Generality
The nitrogenous bases are aromatic heterocyclic organic compounds, containing nitrogen atoms, which take part in the formation of nucleotides.
Fruit of the union of a nitrogenous base, a pentose (ie a sugar with 5 carbon atoms) and a phosphate group, nucleotides are the molecular units that make up the nucleic acids DNA and RNA.
In DNA, the nitrogenous bases are: adenine, guanine, cytosine and thymine; in the RNA, they are the same, except the thymine, in whose place there is a nitrogenous base called uracil.
Unlike those of RNA, the nitrogenous bases of DNA form pairing or base pairs. The presence of these pairing is possible because the DNA has a double-stranded nucleotide structure.
Gene expression depends on the sequence of nitrogenous bases combined with DNA nucleotides.
What are nitrogenous bases?
The nitrogenous bases are the organic molecules, containing nitrogen, which take part in the formation of nucleotides .
Each formed from a nitrogenous base, a sugar with 5 carbon atoms (pentose) and a phosphate group, nucleotides are the molecular units that make up the nucleic acids DNA and RNA .
The nucleic acids DNA and RNA are the biological macromolecules, on which the development and proper functioning of the cells of a living being depend.
NITROGEN BASES OF NUCLEIC ACIDS
The nitrogenous bases that make up the nucleic acids DNA and RNA are: adenine, guanine, cytosine, thymine and uracil .
Adenine, guanine and cytosine are common to both nucleic acids, ie they are part of both DNA nucleotides and RNA nucleotides. Thymine is exclusive to DNA, while uracil is exclusive to RNA .
Making a brief summary, then, the nitrogenous bases that form a nucleic acid (be it DNA or RNA) belong to 4 different types.
ABRREVIATIONS OF NITROGEN BASES
Chemists and biologists considered it appropriate to shorten the names of the nitrogenous bases with a single letter of the alphabet. In this way, they have made the representation and description of nucleic acids on texts easier and faster.
The adenine coincides with the capital letters A; the guanine with the capital letter G; cytosine with a capital letter C; the thymine with the capital letters T; finally, the uracil with the capital letter U.
Classes and structure
There are two classes of nitrogenous bases: the class of the nitrogenous bases that derive from the pyrimidine and the class of the nitrogenous bases that derive from the purine .
Figure: generic chemical structure of a pyrimidine and a purine.
The nitrogenous bases deriving from the pyrimidine are also known with the alternative names of: pyrimidine or pyrimidine nitrogenous bases ; while the nitrogenous bases that derive from the purine are also known with the alternative words of: purine or purine nitrogenous bases .
Cytosine, thymine and uracil belong to the class of pyrimidine nitrogenous bases; adenine and guanine, on the other hand, make up the class of purine nitrogenous bases.
Examples of purine derivatives, other than the nitrogen bases of DNA and RNA
Among the purine derivatives, there are also organic compounds that are not nitrogenous bases of DNA and RNA. For example, compounds such as caffeine, xanthine, hypoxanthine, theobromine and uric acid fall into this category.
WHAT ARE AZOTE BASES FROM THE CHEMICAL VIEWPOINT?
Organic chemists define the nitrogenous bases and all purine and pyrimidine derivatives as heterocyclic aromatic compounds .
- A heterocyclic compound is an organic ring (or cyclic) compound which, in the aforementioned ring, has one or more atoms other than carbon. In the case of purines and pyrimidines, atoms other than carbon are nitrogen atoms.
- An aromatic compound is a ring-shaped organic compound having structural and functional characteristics similar to those of benzene.
STRUCTURE
Figure: chemical structure of benzene.
The chemical structure of the nitrogenous bases derived from the pyrimidine consists, mainly, in a single ring with 6 atoms, 4 of which are carbons and 2 of which are nitrogen.
In fact, a pyrimidine nitrogenous base is a pyrimidine with one or more substituents (ie a single atom or a group of atoms) bound to one of the ring's carbon atoms.
In contrast, the chemical structure of the nitrogenous bases derived from the purine consists, mainly, in a double ring with 9 total atoms, 5 of which are carbons and 4 of which are nitrogen. The aforementioned double ring with 9 total atoms derives from the fusion of a pyridiminic ring (ie the pyrimidine ring) with an imidazole ring (ie the ring of imidazole, another organic heterocyclic compound).
Figure: imidazole structure.
As is known, the pyrimidine ring contains 6 atoms; while the imidazole ring contains 5. With fusion, the two rings share two carbon atoms each and this explains why the final structure contains, specifically, 9 atoms.
LOCATION OF NITROGEN ATOMS IN PURINE AND PYRIMIDINE
To simplify the study and description of organic molecules, organic chemists have thought of assigning an identification number to the coals and to all the other atoms of the supporting structures. The numbering always starts from 1, it is based on very specific assignment criteria (which, here, it is better to omit) and serves to establish the position of each atom, within the molecule.
For pyrimidines, the numerical assignment criteria establish that the 2 nitrogen atoms occupy position 1 and position 3, while the 4 carbon atoms reside in positions 2, 4, 5 and 6.
For purines, on the other hand, the numerical assignment criteria state that the 4 nitrogen atoms occupy positions 1, 3, 7 and 9, while the 5 carbon atoms reside in positions 2, 4, 5, 6 and 8.
