Chromosomes: What They Are, Characteristics And Functioning
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.
They contain the genes that encode our anatomical and personality characteristics. They are not something exclusive 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 the difference is between eukaryotic organisms and prokaryotic organisms.
Chromosomes (from the Greek “chroma”, “color, stain” 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 because when they were discovered it was because they are structures that stain darkly in microscope preparations.
Although the 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 being a criterion widely used within the biological sciences to determine where a species begins and where it ends. The number of chromosomes of a species is specified with a number, it is called Ploidy and is symbolized with 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 which are closely related to proteins of two types, histones and non-histones.
The way chromosomes can be found depends on the phase of the cell They can be found poorly compacted and lax, as in the nuclei of cells in interphase or in the normal state, or highly compacted and visible separately, as happens when the mitotic metaphase occurs, one of the phases of cell division.
Chromatin is the form in which DNA is expressed in the cell nucleus, and you could say that it is what chromosomes are made of. This component is composed of DNA, histone and non-histone proteins, as well as RNA.
1. Histones
Histones are proteins rich in lysine and arginine, which interact with DNA forming 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 that encode histones are grouped in niches or “clusters,” which are repeated tens to hundreds of times. Each cluster contains genes rich in GC (guanine-cytosine) pairs, encoding histones in the following order H1-H2A-H3-H2B-H4.
2. Nucleosome
The chromatin, during interphase, can be observed by electron microscope, presenting a shape similar to that of a necklace or a rosary. Each pearl of 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 marrow and a linker. The medulla is made up of an octamer made of two subunits of histones H2A, H2B, H3 and H4. The DNA is wound around the marrow, 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, nucleosomes can coil to form a solenoid, which constitutes the chromatin fibers of the interphase nuclei (300 Å). They can coil even more, forming supersolenoids with diameters of 6000 Å, forming the fibers of metaphase chromosomes.
3. Non-histone proteins
Non-histone proteins are 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%), high content of acidic amino acids (20-30%), a high proportion of proline (7%) and low content of hydrophobic amino acids.
Parts of chromosomes
The organization of chromatin is not uniform throughout the chromosome. A series of distinct 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. This is the area of the chromosome that interacts with the achromatic spindle fibers from prophase to anaphase, both in mitosis and meiosis. It is responsible for carrying out and regulating the chromosome movements that occur during the phases of cell division.
2. Telomeres
Telomeres are the end-forming parts of chromosomes. They are regions in which there is highly repetitive, non-coding DNA, 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 constrictions in some chromosomes other types of thin regions can be found, called secondary constrictions, 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 in the NOR for much of the cell cycle.
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).
shape of chromosomes
The shape of the chromosomes is the same for all somatic (non-sexual) cells, and characteristic of each species. The shape depends, fundamentally, on the location of the chromosome and its location on the chromatid
As we have already mentioned previously, the chromosome is basically made up of the centromere that divides the chromosome into a short arm 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, 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 it is not 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, with the centromere very far towards one end.
Law of numerical constancy
Normally, in most animal and plant species, All individuals in it have a constant and determined number of chromosomes, which constitute its 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 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 sexual 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), XYY males and XXX females.
The number of chromosomes that diploid species have, as in our case, has two pairs of chromosomes of each type, and is represented as 2n. In haploid organisms, that is, those that contain 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, 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 graciliswhich only has four chromosomes, while other plants, such as the bread wheat plant, have 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 Aulacantha is a microorganism that has 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 It happens in the human species and these chromosomes are called sex chromosomes or heterochromosomes
This is the sex determination system of humans and many other animals:
The females are XX (homogametic female), that is, They have two X chromosomes and will only be able to offer 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 produce sperm with either one or the other.
The union between the egg and the sperm will give individuals either XX or XY being the probability of 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 occurs to 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 strange, 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 the They don’t have something like Y
The 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
The human being has 23 pairs of chromosomes, 22 of them being autosomes and one pair being 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, that is, the number of genes that our species has, is about 3.2 billion DNA base pairs, containing between 20,000-25,000 genes. The human DNA sequence contains encoded information necessary for the expression of the human proteome that is, the set of proteins that human beings synthesize and that cause us to be the way 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, gene fragments, introns… Although, although it was thought that these DNA sequences were chromosomal regions without function Some research has recently cast doubt on this claim.
The chromosome of prokaryotes
Prokaryotic organisms, whose kingdoms are bacteria and archaea, have only one chromosome, circular in shape, 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.
