Main Page | See live article | Alphabetical index In molecular biology, the 5' end and the 3' end are respectively the leading and tail ends of a strand of nucleic acid. These strands are unidirectional, i.e. they may only be read in one direction (for purposes transcription, translation, replication and other processes). The sugar (ribose or deoxyribose) molecules in the nucleic acid are all oriented in the same direction. Their carbon atoms are numbered: the 5' carbon atom is always on the side of the sugar molecule that faces the leading end, while the 3' carbon atom always faces the tail end. This article is from Wikipedia. All text is available under the terms of the GNU Free Documentation License.