Position in nucleotides
The nitrogenous base of a nucleotide always joins the carbon in position 1 of the corresponding pentose, through a covalent N-glycosidic bond .
In particular,
- The nitrogenous bases deriving from the pyrimidine form the N-glycosidic bond, through their nitrogen in position 1 ;
- While the nitrogenous bases that derive from the purine form the N-glycosidic bond, through their nitrogen in position 9 .
In the chemical structure of nucleotides, the pentose represents the central element, to which the nitrogenous base and the phosphate group bind.
The chemical bond that unites the phosphate group to the pentose is of the phosphodiester type and involves an oxygen of the phosphate group and the carbon in position 5 of the pentose.
WHEN AZOTE BASES FORM A NUCLEOSIDE?
The combination of a nitrogenous base and a pentose forms an organic molecule that takes the name of nucleoside .
Thus, it is the addition of the phosphate group that changes nucleosides into nucleotides.
Moreover, according to a particular definition of nucleotides, these organic compounds would be "nucleosides that have one or more phosphate groups linked to carbon 5 of the constituent pentose".
Organization in the DNA
DNA, or deoxyribonucleic acid, is a large biological molecule, formed by two very long strands of nucleotides (or polynucleotide filaments ).
These polynucleotide filaments have some characteristics, which deserve a special mention because they also closely concern the nitrogenous bases:
- They are joined together.
- They are oriented in opposite directions ("antiparallel filaments").
- They wrap one another, as if they were two spirals.
- The nucleotides that constitute them have a disposition such that the nitrogenous bases are oriented towards the central axis of each spiral, while the pentoses and phosphate groups form the external scaffolding of the latter.
The singular arrangement of the nucleotides causes each nitrogenous base of one of the two polynucleotide filaments to unite, through hydrogen bonds, to a nitrogenous base present on the other filament. This union, therefore, creates a combination of bases, combinations that biological and geneticists call pairing or a pair of bases .
It has been stated above that the two filaments are joined together: it is the bonds between the various nitrogenous bases of the two polynucleotide filaments that determine their union.
CONCEPT OF COMPLEMENTARY BETWEEN BASED BASES
By studying the structure of DNA, the researchers realized that the pairing of nitrogenous bases is highly specific . In fact, they noticed that adenine only joins thymine, while cytosine binds only to guanine.
In light of this discovery, they coined the term " complementarity between nitrogenous bases ", to indicate the univocal binding of adenine with thymine and of cytosine with guanine.
The identification of complementary pairing between nitrogenous bases was the key to explaining the physical dimensions of DNA and the particular stability enjoyed by the two polynucleotide filaments.
A decisive contribution to the discovery of the DNA structure (from the spiral winding of the two polynucleotide strands to the pairing of complementary nitrogenous bases) was given by the American biologist James Watson and the English biologist Francis Crick, in 1953.
With the formulation of the so-called " double helix model ", Watson and Crick had an incredible intuition, which represented an epochal turning point in the field of molecular biology and genetics.
In fact, the discovery of the exact DNA structure made possible the study and understanding of the biological processes that see deoxyribonucleic acid as the protagonist: from how the RNA is replicated or shaped to how it generates proteins.
THE TIES THAT HOLD THE COUPLES OF LUNG BASES TOGETHER
Joining two nitrogenous bases in a DNA molecule, forming the complementary pairing, are a series of chemical bonds, known as hydrogen bonds .
Adenine and thymine interact with each other by means of two hydrogen bonds, while guanine and cytosine by means of three hydrogen bonds.
HOW MANY COUPLES OF AZOTATE BASES CONTAIN A HUMAN DNA MOLECULE?
A generic human DNA molecule contains about 3.3 billion basic nitrogenous pairs, which are about 3.3 billion nucleotides per filament.
Figure: chemical interaction between adenine and thymine and between guanine and cytosine. The reader can note the position and number of the hydrogen bonds that hold together the nitrogenous bases of two polynucleotide filaments.
Organization in the RNA
Unlike DNA, RNA, or ribonucleic acid, is a nucleic acid usually composed of a single strand of nucleotides.
Therefore, the nitrogenous bases that constitute it are "unmatched".
However, it should be pointed out that the lack of a complementary nitrogenous base strand does not exclude the possibility that the RNA nitrogenous bases may appear like those of DNA.
In other words, the nitrogenous bases of a single RNA filament can match, according to the laws of complementarity between nitrogenous bases, exactly like the nitrogenous bases of DNA.
The complementary pairing between nitrogenous bases of two distinct RNA molecules is the basis of the important process of protein synthesis (or protein synthesis ).
URACILE REPLACES THE TIMINA
In RNA, uracil replaces DNA thymine not only in structure but also in complementary pairing: in fact, it is the nitrogenous base that specifically binds to adenine, when two distinct RNA molecules appear for functional reasons.
Biological role
The expression of the genes depends on the sequence of nitrogenous bases joined to the nucleotides of the DNA. Genes are more or less long segments of DNA (ie nucleotide segments), which contain the information essential to protein synthesis. Made up of amino acids, proteins are biological macromolecules, which play a fundamental role in regulating the cellular mechanisms of an organism.
The sequence of nitrogenous bases of a given gene specifies the amino acid sequence of the related protein.
