Chromosomes: what they are, characteristics and how they work

Our DNA, which is 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.

In them are the genes that encode our anatomical and personality characteristics. They are not something unique to human beings, since every organism has chromosomes, although in different shapes and quantities.

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

• Related article: "Major cell types of the human body"

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 found. The reason for their name is due to the fact that when they were discovered it was thanks to the fact that they are structures that stain darkly in the preparations for the microscope.

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

The number of chromosomes of individuals of the same species is constantThis being a widely used criterion within biological sciences to determine where a species begins and ends. The number of chromosomes of a species is specified with 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. Human beings have 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 They are closely related to proteins of two types, histones and non-histones.

How chromosomes can be found depends on the phase of the cell. They can be found loosely compacted and lax, as in the nuclei of cells at the interface or in a normal state, or highly compacted and separately visible, as occurs when mitotic metaphase occurs, one of the phases of division mobile.

Chromatin is the form in which DNA manifests itself in the cell nucleus, and you could say that's what chromosomes are made of. This component is made up of DNA, histonic and non-histonic proteins, as well as RNA.

1. Histones

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

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

2. Nucleosome

The chromatin, during the interphase, can be observed by means of an electron microscope, presenting a shape similar to that of a necklace or a rosary. Each pearl on the necklace is a spherical subunit, called a nucleosome, linked 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 medulla is made up of an octamer made of two subunits of the histones H2A, H2B, H3 and H4. Around the marrow the DNA is wound, making almost two turns. The rest of the DNA is part of the linker, interacting with histone H1.

The association of DNA with histones generates nucleosomes, about 100 Å (Ångström) in diameter. In turn, the nucleosomes can be coiled to form a solenoid, which constitutes the chromatin fibers of the interphase nuclei (300 Å). They can twist even further, forming super solenoids with diameters of 6000 Å, forming the fibers of metaphase chromosomes.

3. Nonhistonic proteins

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

Parts of chromosomes

The organization of chromatin is not uniform throughout the chromosome. A series of differentiated elements can be distinguished: centromeres, telomeres, nucleolus organizing regions and chronometers, all of which can 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 fibers of the achromatic spindle from prophase to anaphase, both in mitosis and meiosis. It is responsible for carrying out and regulating the chromosomal movements that occur during the phases of cell division.

2. Telomeres

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

3. Nucleolus organizing regions

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

Those regions are the nucleolus organizing regions (NORs). The ribosomal DNA sequences are encompassed within the nucleolus, which remains encompassed by the NORs for much of the cell cycle.

4. Chromomers

Chromomers are the thick and 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 condensation of chromatin (prophase).

• You may be interested: "Differences between DNA and RNA"

Chromosome shape

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

As we have already mentioned, the chromosome is basically made up of the centromere that divides the chromosome into a short and a long arm. The position of the centromere can vary from chromosome to chromosome, giving them different shapes.

1. Metacentric

It is the prototypical chromosome, the centromere being located in the middle of the chromosome and the two arms have the same length.

2. Submetacentric

The length of one arm of the chromosome is greater than the other, but it is not something very exaggerated.

3. Acrocentric

One arm is very short and the other is very long.

4. Telocentric

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

Law of numerical constancy

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

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, they receive a different number of chromosomes. This is the case of medical conditions such as Down syndrome (trisomy of chromosome 21), Klinefelter (XXY males) XYY males and XXX females.

The number of chromosomes exhibited by diploid speciesAs is our case, it has two pairs of chromosomes of each type, and is represented as 2n. In haploid organisms, that is, that contain only one set of each chromosome, they are represented by the letter n. There are polyploid species, which present more than two sets of each chromosome, being represented as 3n, 4n ...

As 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 the Haplopappus gracilis, which only has four chromosomes, while other vegetables, such as the bread wheat plant, have 42, more than our species, but it is still a vegetable without a brain or other organs. The organism with the most chromosomes known to date is called Aulacantha, is a microorganism that has 1600 chromosomes

Sex chromosomes

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

XY determination system

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

Females are XX (homogeneous female), that is, have two X chromosomes and will only be able to deliver eggs with the X chromosome.

The males, on the other hand, are XY (heterogametic male), having an X and a Y chromosome, and being able to give sperm with either one or the other.

The union between the ovum and the sperm will give individuals or XX or XY, the probability being 50% of being of one or the other biological sex.

ZW determination system

This is that of other species, such as butterflies or birds. The opposite is the case in 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 did not turn out to be very rare, this one will surely not leave anyone indifferent.

It occurs mainly in fish and amphibians, and also in some other insects, since they do not have a sex chromosome other than X, that is, do not have something like Y.

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

Human chromosomes

Humans have 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 the sex chromosomes XY or XX, respectively.

The total size of the human genome, that is, the number of genes that our species has, is about 3,200 million DNA base pairs, containing between 20,000-25,000 genes. The human DNA sequence encoded contains the information necessary for the expression of the human proteome, that is to say, the set of proteins that human beings synthesize and that is the cause that we are as we are.

It has been estimated that about 95% of the DNA related to genes would correspond to non-coding DNA, usually called “junk DNA”: pseudogenes, fragments of genes, introns... Although, although it was thought that these DNA sequences were chromosomal regions with no function, lately research has questioned this affirmation.

The prokaryotic chromosome

Prokaryotic organisms, whose kingdoms are that of bacteria and archaea, have only one chromosome, in a circular form, 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 scattered throughout the cytoplasm of the organism, since these living beings do not have a defined nucleus.

Bibliographic references:

• Olins, D. AND.; 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.