What is the structure of DNA as proposed by Watson and Crick?
Topic: Watson and Crick model of DNA
Answer
DNA is a DOUBLE HELIX — two polynucleotide strands coiled around each other like a twisted ladder. The sugar-phosphate chains form the uprights and the paired nitrogenous bases, joined by hydrogen bonds, form the rungs. The two strands run in opposite directions (antiparallel).
The phrase that must appear is DOUBLE HELIX.
The twisted-ladder picture is the most useful way to hold the structure: the ALTERNATING sugar and phosphate molecules make the two side rails, and each rung is a pair of bases — A with T, G with C — held by hydrogen bonds.
Watson and Crick proposed it in 1953, building on the X-ray diffraction photographs taken by Rosalind Franklin, and on Chargaff’s finding that A always equals T and G always equals C.
WHY THE STRUCTURE MATTERS: because the strands are complementary, each can act as a template for rebuilding the other — which is how DNA replicates, and the reason the structure explained heredity the moment it was proposed.