Monomer of DNA. Which monomers form a DNA molecule?

Nucleic acids, especially DNA, are well known in science. This is explained by the fact that they are the substances of the cell on which the storage and transfer of her hereditary information depends. DNA, discovered as far back as 1868 by F. Micher, is a molecule with pronounced acid properties. The scientist identified it from the nuclei of leukocytes - the cells of the immune system. During the next 50 years, nucleic acid studies were carried out sporadically, since most biochemists were considered to be the main organic substances responsible, in particular, for hereditary traits, proteins.

Since the decoding of the DNA structure carried out by Watson and Crick in 1953, serious studies have begun that have shown that deoxyribonucleic acid is a polymer and DNA monomers are nucleotides. Their types and structure will be studied by us in this paper.

Nucleotides as structural units of hereditary information

One of the fundamental properties of living matter is the preservation and transmission of information about the structure and functions of both the cell and the whole organism. This role is performed by deoxyribonucleic acid, and DNA monomers - nucleotides are original "bricks", of which a unique construction of the substance of heredity is constructed. Let's consider what signs guided the living nature, creating a superhelix of nucleic acid.

How nucleotides are formed

To answer this question, we need some knowledge from the field of the chemistry of organic compounds. In particular, we recall that in nature there is a group of nitrogen-containing heterocyclic glycosides, connected with monosaccharides - pentoses (deoxyribose or ribose). They are called nucleosides. For example, adenosine and other types of nucleosides are present in the cytosol of the cell. They enter into an esterification reaction with molecules of orthophosphoric acid. The products of this process will be nucleotides. Each DNA monomer, and four of them, has a name, for example, guanine, thymine and cytosine nucleotide.

Purine monomers of DNA

In biochemistry, a classification is adopted that separates DNA monomers and their structure into two groups: for example, purine is adenine and guanine nucleotides. They contain in their composition derivatives of purine - an organic substance having the formula C ₅ H ₄ N ₄ . The DNA monomer, the guanine nucleotide, also contains a purine nitrogen base coupled to the deoxyribose N-glycosidic linkage in the concrete configuration.

Pyrimidine nucleotides

Nitrogen bases, called cytidine and thymidine, are derivatives of the organic substance of pyrimidine. Its formula is C ₄ H ₄ N ₂ . The molecule is a six-membered flat heterocycle containing two nitrogen atoms. It is known that instead of the thymine nucleotide in ribonucleic acid molecules, such as rRNA, tRNA, and mRNA, a uracil monomer is contained. In the process of transcription, during the deletion of information from the DNA gene to the mRNA molecule, the thymine nucleotide is replaced by adenine, and the adenine nucleotide is replaced by uracil in the synthesized mRNA chain. That is, the following entry will be fair: A - Y, T - A.

The Chargaff Rule

In the previous section, we have already touched on the principles of the correspondence of monomers in DNA strands and in the gene-iRNA complex. The well-known biochemist E.Chargaff has established a completely unique property of deoxyribonucleic acid molecules, namely, that the amount of adenine nucleotides in it is always equal to thymine, and guanine - to cytosine. The main theoretical basis of the principles of Chartaff was the research of Watson and Crick, who established which monomers form a DNA molecule and what spatial organization they have. Another regularity, deduced by Chargaff and called the principle of complementarity, indicates the chemical affinity of purine and pyrimidine bases and their ability to interact with each other to form hydrogen bonds. This means that the arrangement of monomers in both DNA chains is strictly determined: for example, on the other hand, there can be only T of the first DNA chain and between the two hydrogen bonds. In contrast to the guanine nucleotide, only the cytosine can be located. In this case, between the nitrogenous bases, three hydrogen bonds are formed.

Role of nucleotides in the genetic code

To carry out the reaction of protein biosynthesis occurring in ribosomes, there is a mechanism for translating information about the amino acid composition of the peptide from the sequence of nucleotides of the mRNA into the amino acid sequence. It turned out that three adjacent monomers carry information about one of the 20 possible amino acids. This phenomenon was called the genetic code. In solving problems in molecular biology, it is used to determine both the amino acid composition of a peptide and to clarify the question: which monomers form a DNA molecule, in other words, what is the composition of the corresponding gene. For example, the triplet (codon) AAA in the gene encodes the amino acid phenylalanine in the protein molecule, and in the genetic code it will correspond to the triplet UUU in the mRNA chain.

Interaction of nucleotides during DNA reduplication

As it was explained earlier, structural units, DNA monomers are nucleotides. Their specific sequence in chains is the matrix for the synthesis of the daughter molecule of deoxyribonucleic acid. This phenomenon occurs in the S-phase of the cell's interphase. The sequence of nucleotides of the new DNA molecule is assembled on the mother circuits by the enzyme of the DNA polymerase, taking into account the principle of complementarity (A-T, D-C). Replication refers to the reactions of matrix synthesis. This means that the DNA monomers and their structure in the mother circuits are the basis, that is, the matrix for its daughter copy.

Can the structure of the nucleotide change

By the way, let's say that deoxyribonucleic acid is a very conservative structure of the cell nucleus. This is a logical explanation: the hereditary information stored in the core chromatin should be unchanged and copied without distortion. Well, the cellular genome is constantly "under the gun" of environmental factors. For example, such aggressive chemical compounds as alcohol, drug, radioactive radiation. All of them are so-called mutagens, under the influence of which any monomer of DNA can change its chemical structure. Such distortion in biochemistry is called a point mutation. The frequency of their appearance in the genome of the cell is quite high. Mutations are corrected by well-functioning work of the cellular repair system, which includes a set of enzymes.

Some of them, for example restriction enzymes, "cut out" the damaged nucleotides, polymerases ensure the synthesis of normal monomers, ligases "sew" the restored sections of the gene. If the above mechanism for some reason does not work in the cell and the defective DNA monomer remains in its molecule, the mutation is picked up by matrix synthesis processes and phenotypically manifested as proteins with impaired properties that are unable to perform the necessary functions inherent in them in the cellular metabolism. This is a serious negative factor, which reduces the viability of the cell and shortens the duration of its life.