(Chemistry) Designates the carbon atom of a ribose molecule which attaches to a phosphate moiety in nucleic acids, nucleotides and nucleosides.
(Molecular biology) Designates the upstream 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:
5'-ACGTATAAGCTCGATAGC-3'
3'-TGCATATTCGAGCTATCG-5'
The sequence of this molecule could be written as either "ACGTATAAGCTCGATAGC" or "GCTATCGAGCTTATACGT" - the sequence from 5' to 3' read from each complementary strand.