The so-called primary structure of DNA refers to the connection and arrangement order of four nucleotides, which indicates the chemical composition of DNA molecules. DNA not only has strict chemical composition, but also has a special advanced structure, which mainly exists in the form of regular double helix. Basic characteristics of extended data dna molecules

1, the DNA molecule is formed by coiling two parallel deoxynucleotide chains.

2. deoxyribose and phosphoric acid in 2.DNA molecules are alternately connected and arranged on the outside to form the basic skeleton, and bases are arranged on the inside.

3. The' bases on the two chains form base pairs through hydrogen bonding, and their composition has certain rules. This is the pairing of purine and pyrimidine. Adenine (a) can only be paired with thymine (t), and guanine (g) can only be paired with cytosine (c). If the base on one chain is C, then the base paired with it on the other chain must be G. This one-to-one correspondence between bases is called the principle of base complementary pairing. Although there are only four bases that make up DNA molecules, and their pairing methods are only A and T, C and G, the bases can be arranged at will, which constitutes the diversity of DNA molecules.