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Anti-parallel strands

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 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 (anti-parallel) directions. At each end of the double helix, one strand is 5' and the other is 3'. In the illustration, the 3' end is shown with a red arrowhead and the 5' end with the yellow connector.