THE STRUCTURE OF NUCLEIC ACIDS

The primary structure of nucleic acids is the sequence of mononucleotides in the polynucleotide chain of DNA and RNA. Monomers in nucleic acid molecules are connected by an ester bond formed by the phosphate residue of one mononucleotide and 3'-hydroxyl group of the pentose residue of another mononucleotide ( 3', 5'-phosphodiester bond ) (fig. 8). To study the chemical composition of NA sequencing is used. It is NA splitting into fragments by enzymes or chemical reagents, and analysis of the products by electrophoresis, chromatography, etc.

Fig. 8. The primary structure of DNA

Secondary structure. DNA is composed of two strands, forming a double helix in which the two polynucleotide chains twisted around the same axis. The bases are inside, and carbohydrate components are outside (fig. 9). Bases are stacked in pairs: purine from the one strand and pyrimidine - from another. The interaction of pairs of A-T and G-C called complementarity, and the corresponding bases - complementary. The stability of A - T pairs is provided by two hydrogen bonds, and pairs of G - C - by three hydrogen bonds. DNA strands are complementary to each other.

The forces of hydrophobic interactions ( stacking interactions ) occure between the bases, assembled in a stack along the DNA molecule. They make a great contribution to the stabilization of the double helix. The intensity of stacking:

Both chains in the DNA molecule have opposite polarity: internucleotide bond in one chain has a direction of 5' ® 3', the other - 3'®5'.

Fig. 9. Secondary structure of DNA
Fig. 10. Complementary of strands in DNA.

The configuration of the double helix of DNA varies from the quantity of water and ionic strength of solution. Existence more than 10 forms of DNA was established by X-ray analysis.

RNA molecules are constructed from a single polynucleotide chain. In this chain there are complementary areas, which form a double helix. At the same time the pair A - U and G - C are connected by hydrogen bonds. Helical regions of RNA ( hairpin ) contain 20-30 base pairs and alternate with non-helical parts.

R. Holley suggested two-dimensional cloverleaf model for t-RNA. There is a helix polynucleotide chain on itself in strictly fixed locations. Features of the t-RNA structure are directly related to the processes of translation, so they are considered in more detail in the section of protein biosynthesis.

The secondary structure of the messenger and ribosomal RNA is not studied as well. Secondary structure of r-RNA and m-RNA is characterized by helix formation on itself.

Some nucleotide sequences in the secondary structure of DNA and RNA are called palindromic (inverted repeations). These repeations are the basis for the formation of hairpins or triple helices.

The tertiary structure of nucleic acids. DNA double helix in some areas may be further coiling to form a superhelix. Some viruses detected single-stranded DNA of linear and circular forms. The tertiary structure of the m-RNA and t-RNA is packed more compact by folding of various parts of the molecule.

Nucleic acids are parts of nucleoproteins (NP). They perform functions of storage and transmission of genetic information.

There are 2 types of NP, which differ from each other in composition, size, and physical and chemical properties - deoxyribonucleoproteins (DNP, contain DNA) and ribonucleoproteins (RNP, contain RNA). DNP predominantly located in the cells nuclei and PNR - in the cytoplasm. Typical representatives of the NP are ribosome (ribosomal RNA complexes with proteins). DNP-chromatin is the complex of DNA with histone and non-histone proteins.

Histones are strongly alkaline proteins of low molecular weight, soluble in dilute acids. They contain large amounts of lysine and arginine. 5 classes of histones vary in size, amino acid composition and amount of charge (always positive). They take part in the structural organization of chromatin by neutralizing negatively charged phosphate groups of DNA by positive charges of amino acid residues.

The nature of non-histone proteins is not found out enough.

Many chromatin proteins are characterized by specific structural motifs that provide their binding to DNA: “ leucine zipper”, “ α -helix - turn - α -helix”, " zinc finger".

Test Questions

1. Give a characteristic of amino acids as structural monomers of proteins. What are the features of the peptide bond formation?

2. Give a definition of the primary structure of protein. What is the determining role of primary structure in the formation of higher levels of organization of the protein molecule?

3. What links stabilize the secondary structure of protein? What types of secondary protein structure are you know?

4. What is meant by the tertiary structure of protein? What types of bonds stabilize it?

5. Give a characteristic of quaternary structure of proteins. What are the monomers and oligomers?

6. What is the relationship of structure and function of proteins?

7. Name the major functions of proteins in the body.

8. What methods can determine the molecular weight of protein?

9. Does the solubility of proteins depend from their amino acid composition?

10. What is meant by denaturation and renaturation of proteins? Which agents cause denaturation?

11. What is the isoelectric point of proteins?

12. How is the process of salting out of proteins use in medicine?

13. Name the functions of albumin and globulins of blood plasma.

14. Give examples of fibrous proteins.

15. Give the classification of complex proteins.

16. Consider the structure and function of hemoglobin A.

ENZYMES

Enzymes are substances mainly of protein nature, which have catalytic activity. The phenomena of fermentation and digestion have long been known. The term " enzyme" goes from the Greek En zyme - in yeast, as well as the " ferment" from Lat. Fermentatio - fermentation.

The science that studies the enzymes is called enzymology.

Although laboratory synthesis of several enzymes has carried out (ribonuclease, lysozyme), the only way to get enzymes is their excretion from biological objects.

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