Answer:
it means that DNA molecule has two completing strands;which means :if you imagine a DNA strand that has AATGC ,for example, the opposite strand must have TTACG.because: a purine should be paired with a pyrimidine.now you may wonder that cytosin,tymin and uracil are pyrimidines so why A must pair with T.first you have to omit uracil because we dont have uracil in DNA and the reason for omitting cytosin is that A has 2 hydrogen bonds with T, meanwhile C needs 3 hydrogen bonds .this that C and G have 3 hydrogen bonds is that in their molecular base structure there are some elecronegative atoms .there are three electrnegative atoms in C and G but there are 2 in A nd T.