A single DNA strand has two ends: one end has 5’-phosphate group and other has 3’-OH group. The DNA chains are arranged in such a way that you can never have two 5’ (five prime) or 3’ (three prime) at one end. This arrangement is called antiparallel because the strands run in opposite directions to each other.

The backbone of each polynucleotide strand consists of deoxyribose sugar alternating with phosphate groups that links the 3' carbon of one sugar to 5' carbon of next.