Directionality, in molecular biology, refers to the en-to-end chemical orientation of a single strand of nucleic acid. The chemical convention of naming carbon atoms in the nucleotide sugar-ring numerically gives rise to a 5' end and a 3' end (usually pronounced "five prime end" and "three prime end"). The relative positions of structures along a strand of nucleic acid, including genes, transcription factors, and polymerases are usually noted as being either upstream (towards the 5' end) or downstream (towards the 3' end).