From the physical and chemical point of view, the fundamental reason why nucleic acid bases are complementary is that bases are heterocyclic aromatic molecules. The aromaticity of bases ensures that the atoms on all base heterocyclic rings are * * * planes, and the base accumulation phenomenon will occur between two adjacent bases on the same chain through interaction (that is to say, the so-called base accumulation force is actually the overlap of electron clouds between two layers of aromatic rings), which ensures that the bases in the double-stranded or multi-stranded structure of nucleic acids can be arranged in layers and orderly. The highly electronegative nitrogen and oxygen atoms on the heterocyclic ring ensure that the base can provide enough hydrogen bond donor and acceptor atoms, so that two or more hydrogen bonds can be formed between the bases in the same layer, so that the bases in the same layer can be paired. In fact, base pairing is very diverse. Middle school only teaches A-T C-G cis-watson-crick pairing. However, in addition to this classic pairing form, there are many non-classic pairing forms. For details, please refer to Zhihu column of @ Leng Yeo. As for the phosphoribosyl skeleton, it is not the key to determine whether nucleic acids can complement each other. In fact, scientists have tried to modify the phosphoribosyl skeleton to make it unrecognizable (such as peptide nucleic acid and threone nucleic acid). Suppose there is an arabinose nucleic acid, except that the pentasaccharide is arabinose, and its primary structure is the same as RNA. What is the impact on its high-level structure? ), you can still base pair.

The situation in RNA will be much more complicated, so consider DNA first. It seems to be very "coincidence" that the bases in DNA are "just" dominated by AT/CG complementary pairing, while other forms are not dominated by free energy. It is probably meaningless to simply discuss the ontology of free energy of nucleic acid pairing, because it can only explain "born this way". Let's simply consider it from two aspects: metabolism and evolution.