In a DNA two polynucleotide chains are coiled together to form a helix. Sugar -phosphate form backbone of this helix while bases project inwards to each other. 
Complementary bases pair with each other through hydrogen bonds. Purine always pair with pyrimidine. The helix is right handed. The pitch is 3.4 nm and 10 base pairs are present in each turn. The plan of one base pair stacks over the other in a double helix.This provides stability along with hydrogen bonding.

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