All life on the planet consists of many cellsmaintain the orderliness of their organization due to the genetic information contained in the nucleus. It is preserved, realized and transmitted by complex high-molecular compounds - nucleic acids consisting of monomeric units - nucleotides. The role of nucleic acids can not be overemphasized. The stability of their structure determines the normal vital activity of the organism, and any deviations in the structure inevitably lead to a change in the cellular organization, the activity of physiological processes and the viability of the cells as a whole.
The concept of nucleotide and its properties
Each DNA or RNA molecule is assembled from oversmall monomeric compounds - nucleotides. In other words, a nucleotide is a building material for nucleic acids, coenzymes and many other biological compounds that are urgently needed by a cell in the course of its vital activity.
The main properties of these essential substances include:
• storing information about the structure of the protein and inherited traits;
• controlling growth and reproduction;
• participation in the metabolism and many other physiological processes occurring in the cell.
The composition of the nucleotide
Speaking of nucleotides, it is impossible not to 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.
The 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 - ribose. Each molecule of pentose has five carbon atoms, four of them, together with the oxygen atom, form a five-membered ring, and the fifth one belongs to the HO-CH2 group.
The position of each carbon atom in the moleculepentoses are denoted by the Arabic numeral with a stroke (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 location in the ring serve as a kind of indication of the correct direction.
On the hydroxyl group, the phosphoric acid residue is attached to the third and fifth carbon atoms (3C´ and 5C´). It determines the chemical affiliation of DNA and RNA to the group of acids.
A nitrogenous base is attached to the first carbon atom (1C´) in the sugar molecule.
The species composition of nitrogenous bases
DNA nucleotides on the nitrogenous base are represented by four types:
• adenine (A);
• guanine (D);
• cytosine (C);
• thymine (T).
The first two belong to the class of purines, the last two - pyrimidines. Molecular weight purine is always heavier than pyrimidine.
RNA nucleotides on the nitrogenous base are presented:
• adenine (A);
• guanine (D);
• cytosine (C);
• uracil (V).
Uracil, like thymine, is a pyrimidine base.
In scientific literature it is often possible to find another designation of nitrogenous bases - in Latin letters (A, T, C, G, U).
Let us dwell 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) are composed of pyrimidine andimidazole or two heterocycles. The molecule of purine bases is represented by four nitrogen atoms and five carbon atoms. Each atom is numbered from 1 to 9.
As a result of combining the nitrogenous base and the pentose residue, a nucleoside is formed. A nucleotide is a compound of a nucleoside and a phosphate group.
The formation of phosphodiester bonds
It is important to understand the question of how nucleotides join into a polypeptide chain and form a nucleic acid molecule. This happens due to the so-called phosphodiester bonds.
The interaction of two nucleotides gives dinucleotide.The formation of a new compound occurs by condensation, when a phosphodiester bond occurs between the phosphate residue of one monomer and the hydroxyl group of the pentose of the other.
Polynucleotide synthesis - repeated repetitionthis reaction (several million times). The polynucleotide chain is constructed through the formation of phosphodiester bonds between the third and fifth carbons of sugars (3C´ and 5C´).
The assembly of a polynucleotide is a complex process involving the participation of the enzyme DNA polymerase, which ensures chain growth from only one end (3´) with a free hydroxy group.
DNA molecule structure
A DNA molecule, as well as a protein, can have a primary, secondary and tertiary structure.
Nucleotide sequence in the DNA chaindefines 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 regularity acts: adenine of one chain corresponds to thymine of another, guanine to cytosine, and vice versa. Pairs of adenine and thymine or guanine and cytosine are formed by two hydrogen bonds in the first and three in the latter case. Such a combination of nucleotides provides a strong connection between the chains and an equal distance between them.
Knowing the sequence of nucleotides of one DNA strand, according to the principle of complementarity or addition, one can complete the second one.
Tertiary DNA structure is formed bycomplex three-dimensional bonds, which makes its molecule more compact and able to be placed in a small cell volume. For example, the length of the E. coli DNA is more than 1 mm, while the cell length is less than 5 microns.
The number of nucleotides in the DNA, namely theirquantitative ratio, subject to the rule Chergaffa (the number of purine bases is always equal to the number of pyrimidine). The distance between the nucleotides is a constant value equal to 0.34 nm, as well as their molecular weight.
RNA molecule structure
RNA is represented by a single polynucleotide strand,formed through covalent bonds between pentose (in this case, ribose) and the phosphate residue. In length, it is much shorter than DNA. The species composition of nitrogenous bases in the nucleotide also has differences. 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 3000up to 5000 nucleotides. As a necessary structural component, it participates in the formation of the active center of the ribosome, the site of one of the most important processes in the cell, protein biosynthesis.
• Транспортная (тРНК) – состоит в среднем из 75 - 95 nucleotides, carries the transfer of the desired amino acid to the site of synthesis of the polypeptide in the ribosome. Each type of tRNA (at least 40) has its own, unique sequence of monomers or nucleotides.
• Informational (mRNA) - on the nucleotide composition is very diverse. It 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
The nucleotides in the cell perform a number of important functions:
• are used as building blocks for nucleic acids (purine and pyrimidine nucleotides);
• participate in many metabolic processes in the cell;
• are part of ATP - the main source of energy in the cells;
• act as carriers of reducing equivalents in cells (NAD +, NADP +, FAD, FMN);
• perform the function of bioregulators;
• may be considered as second messengers of extracellular regular synthesis (for example, cAMP or cGMP).
A nucleotide is a monomeric unit that formsmore complex compounds are 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 vital activity of the cells and the organism as a whole.
