A DNA molecule is a polynucleotide whose monomer units are four deoxyribonucleotides (dAMP, dGMP, dCMP and dTMP). The ratio and sequence of these nucleotides in the DNA of different organisms are different. In addition to the main nitrogenous bases, DNA also contains other deoxyribonucleotides with minor bases: 5-methylcytosine, 5-hydroxymethylcytosine, 6-methylaminopurine.
After it became possible to use the method of X-ray crystallography to study biological macromolecules and obtain perfect radiographs, it was possible to determine the molecular structure of DNA. The indicated method is based on the fact that a beam of parallel x-rays incident on a crystalline cluster of atoms forms a diffraction pattern, which mainly depends on the atomic mass of these atoms and their location in space. In the 40s of the last century, a theory was put forward on the three-dimensional structure of a DNA molecule. W. Astbury proved that deoxyribonucleic acid is a stack of superimposed planar nucleotides.
The primary structure of a DNA molecule
By the primary structure of nucleic acids is meant the sequence of nucleotides in the polynucleotide DNA chain. Nucleotides bind to each other via phosphodiester bonds, which are formed between the OH group at position 5 of the deoxyribose of one nucleotide and the OH group at position 3 of the pentose of the other.
The biological properties of nucleic acids are determined by the qualitative ratio and sequence of nucleotides along the polynucleotide chain.
The nucleotide composition of DNA in organisms of different taxonomic groups is specific and is determined by the ratio (G + C) / (A + T). Using the coefficient of specificity, the degree of heterogeneity of the nucleotide composition of DNA in organisms of various origins was determined. So, in higher plants and animals, the ratio (G + C) / (A + T) varies slightly and has a value greater than 1. For microorganisms, the specificity coefficient varies over a wide range - from 0.35 to 2.70. At the same time, somatic cells of this biological species contain DNA of the same nucleotide composition, i.e., it can be said that the DNA content of one species is identical in the content of HC pairs of bases.
Determining the heterogeneity of the nucleotide composition of DNA by the specificity coefficient does not yet provide information on its biological properties. The latter is due to the different sequence of individual nucleotide regions in the polynucleotide chain. This means that the genetic information in the DNA molecules is encoded in a specific sequence of its monomeric units.
A DNA molecule contains nucleotide sequences designed to initiate and terminate DNA synthesis processes (replication), RNA synthesis (transcription), and protein synthesis (translation). There are nucleotide sequences that serve to bind specific activating and inhibitory regulatory molecules, as well as nucleotide sequences that do not carry any genetic information. There are also modified regions that protect the molecule from the action of nucleases.
The problem of the nucleotide sequence of DNA has not yet been completely resolved. Determining the nucleotide sequence of nucleic acids is a time-consuming procedure involving the use of the method of specific nuclease cleavage of molecules into individual fragments. To date, the complete nucleotide sequence of nitrogen bases has been established for most tRNAs of different origin.
DNA molecule: secondary structure
Watson and Crick designed the deoxyribonucleic acid double helix model . According to this model, two polynucleotide chains encircle each other, and a peculiar spiral is formed.
The nitrogenous bases in them are located inside the structure, and the phosphodiester core is outside.
DNA molecule: tertiary structure
Linear DNA in the cell has the shape of an elongated molecule, it is packaged in a compact structure and occupies only 1/5 of the cell volume. For example, the length of the DNA of a human chromosome reaches 8 cm, and is packed so that it fits on the chromosome with a length of 5 nm. Such folding is possible due to the presence of spiral DNA structures. It follows from this that the double-stranded DNA helix in space can undergo further folding into a certain tertiary structure - the supercoil. The supercoiled DNA conformation is characteristic of the chromosomes of higher organisms. Such a tertiary structure is stabilized due to covalent bonds with amino acid residues that make up the proteins that form the nucleoprotein complex (chromatin). Therefore, the DNA of eukaryotic cells is associated with proteins of a mainly basic nature - histones, as well as acidic proteins and phosphoproteins.