DNA model described by Watson and Crick was
The DNA molecule consists of two polynucleotide chains or strands that spirally twisted around each other and coiled around a common axis to form a right-handed double-helix. The two strands are antiparallel i.e. they ran in opposite directions so that the 3' end of one chain facing the 5' end of the other. The sugar-phosphate backbones remain on the outside, while the core of the helix contains the purine and pyrimidine bases. Ref-Sathyanarayana 4/e p400
Right handed parallel
Left handed anti parallel
Right handed anti parallel
Right handed parallel
Metabolism of nucleic acids