Answer:
DNA consists of two complementary strands which runs anti parallely to each other and forms a helix like structure due to their twisting angle geometry
Complementary strand means, if you are given the nitrogenous base sequence of any one strand, you can easily find the sequence of other strand by the simple rule of "A binds with T and C binds with G" i.e. purine binds with pyrimindines in other words.