Anti parallel strands
DNA has a double helix structure, this consists of two sugar phosphate backbones. Each backbone is made up of alternating deoxyribose sugar molecules and phosphate groups, and is different at each end, while one end is 5', the other end on the same strand is 3'. The 3' carbon of one sugar is linked to the 5' carbon of the next sugar in the chain by the phosphate group. The backbone strands are said to be 'antiparallel' the 5' end of one strand is paired with the 3' end of the other strand. Because the strands are antiparallel, one base from a base pair will attach to the sugar above the plane of pairing where the other base will attach to the sugar below the plane of pairing.