The nucleotide is what? The composition, structure, number and sequence of nucleotides in the DNA chain

All living things on the planet consist of a multitude of cells that support the orderliness of their organization at the expense of 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 the physiological processes and the viability of the 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 extremely necessary for the cell in the process of its vital activity.

The main properties of these irreplaceable substances include:

• storage of information on protein structure and inherited characteristics;
• control over growth and reproduction;
• participation in the metabolism and many other physiological processes taking place in the cell.

Nucleotide composition

Speaking about nucleotides, one can not help but dwell on such an important issue as their structure and composition.

Each nucleotide consists of:

• sugar residue;
• nitrogen base;
• a phosphate group or a phosphoric acid residue .

It can be said that a nucleotide is a complex organic compound. Depending on the species composition of the nitrogen bases and the type of pentose in the nucleotide structure, the nucleic acids are subdivided into:

• deoxyribonucleic acid, or DNA;
• Ribonucleic acid, or RNA.

The composition of nucleic acids

In nucleic acids, the sugar is 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 an oxygen atom form a five-membered ring, and the fifth is in the group HO-CH2.

The position of each carbon atom in the pentose molecule is denoted by an Arabic numeral with a prime (1C', 2C', 3C', 4C', 5C'). Since all the processes of reading the hereditary information from the nucleic acid molecule have a strict orientation, the numbering of carbon atoms and their arrangement in the ring serves as a kind of indicator of the correct direction.

On the hydroxyl group, a phosphoric acid residue is attached to the third and fifth carbon atoms (3C 'and 5C'). He determines the chemical affiliation of DNA and RNA to the group of acids.

The nitrogen base is attached to the first carbon atom (1C ') in the sugar molecule.

Species composition of nitrogenous bases

Nucleotides of DNA along the nitrogenous base are represented by four types:

• adenine (A);
• with guanine (D);
• cytosine (C);
• Thymine (T).

The first two belong to the class of purines, the latter two belong to pyrimidines. By molecular weight, purine is always heavier than pyrimidine.

RNA nucleotides on the nitrogenous base are:

• adenine (A);
• with guanine (D);
• cytosine (C);
• uracil (Y).

Uracil, just like thymine, is a pyrimidine base.

In the scientific literature it is often possible to find another designation of nitrogenous bases - Latin letters (A, T, C, G, U).

More details on the chemical structure of purines and pyrimidines.

Pyrimidines, namely cytosine, thymine and uracil, in their composition 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 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 the connection of the nitrogenous base and the pentose residue, a nucleoside is formed. A nucleotide is a compound of a nucleoside and a phosphate group.

Formation of phosphodiester bonds

It is important to understand the question of how nucleotides are connected to a polypeptide chain and form a nucleic acid molecule. This is due to the so-called phosphodiester bonds.

The interaction of two nucleotides yields a dinucleotide. The formation of a new compound occurs by condensation when a phosphodiester bond occurs between the phosphate moiety of one monomer and the hydroxy group of the pentose of the other.

Synthesis of polynucleotide - repeated repetition of this reaction (several million times). The polynucleotide chain is constructed by forming phosphodiester bonds between the third and fifth carbohydrates of sugars (3C 'and 5C').

The assembly of the polynucleotide is a complex process that involves the enzyme of the DNA polymerase, which provides chain growth at only one end (3 ') with a free hydroxy group.

Structure of the DNA molecule

A DNA molecule, as well as a protein, can have a primary, secondary, and tertiary structure.

The sequence of nucleotides in the DNA chain determines its primary structure. The secondary structure is formed due to hydrogen bonds, the basis of which is the principle of complementarity. In other words, in the synthesis of the double helix of DNA, there is a definite regularity: adenine of one chain corresponds to thymine of another, guanine to cytosine, and vice versa. Couples of adenine and thymine or guanine and cytosine are formed due to two in the first and three in the latter case of hydrogen bonds. Such a nucleotide compound provides a strong bond 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 complementation, the second one can be completed.

Tertiary DNA structure is formed due to complex three-dimensional bonds, which makes its molecule more compact and capable of being located in a small cell volume. For example, the length of the E. coli DNA is more than 1 mm, whereas the cell length is less than 5 μm.

The number of nucleotides in DNA, namely their quantitative ratio, obeys the Chergaff rule (the number of purine bases is always equal to the amount of pyrimidine bases). The distance between nucleotides is a constant value of 0.34 nm, like their molecular weight.

Structure of the RNA molecule

RNA is represented by a single polynucleotide chain formed through covalent bonds between pentose (in this case ribose) and a phosphate residue. In length, it is much shorter than DNA. There are also differences in the species composition of the nitrogenous bases in the nucleotide. RNA instead of the pyrimidine base of thymine is used by uracil. Depending on the functions performed in the body, RNA can be of three types.

• Ribosomal (rRNA) - usually contains from 3,000 to 5,000 nucleotides. As the necessary structural component takes part in the formation of the active center of the ribosomes, the location of one of the most important processes in the cell is protein biosynthesis.
• Transport (tRNA) - consists on the average of 75 - 95 nucleotides, carries out the transfer of the desired amino acid to the place of synthesis of the polypeptide in the ribosome. Each type of tRNA (at least 40) has its own sequence of monomers or nucleotides inherent in it.
• Information (mRNA) - by nucleotide composition is very diverse. It transfers genetic information from DNA to ribosomes, acts as a matrix for the synthesis of a protein molecule.

Role of nucleotides in the body

Nucleotides in the cell perform a number of important functions:

• are used as structural blocks for nucleic acids (nucleotides of purine and pyrimidine series);
• participate in many metabolic processes in the cell;
• are part of ATP - the main source of energy in cells;
• act as carriers of replacement equivalents in cells (NAD +, NADP +, FAD, FMN);
• perform the function of bioregulators;
• can be considered as second messengers of extracellular regular synthesis (eg, cAMP or cGMP).

A nucleotide is a monomeric unit that forms more complex compounds-nucleic acids, without which it is impossible to transfer genetic information, store it, and reproduce it. Free nucleotides are the main components involved in signal and energy processes that support the normal vital activity of cells and the organism as a whole.