(Chemistry) Designates the carbon atom of a ribose molecule which attaches to the hydroxyl moiety in nucleic acids, nucleotides and nucleosides which can be joined to the 5' phosphate group of other nucleic acid, nucleotide and nucleoside molecules.

(Molecular biology) Designates the downstream end of a DNA or RNA sequence. The strands of nucleic acids such as DNA and RNA are directional, with one end terminating in a phosphate group joined to the 5' carbon of a ribose molecule, and the other end terminating in a hydroxyl group joined to the 3' carbon of another ribose molecule. Traditionally, sequences of DNA and RNA are written from 5' to 3'. For instance, take the following DNA molecule:


The sequence of this molecule could be written as either "ACGTATAAGCTCGATAGC" or "GCTATCGAGCTTATACGT" - the sequence from 5' to 3' read from each complementary strand.