Structural Biochemistry/Nucleic Acid/Connection between monomers

DNA and RNA Backbone

In macromolecules, such as DNA and RNA, there are linear polymers built and connected together by monomers. These monomers are known as nucleotides, and they consist of a nitrogenous base, a sugar, and a phosphate group. The chains and bonds between these nucleotides form the backbone of DNA and RNA, and these backbones allow the formation of unique genetic sequences. In DNA and RNA backbones, the monomers are connected by phosphodiester bridges. Specifically, the bridges are formed between the 3'-hydroxyl group of either the ribose sugar in RNA or deoxyribose sugar in DNA, and the 5'-hydroxyl group of the adjacent sugar; essentially called a 3'-5' phosphodiester bond. Chemically, to make this bond, the 3'-hydroxyl group of a sugar undergoes esterification with a phosphate group. That phosphate group then gets attacked by the 3'-hydroxyl group to form the phosphodiester bridge.

Once the phosphodiester bond is established, the backbone needs to be preserved in order to maintain the genetic information of the nucleotide sequence. Thus, no more nucleophilic attacks may occur on the backbone. In order to prevent nucleophilic attacks, the phosphate group on the phosphodiester bond has a negative charge which is used to prevent other nucleophilic species such as hydroxyl groups from attacking. The fact that DNA lacks a hydroxyl group on the 2' carbon means that it is more resistant to nucleophilic attacks, and thus, is the more stable hereditary material than RNA is.