Question

Double-helix structure of DNA as proposed by Watson and Crick.

Hint:

{Watson--Crick Model Key Points:} Double helix, antiparallel strands, complementary base pairing (A--T, G--C).

Answer 1

Concept: In 1953, James Watson and Francis Crick proposed the double-helix model of DNA, which explains the molecular structure and mechanism of genetic inheritance. Their model was based on X-ray diffraction data (notably by Rosalind Franklin) and Chargaff’s base pairing rules.
Explanation: According to the Watson--Crick model, the important features of the DNA double helix are:

• DNA consists of two long polynucleotide strands coiled around each other to form a right-handed double helix.
• Each strand is made of repeating units called nucleotides, consisting of:
  • A deoxyribose sugar
  • A phosphate group
  • A nitrogenous base
• The two strands run in opposite directions (antiparallel), i.e., one 5' $\rightarrow$ 3' and the other 3' $\rightarrow$ 5'.
• The sugar-phosphate backbone forms the outer framework, while nitrogenous bases project inward.
• Complementary base pairing occurs:
  • Adenine pairs with thymine via two hydrogen bonds
  • Guanine pairs with cytosine via three hydrogen bonds
• The helix has a uniform diameter of about 2 nm and one complete turn every 3.4 nm (approximately 10 base pairs per turn).

This model explained how genetic information is stored and replicated, making it a cornerstone of molecular biology.