In molecular biology, the 5' end and the 3' end (pronounced 5-prime and 3-prime) 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.(see figure).

Nucleic Acids can only be synthesized in vivo in the 5' -> 3' direction. Traditionally DNA and RNA sequences are written going from 5' to 3'.

Phisiologically, the importance of the 3' end is the presence of a hydroxyl group. The 3'-hydroxyl is nessecary in ligation . Molecular biologists can use nucleotides that lack a 3'-hydroxyl (dideoxyribonucleotides) to interupt the replication of DNA. This dideoxy termination method was used to create a system to sequence DNA. The dideoxy termination method was developed by Frederick Sanger, and is often refered to as the Sanger Method.