Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Directionality, in molecular biology and biochemistry, refers to the end-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 and various protein binding sites, are usually noted as being either upstream (towards the 5'-end) or downstream (towards the 3'-end).