If you imagine the appearance of a DNA molecule, then it resembles a twisted spiral, which is formed by two polynucleotide chains that are twisted together and simultaneously around one axis common to them.
As a rule, the structure of DNA is considered in the framework of system analysis, while the main one is a strictly established (for a normal state) order of relative position of deoxyribonucleoside monophosphates - dNMPs that make up polynucleotide chains.
In a non-mutated cell, mononucleotides are connected by phosphodiester bonds, while the ends of the polynucleotide chain themselves have two variants of arrangement of material groups: a phosphate group is located at the 5'-end of the chain, and the OH group is located at the 3'-end of the chain.
If we consider a molecule consisting of two chains, then the DNA structure will be such that the polynucleotide chains will be antiparallel to each other. At the same time, chains in such a structure will be retained due to the hydrogen bonds that exist between the bases AT and GC lying in one plane perpendicular to the main axis of the molecule’s helix. Those hydrophobic interactions that form between the bases of such a molecule ensure the stability of the entire double helix. For such a molecule, the structure of DNA is characterized by the complementarity of polynucleotide chains, but not their identity, since their nucleotide composition is different.
Further, considering what the structure of DNA is, it should be pointed out that each individual molecule is “packed” into its own, strictly unique, separate chromosome. These chromosomes contain a variety of proteins that correspond to strictly defined sequences of the structure of the DNA molecule. These proteins are classified into 2 categories: histones and non-histone proteins. In combination with the nuclear DNA of cells, these proteins are called chromatin.
Describing the structure of DNA, it should be pointed out that chromatin consists of five types of histones, the cumulative positive charge of which provides histones with a very strong bond with DNA. The histone complex and a specific section of the DNA molecule, containing 146 nucleotide pairs, interact, as a result of which nucleosomes are formed.
Non-histone proteins included in the structure of dna molecules are various types of regulatory proteins associated with certain DNA sequences. Also, the structure of the DNA molecule is supplemented by enzymes that provide biosynthesis.
Investigation of the structure of DNA and RNA in the framework of system analysis should only be done in a complex, that is, when considering the structure of DNA, it is necessary to consider the structure of RNA.
Its primary structure, as in the case of a DNA molecule, is an algorithm for alternating ribonucleoside monophosphates, and it must be borne in mind that, unlike a DNA molecule, all types of RNA have only one polynucleotide chain. In the structure of the RNA molecule, its individual chains form the so-called “hairpins” - spiral loops that are created by the bases AU and GC and are stabilized by hydrogen bonds.
As a rule, the average largest DNA molecule includes about 150 million nucleotide pairs, and its length is four centimeters. In laboratory analysis, such molecules are extremely inconvenient for research, because when it is isolated from the tissue, the molecule, as a rule, is highly fragmented and becomes much smaller in size. To eliminate this inconvenience, the PCR method is used in the studies - carrying out a polymerase chain reaction, in the framework of which selective synthesis of individual sections of the DNA molecule takes place and its necessary fragments are isolated.