All life on the planet consists of many cells that maintain the orderliness of their organization due to the genetic information contained in the nucleus. It is stored, implemented and transmitted by complex high-molecular compounds - nucleic acids, consisting of monomer units - nucleotides. The role of nucleic acids cannot be overestimated. The stability of their structure determines the normal vital activity of the organism, and any deviations in the structure will inevitably lead to a change in the cellular organization, the activity of physiological processes and the viability of cells as a whole.
The concept of a nucleotide and its properties
Each molecule of DNA or RNA is assembled from smaller monomeric compounds - nucleotides. In other words, a nucleotide is a building material for nucleic acids, coenzymes and many other biological compounds that are essential for a cell in the course of its life.
To the main properties of these irreplaceablesubstances can be attributed:
• storage of information about protein structure and inherited traits;
• control over growth and reproduction;
• participation in metabolism and many other physiological processes occurring in the cell.
Nucleotide composition
Speaking of nucleotides, one cannot but dwell on such an important issue as their structure and composition.
Each nucleotide consists of:
• sugar residue;
• nitrogenous base;
• phosphate group or phosphoric acid residue.
It can be said that a nucleotide is a complex organic compound. Depending on the species composition of nitrogenous bases and the type of pentose in the nucleotide structure, nucleic acids are divided into:
• deoxyribonucleic acid, or DNA;
• ribonucleic acid, or RNA.
Composition of nucleic acids
In nucleic acids, sugar is represented by pentose. This is a five-carbon sugar, in DNA it is called deoxyribose, in RNA it is called ribose. Each pentose molecule has five carbon atoms, four of which, together with an oxygen atom, form a five-membered ring, and the fifth is part of the HO-CH2 group.
The position of each carbon atom in a pentose molecule is indicated by an Arabic numeral with a prime (1C´, 2C´, 3C´, 4C´, 5C´). Since all the processes of reading hereditary information from a nucleic acid molecule have a strict direction, the numbering of carbon atoms and their arrangement in the ring serve as a kind of indicator of the right direction.
According to the hydroxyl group toa phosphoric acid residue is attached to the third and fifth carbon atoms (3С´ and 5С´). It determines the chemical affiliation of DNA and RNA to the group of acids.
A nitrogenous base is attached to the first carbon atom (1С´) in a sugar molecule.
Species composition of nitrogenous bases
DNA nucleotides by nitrogenous base are represented by four types:
• adenine (A);
• guanine (G);
• cytosine (C);
• thymine (T).
The first two are purines, the last two are pyrimidines. By molecular weight, purines are always heavier than pyrimidines.
RNA nucleotides by nitrogenous base are represented by:
• adenine (A);
• guanine (G);
• cytosine (C);
• uracil (U).
Uracil, like thymine, is a pyrimidine base.
In the scientific literature, one can often find another designation of nitrogenous bases - in Latin letters (A, T, C, G, U).
Let's dwell in more detail on the chemical structure of purines and pyrimidines.
Pyrimidines, namely cytosine, thymine and uracil, are represented by two nitrogen atoms and four carbon atoms, forming a six-membered ring. Each atom has its own number from 1 to 6.
Purines (adenine and guanine) consist of pyrimidine and imidazole or two heterocycles. The purine base molecule is represented by four nitrogen atoms and five carbon atoms. Each atom is numbered from 1 to 9.
As a result of the connection of nitrogenousa base and a pentose residue form a nucleoside. A nucleotide is a combination of a nucleoside and a phosphate group.
Formation of phosphodiester bonds
It is important to understand the question of how nucleotides are connected in a polypeptide chain and form a nucleic acid molecule. This happens due to the so-called phosphodiester bonds.
The interaction of two nucleotides gives a dinucleotide. The formation of a new compound occurs by condensation, when a phosphodiester bond occurs between the phosphate residue of one monomer and the hydroxy group of the pentose of another.
Synthesis of a polynucleotide is repeated repetition of this reaction (several million times). The polynucleotide chain is built through the formation of phosphodiester bonds between the third and fifth carbons of sugars (3С´ and 5С´).
Polynucleotide assembly is a complex process that occurs with the participation of the DNA polymerase enzyme, which ensures the growth of the chain only from one end (3´) with a free hydroxy group.
Structure of the DNA molecule
A DNA molecule, like a protein, can have a primary, secondary and tertiary structure.
The sequence of nucleotides in a DNA chain determines its primary structure. The secondary structure is formed by hydrogen bonds, which are based on the principle of complementarity. In other words, during the synthesis of the DNA double helix, a certain pattern operates: adenine of one chain corresponds to the thymine of the other, guanine to cytosine, and vice versa. Pairs of adenine and thymine or guanine and cytosineare formed due to two in the first and three in the last case hydrogen bonds. Such a connection of nucleotides ensures a strong connection of the chains and an equal distance between them.
Knowing the nucleotide sequence of one DNA strand, you can complete the second one by the principle of complementarity or addition.
The tertiary structure of DNA is formed by complex three-dimensional bonds, which makes its molecule more compact and able to fit in a small cell volume. So, for example, the length of E. coli DNA is more than 1 mm, while the length of the cell is less than 5 microns.
The number of nucleotides in DNA, namely their quantitative ratio, obeys the Chergaff rule (the number of purine bases is always equal to the number of pyrimidine bases). The distance between nucleotides is a constant value equal to 0.34 nm, as is their molecular weight.
The structure of the RNA molecule
RNA is represented by a single polynucleotide chain formed through covalent bonds between a pentose (in this case, ribose) and a phosphate residue. It is much shorter than DNA in length. There are also differences in the species composition of nitrogenous bases in the nucleotide. In RNA, uracil is used instead of the pyrimidine base of thymine. Depending on the functions performed in the body, RNA can be of three types.
• Ribosomal (rRNA) - usually contains from 3000 to 5000 nucleotides. As a necessary structural component, it takes part in the formation of the active center of ribosomes, the site of one of the most important processes in the cell- protein biosynthesis.
• Transport (tRNA) - consists of an average of 75 - 95 nucleotides, transfers the desired amino acid to the site of polypeptide synthesis in the ribosome. Each type of tRNA (at least 40) has its own, unique sequence of monomers or nucleotides.
• Informational (mRNA) - in terms of nucleotide composition is very diverse. Transfers genetic information from DNA to ribosomes, acts as a matrix for the synthesis of a protein molecule.
The role of nucleotides in the body
Nucleotides in the cell perform a number of important functions:
• are used as building blocks for nucleic acids (nucleotides of the purine and pyrimidine series);
• are involved in many metabolic processes in the cell;
• are part of ATP - the main source of energy in cells;
• act as carriers of reducing equivalents in cells (NAD+, NADP+, FAD, FMN);
• perform the function of bioregulators;
• can be considered as second messengers extracellular regular synthesis (for example, cAMP or cGMP).
Nucleotide is a monomeric unit that forms more complex compounds - nucleic acids, without which the transfer of genetic information, its storage and reproduction is impossible. Free nucleotides are the main components involved in signaling and energy processes that support the normal functioning of cells and the body as a whole.
