Single strands of nucleic acids are directional, having two chemically distinct ends defined by specific functional groups attached to the sugar component of the nucleotide.
- The 5′ end of the strand is characterized by a free phosphate group attached to the 5′ carbon of the sugar (ribose in RNA or deoxyribose in DNA). This phosphate group is what links nucleotides together via phosphodiester bonds to the 3′ hydroxyl of the adjacent nucleotide
- The 3′ end has a free hydroxyl group (-OH) attached to the 3′ carbon of the sugar. This 3′ hydroxyl is essential for the addition of new nucleotides during nucleic acid synthesis, as new nucleotides are added to this end
Thus, the directionality of a nucleic acid strand is defined by the phosphate group at the 5′ end and the hydroxyl group at the 3′ end. This polarity is crucial for biological processes such as DNA replication and transcription, which proceed in the 5′ to 3′ direction
