The DNA molecule is composed of two strands held together by hydrogen bonds. A single strand is different at its two ends. One end is called 5' (5 prime), the other end is called 3' (3 prime).
The names come from the notation for the two sugar carbon atoms which participate in the phosphodiester bonds. The different strands in the helix run in opposite directions (anti-parallel) such that at each end of the double helix, one strand is 5' and the other is 3'.
