A single-stranded non-circular DNA molecule has two non-identical ends, the 3 end and the 5 end (usually pronounced "three prime end" and "five prime end").  The numbers refer to the numbering of carbon atoms in the deoxyribose, which is a sugar forming an important part of the backbone of the DNA molecule. In the backbone of DNA the 5 carbon of one deoxyribose is linked to the 3 carbon of another by a phosphodiester bond linkage. The 5 carbon of this deoxyribose is again linked to the 3 carbon of the next, and so forth.