Nucleic acid is a long-chain polynucleotide compound with no side chain formed by connecting nucleotides through 3′,5′- phosphodiester bond.At first, small molecules It developed out of control and gradually opened up a sky of its own. https://www.all-chemistry.com/

　　Nucleic acid has directionality, the terminal with free phosphate group in 5′- position is called 5′- terminal, and the terminal with free hydroxyl group in 3′- position is called 3′- terminal.

　　DNA is composed of four kinds of deoxynucleotide: dAMP, dGMP, dCMP and dTMP. Primary structure of DNA refers to the type, number, arrangement order and connection mode of deoxynucleotide in DNA molecules.

　　RNA consists of four ribonucleotides: AMP, GMP, CMP and UMP. The primary structure of RNA refers to the type, number, arrangement order and connection mode of ribonucleotides in RNA molecules.