Alberta Grade 9 Science PAT Practice Test

Understanding who proposed the DNA double helix is about recognizing the scientists who put together the model of DNA’s structure. James Watson and Francis Crick proposed the double helix in 1953, building a picture of two long strands wound around each other with a sugar-phosphate backbone on the exterior and bases pairing inside. This arrangement, with A pairing with T and G pairing with C, explains how genetic information can be accurately copied because each strand can serve as a template for making the other. Their model drew on Chargaff’s base-pairing rules and X-ray diffraction data collected by Rosalind Franklin (and Maurice Wilkins), which pointed to a helical shape. Darwin and Mendel made foundational contributions to biology, but not to the specific structure of DNA.