Watson and Crick observed that the two DNA strands are antiparallel, and have opposite polarity. This means that 5' phosphate of one strand faces the 3'hydroxyl group of other strand and that the 5' phosphate groups of two strands are present in opposite position. The antiparallel arrangement of two helices allows hydrogen bonding between amino and carbonyl group of complementary base pairs. This led them to the hypothesis of the semiconservative mode of DNA replication wherein two strands of DNA first separate from each other followed by copying of each template strand to form two DNA molecules each carrying one parental strand and one newly synthesized strand.