Molecular biologists use several shorthands when referring to nucleic acid molecules such as DNA and RNA.
Additionally the positions of the carbons in the ribose sugar that forms the backbone of the nucleic acid chain are numbered as follows:
Since the hydroxyl group attached to the 3' carbon of one base attaches to the phosphate group attached to the 5' carbon of the next base, 5' and 3' are often used to indicate the polarity of a DNA strand, e.g. "5' to 3'" and "3' to 5'". This is especially important as regards polymerization, where polymerization enzymes move exclusively from 5' to 3' along a DNA strand.