5' end
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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).
Relative position is usually noted as upstream (towards the 5' end) or downstream (towards the 3' end).
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'.
Physiologically, the importance of the 5' end is the presence of a phosphate. The 5'-phosphate is the leading end of a nucleic acid strand and is necessary for ligation. Molecular biologists can remove the 5'-phosphate with phosphatases to prevent an end of DNA attaching inadvertently.