FIELD: cytology and molecular biology. SUBSTANCE: invention provides compositions for stabilizing polynucleic acids and to increase capability thereof of penetrating through cellular membranes and manifesting their activity inside cells. A novel polynucleotide composition contains covalently modified polynucleotide or nucleic acid molecule and copolymer of polymer segments, wherein those segments include (i) at least one polyether segment, which represents ethylene oxide polymer or copolymer or block copolymer of the latter with isopropylene oxide, each of indicated polyether segments containing from about 5 to approximately 400 monomer units, and (ii) at least one polycationic segment representing cationic homopolymer, copolymer or block copolymer, which could be reaction products of at least three amino-containing monomers or quaternary salts thereof, indicated amino- containing monomers containing: identical or different units of formula -NH-R°- wherein R° represents straight-chain alkylene with 2 to 6 carbon atoms; cationic amino acid; motif (-OPO(NH-R9-NH2)O-R8-), wherein R9 represents straight-chain aliphatic group with 1 to 12 carbon atoms and R8 is group -(CH2)n-CH(R13)-, in which n is integer from 0 to 5 and R13 hydrogen, C3-C8-cycloalkyl, or C1-C6-alkyl; or 4- vinylpyridine. EFFECT: enhanced desired properties of composition. 15 cl, 23 ex
