Nucleic acids, especially DNA, are quite well known in science. This is explained by the fact that they are the substances of the cell, on which the storage and transmission of its hereditary information depends. DNA, discovered back in 1868 by F. Miescher, is a molecule with pronounced acidic properties. The scientist isolated it from the nuclei of leukocytes - cells of the immune system. Over the next 50 years, studies of nucleic acids were carried out sporadically, since most biochemists considered proteins to be the main organic substances responsible, among other things, for hereditary traits.
Since the deciphering of the structure of DNA by Watson and Crick in 1953, serious research began, which found out that deoxyribonucleic acid is a polymer, and nucleotides serve as DNA monomers. Their types and structure will be studied by us in this work.
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 organismgenerally. This role is played by deoxyribonucleic acid, and DNA monomers - nucleotides are a kind of "bricks" from which the unique structure of the substance of heredity is built. Let's consider what signs wildlife was guided by when creating a nucleic acid supercoil.
How nucleotides are formed
To answer this question, we need some knowledge of organic chemistry. In particular, we recall that in nature there is a group of nitrogen-containing heterocyclic glycosides combined with monosaccharides - pentoses (deoxyribose or ribose). They are called nucleosides. For example, adenosine and other types of nucleosides are present in the cytosol of a cell. They enter into an esterification reaction with orthophosphoric acid molecules. The products of this process will be nucleotides. Each DNA monomer, and there are four types, has a name, such as guanine, thymine, and cytosine nucleotides.
Purine monomers of DNA
In biochemistry, a classification is adopted that divides DNA monomers and their structure into two groups: for example, adenine and guanine nucleotides are purine. They contain derivatives of purine, an organic substance with the formula C5H4N44
. The DNA monomer, a guanine nucleotide, also contains a purine nitrogenous base connected to deoxyribose by an N-glycosidic bond in the beta configuration.
Pyrimidine nucleotides
Nitrogenous bases,called cytidine and thymidine, are derivatives of the organic substance pyrimidine. Its formula is C4H4N2. The molecule is a six-membered planar heterocycle containing two nitrogen atoms. It is known that instead of a thymine nucleotide, ribonucleic acid molecules, such as rRNA, tRNA, and mRNA, contain a uracil monomer. During transcription, during the transfer 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 record will be fair: A - U, T - A.
Chargaff rule
In the previous section, we have already partially touched on the principles of correspondence between monomers in DNA chains and in the gene-mRNA complex. The famous biochemist E. Chargaff established a completely unique property of deoxyribonucleic acid molecules, namely, that the number of adenine nucleotides in it is always equal to thymine, and guanine - to cytosine. The main theoretical basis of Chargaff's principles was the research of Watson and Crick, who established which monomers form the DNA molecule and what spatial organization they have. Another pattern, derived by Chargaff and called the principle of complementarity, indicates the chemical relationship of purine and pyrimidine bases and their ability to form hydrogen bonds when interacting with each other. This means that the arrangement of monomers in both DNA strands is strictly determined: for example, opposite A of the first DNA strand can beonly T is different and two hydrogen bonds arise between them. Opposite the guanine nucleotide, only cytosine can be located. In this case, three hydrogen bonds form between the nitrogenous bases.
The role of nucleotides in the genetic code
To carry out the reaction of protein biosynthesis occurring in ribosomes, there is a mechanism for transferring information about the amino acid composition of the peptide from the mRNA nucleotide sequence into the amino acid sequence. It turned out that three adjacent monomers carry information about one of the 20 possible amino acids. This phenomenon is 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 AAA triplet (codon) in the gene encodes the amino acid phenylalanine in the protein molecule, and in the genetic code it will correspond to the UUU triplet in the mRNA chain.
Interaction of nucleotides in the process of DNA reduplication
As it was found out earlier, structural units, DNA monomers are nucleotides. Their specific sequence in the chains is the template for the process of synthesis of the daughter molecule of deoxyribonucleic acid. This phenomenon occurs in the S-stage of cell interphase. The nucleotide sequence of a new DNA molecule is assembled on the parent chains under the action of the DNA polymerase enzyme, taking into account the principlecomplementarity (A - T, D - C). Replication refers to the reactions of matrix synthesis. This means that the DNA monomers and their structure in the parent chains serve as the basis, that is, the matrix for its child copy.
Can the structure of a nucleotide change
By the way, let's say that deoxyribonucleic acid is a very conservative structure of the cell nucleus. There is a logical explanation for this: the hereditary information stored in the chromatin of the nucleus must 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, drugs, radioactive radiation. All of them are so-called mutagens, under the influence of which any DNA monomer can change its chemical structure. Such a distortion in biochemistry is called a point mutation. The frequency of their occurrence in the cell genome is quite high. Mutations are corrected by the well-functioning work of the cellular repair system, which includes a set of enzymes.
Some of them, for example, restrictases, "cut out" damaged nucleotides, polymerases provide the synthesis of normal monomers, ligases "sew" the restored sections of the gene. If, for some reason, the mechanism described above does not work in the cell and the defective DNA monomer remains in its molecule, the mutation is picked up by the processes of matrix synthesis and phenotypically manifests itself in the form of proteins with impaired properties that are unable to perform the necessary functions inherent in them incellular metabolism. This is a serious negative factor that reduces the viability of the cell and shortens its lifespan.
