pairs of DNA.formed by a medulla and a linker. The pith is formed by an octamer made of two subunits of histones H2A, H2B, H3 and H4. Around the pith the DNA is wound, making almost two turns. The rest of the DNA is part of the ligand, interacting with histone H1.

The association of the DNA with the histone generates nucleosomes, about 100 Å (Ångström) in diameter. In turn, nucleosomes can coil to form a solenoid, which forms the chromatin fibers of the interphase nuclei (300 Å). They can coil even further, forming supersolenoids with diameters of 6000 Å, forming the metaphase chromosome fibers.

3. Non-histone proteins

Non-histone proteins are proteins proteins other than histones that are extracted from the chromatin of nuclei with sodium chloride. (NaCl), have a high content of basic amino acids (25%), a high content of acid amino acids (20-30%), a high proportion of proline (7%) and a low content of hydrophobic amino acids.

Parts of chromosomes

The organization of chromatin is not uniform throughout the chromosome. A number of distinct elements can be distinguished: centromeres, telomeres, nucleolus organizing regions and chronomeres, all of which may contain specific DNA sequences.

1. Centromeres

The centromere is the part of the chromosome that, when stained, appears less stained compared to the rest. It is the area of the chromosome that interacts with the achromatic spindle fibers from prophase to anaphase. from prophase to anaphase in both mitosis and meiosis. It is responsible for carrying out and regulating the chromosome movements that occur during the phases of cell division.

Telomeres

Telomeres are the limb-like parts of chromosomes. They are regions in which there is non-coding DNA, highly repetitive, whose main function is the structural stability of chromosomes in eukaryotic cells.

3. Organizing regions of the nucleolus

In addition to centromeres and telomeres, which are called primary constrictionsIn addition to centromeres and telomeres, which are called primary constrictions, another type of thin regions, called secondary constrictions, can be found on some chromosomes, which are closely related to the presence of ribosomal DNA sequences.

These regions are the nucleolus organizing regions (NOR). The ribosomal DNA sequences are enclosed within the nucleolus, which remains enclosed by the NORs for much of the cell cycle.

4. Chromomeres

Chromomeres are the thick, compact regions of the chromosome.which are distributed more or less uniformly along the chromosome, and can be visualized during the phases of mitosis or meiosis with less chromatin condensation (prophase).

Chromosome shape

The shape of chromosomes is the same for all somatic (non-sex) cells, and is characteristic of each species. The shape depends mainly on the location of the chromosome and its location on the chromatid..

As mentioned above, the chromosome is basically made up of the centromere, which divides the chromosome into a short and a long arm. The position of the centromere can vary from chromosome to chromosome, giving them different forms.

1. Metacentric

This is the prototypical chromosome, with the centromere located in the middle of the chromosome and the two arms having the same length.

2. Submetacentric

The length of one arm of the chromosome is longer than the other, but this is not very exaggerated.

3. Acrocentric

One arm is very short and the other very long.

4. Telocentric

One arm of the chromosome is very short, having the centromere very far toward one end.

Law of numerical constancy

Normally, in most animal and plant species, all individuals of the same species have a constant and determined number of chromosomes, which constitute their karyotype.which constitute their karyotype. This rule is called the law of numerical constancy of chromosomes. For example, in the case of human beings, the vast majority of us have 23 pairs of chromosomes.

However, it is true that there are individuals who, due to errors in the distribution of chromosomes during the formation of gametes or sex cells, receive a different number of chromosomes. This is the case of medical conditions such as Down syndrome (trisomy of chromosome 21), Klinefelter (XXY males), XYYY males and XXX females.

The number of chromosomes present in diploid species, as in our case, has two chromosomes.as in our case, has two pairs of chromosomes of each type, and is represented as 2n. In haploid organisms, i.e., containing only one set of each chromosome, they are represented by the letter n. There are polyploid species, which have more than two sets of each chromosome, and are represented as 3n, 4n....

Surprising as it may seem, there is no relationship between the number of chromosomes and their degree of complexity. There are plant species, such as Haplopappus graciliswhich has only four chromosomes, while other plants, such as the flour wheat plant has 42, more than our species, but it is still a plant without a brain or other organs. The organism with the most chromosomes known to date is called Aulacanthait is a microorganism with 1600 chromosomes.

Sex chromosomes

In many organisms, one of the pairs of homologous chromosomes is different from the rest, and determines the sex of the individual. This happens in the human species and these chromosomes are called sex chromosomes or heterochromosomes..

XY determination system

This is the proper sex determination system of humans and many other animals:

Females are XX (homogametic female), ie, females have two X chromosomes and can only provide eggs with the X chromosome..

Males, on the other hand, are XY (heterogametic male), having one X and one Y chromosome, and being able to provide sperm with either one or the other.

The union between the egg and the sperm will give either XX or XY individuals.with a 50% probability of being of one or the other biological sex.

ZW determination system

This is the one for other species, such as butterflies or birds.. It is the opposite of the previous case, and for this reason it is preferred to use other letters to avoid confusion.

Males are ZZ (homogametic male), and females are ZW (heterogametic female).

XO determination system

And if the previous system was not very rare, this one will certainly not leave anyone indifferent.

It occurs mainly in fish and amphibians, and also in some insects, since they do not have a sex chromosome different from X, i.e., they do not have something similar to Y.

Sex is determined by whether they have two Xs or only one. The male is XO, meaning that it has only one sex chromosome, X, while the female is XX, having two.

Human chromosomes

The human being has 23 pairs of chromosomes, 22 of which are autosomes and one pair of sex chromosomes.. Depending on whether you are male or female, you have XY or XX sex chromosomes, respectively.

The total size of the human genome, i.e. the number of genes in our species, is about 3.2 billion base pairs of DNA, containing between 20,000-25,000 genes. The human DNA sequence contains the encoded information necessary for the expression of the human proteome, i.e. the set of proteins that are the basis for the expression of the human proteome.that is, the set of proteins synthesized by the human being, which is the reason why we are the way we are.

It has been estimated that about 95% of gene-related DNA corresponds to non-coding DNA, usually called "junk DNA": pseudogenes, gene fragments, introns... Although it was thought that these DNA sequences were chromosomal regions without any function, research has recently cast doubt on this assertion.

The prokaryote chromosome

Prokaryotic organisms, whose kingdoms are bacteria and archaea, have only one chromosome, in circular form, although there are exceptions to this rule.although it is true that there are exceptions to this rule. This type of chromosome, usually called a bacterial chromosome, can contain about 160,000 base pairs.

This chromosome is dispersed throughout the cytoplasm of the organism, since these living beings do not have a defined nucleus.

Bibliographical references:

• Olins, D. E.; Olins, A. L. (2003), Chromatin history: our view from the bridge, Nature Reviews Molecular Cell Biology 4 (10): 809-13.
• Crow, E. W.; Crow, J. F. (2002), 100 Years Ago: Walter Sutton and the Chromosome Theory of Heredity, Genetics 160 (1): 1-4
• Daintith, John, et al., (1994), Biographical Encyclopedia of Scientists, second edition. Bristol, UK: Institute of Physics Publishing.