1. Introduction

DNA is a helix, not a flat spiral. This distinction is fundamental: a helix is a three-dimensional curve that winds around a cylindrical axis at constant pitch, while a spiral is a flat curve that moves away from a center. The DNA double helix is one of the most important structures in molecular biology.

2. Structure of the Double Helix

The DNA double helix, described by Watson and Crick in 1953 from Rosalind Franklin's X-ray diffraction data, is composed of two antiparallel strands wound around each other. Each strand is a polymer of nucleotides (adenine, thymine, guanine, cytosine) linked by a sugar-phosphate backbone. The two strands are held together by hydrogen bonds between complementary bases (A-T, G-C).

3. Why a Helix?

The helical shape of DNA results from several physicochemical constraints. Base stacking is stabilized by van der Waals interactions and hydrophobic effects: the flat bases stack like coins, and this geometry imposes a rotation of ~36° between successive bases. Hydrogen bonds between complementary strands constrain the interstrand distance. The aqueous environment favors exposure of the charged backbone to water and burial of hydrophobic bases inside.

4. A, B and Z Forms

DNA can adopt several conformations depending on conditions. Form B (Watson-Crick) is the most common in cells: right-handed helix, 10 base pairs per turn, pitch of 3.4 nm. Form A appears under low hydration conditions: wider and shorter helix. Form Z is a rare left-handed helix that appears in certain GC-rich sequences under torsional stress.

5. Other Biological Helices

  • Protein α-helix: 3.6 amino acids per turn, stabilized by intramolecular H-bonds
  • Collagen triple helix: three intertwined polypeptide chains
  • RNA: local helices in secondary structures (stem-loops)
  • Microtubules: helical protofilaments forming a hollow cylinder

6. Conclusion

The DNA double helix is an optimal solution to a molecular engineering problem: storing linear information in a compact, stable and accessible way. Its helical shape is not arbitrary but results from precise thermodynamic constraints. It is the most famous example of how physical chemistry determines the structure of biological molecules.