Directionality, in
molecular biology and
biochemistry, is the end-to-end chemical orientation of a single strand of
nucleic acid. In a single strand of
DNA or
RNA, the chemical convention of naming carbon atoms in the
nucleotide sugar-ring means that there will be a
5'-end, which frequently contains a
phosphate group attached to the 5' carbon of the
ribose ring, and a
3'-end (usually pronounced "five prime end" and "three prime end"), which typically is unmodified from the ribose -OH substituent. In a
DNA double helix, the strands run in opposite directions to permit
base pairing between them, which is essential for replication or transcription of the encoded information.