DNA molecule: levels of structural organization

A DNA molecule is a polynucleotide whose monomer units are four deoxyribonucleotides (dAMP, dGMP, dCMF and dTMP). The relationship and sequence of these nucleotides in the DNA of different organisms are different. In addition to the main nitrogenous bases, there are other deoxyribonucleotides in the DNA with minor bases: 5-methylcytosine, 5-hydroxymethylcytosine, 6-methylaminopurine.

After the possibility of using the X-ray crystallography method for studying biological macromolecules and obtaining perfect X-ray patterns, it was possible to clarify the molecular structure of DNA. This 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, their location in space. In the 40s of the last century, a theory was advanced about the three-dimensional structure of the DNA molecule. U. Astbury proved that deoxyribonucleic acid is a stack of superimposed planar nucleotides.

The primary structure of the DNA molecule

By the primary structure of nucleic acids is meant the sequence of the arrangement of nucleotides in the polynucleotide chain of DNA. Nucleotides are linked together by phosphodiester bonds that are formed between the OH group at position 5 of the deoxyribose of one nucleotide and the OH group at the 3 position of the pentose of the other.

The biological properties of nucleic acids are determined by the qualitative relationship 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 (Г + Ц) / (А + Т). Using the specificity factor, the degree of heterogeneity of the nucleotide composition of DNA in organisms of various origins was determined. Thus, in higher plants and animals, the ratio (T + U) / (A + T) varies insignificantly 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 a given biological species contain DNA of the same nucleotide composition, that is, one can say that the DNA of one species is identical in content of HS pairs of bases.

The determination of the heterogeneity of the DNA nucleotide composition by the specificity coefficient does not yet provide information on its biological properties. The latter is due to a different sequence of individual nucleotide sites in the polynucleotide chain. This means that genetic information in DNA molecules is encoded in a specific sequence of its monomeric units.

The DNA molecule contains nucleotide sequences intended for initiation and termination of DNA synthesis (replication), RNA synthesis (transcription), 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 areas that protect the molecule from the action of nucleases.

The problem of the nucleotide sequence of DNA has not been fully resolved to date. Determination of the nucleotide sequence of nucleic acids is a laborious procedure involving the application of a method of specific nuclease cleavage of molecules into separate fragments. To date, the complete nucleotide sequence of nitrogen bases has been established for most tRNAs of different origins.

DNA molecule: secondary structure

Watson and Crick designed a model of the double helix of deoxyribonucleic acid. According to this model, two polynucleotide chains wrap around one another, thus forming a kind of spiral.

Nitrogen bases in them are located inside the structure, and the phosphodiester backbone is outside.

DNA molecule: tertiary structure

The linear DNA in the cell has the shape of an elongated molecule, it is packed into a compact structure and occupies only 1/5 of the cell's volume. For example, the length of human chromosome DNA reaches 8 cm, and is packed so that it fits in a chromosome with a length of 5 nm. This packing is possible due to the presence of spiralized DNA structures. From this it follows that a double-stranded DNA helix in space can undergo further folding into a certain tertiary structure - superhelix. Superspiral conformation of DNA is characteristic of chromosomes of higher organisms. Such a tertiary structure is stabilized by covalent bonds with the amino acid residues that make up the proteins that form the nucleoprotein complex (chromatin). Consequently, the DNA of eukaryotic cells is associated with proteins of mainly basic nature - histones, as well as acidic proteins and phosphoproteins.