What groups form the middle of a nucleic acid chain?

The middle of a nucleic acid chain is primarily composed of sugar and phosphate groups. In DNA, the sugar is deoxyribose, while in RNA, it is ribose. These sugars are linked by phosphate groups, forming the backbone of the nucleic acid strand, with nitrogenous bases attached to the sugars. This structure allows for the formation of the double helix in DNA and the single-stranded configuration in RNA.