Base Pairing in DNA:The Watson-Crick Model In 1953,James Watson and Francis Crick proposed the secondary structure of DNA m DNA under physiological conditions consists of two polynucleotide strands Strands run in opposite directions and coil around each other in a double helix The helix is 20 A wide The two strands are complementary and are held together by hydrogen bonds between specific pairs of bases A with T C with G In 1953, James Watson and Francis Crick proposed the secondary structure of DNA ▪ DNA under physiological conditions consists of two polynucleotide strands ▪ Strands run in opposite directions and coil around each other in a double helix ▪ The helix is 20 Å wide ▪ The two strands are complementary and are held together by hydrogen bonds between specific pairs of bases ▪ A with T ▪ C with G Base Pairing in DNA: The Watson-Crick Model