Page "DNA replication" Paragraph 6

DNA strands have a directionality, and the different ends of a single strand are called the " 3 ' ( three-prime ) end " and the " 5 ' ( five-prime ) end " with the direction of the naming going 5 prime to the 3 prime region.

The strands of the helix are anti-parallel with one being 5 prime to 3 then the opposite strand 3 prime to 5.

These terms refer to the carbon atom in deoxyribose to which the next phosphate in the chain attaches.

Directionality has consequences in DNA synthesis, because DNA polymerase can synthesize DNA in only one direction by adding nucleotides to the 3 ' end of a DNA strand.