hélix

A helix, written hélix in French, is a three-dimensional spiral shape or curve that winds around a central axis while advancing along it. Helices occur in mathematics, biology, and engineering, and they are distinguished by a constant progression along the axis as the curve circles the axis.

Mathematically, a circular helix is the simplest form: a space curve with constant curvature and constant nonzero torsion, generated by a point moving at a constant speed around a fixed cylinder while ascending along its axis.

Biology features two well-known helices: the protein alpha-helix and the DNA double helix. The alpha-helix is a right-handed secondary structure in proteins, stabilized by hydrogen bonds between the backbone atoms of residues four apart. It typically contains about 3.6 amino acids per turn and a pitch of roughly 0.5 nanometers. The DNA double helix consists of two antiparallel strands wound around a common axis, with base pairs forming the interior. In canonical B-DNA the helix is right-handed, about 2 nanometers in diameter, with a rise of 0.34 nanometers per base pair and about 10 base pairs per turn, yielding a pitch near 3.4 nanometers per full rotation.

Other examples include helically wound springs, screws, and many helical nanostructures used in materials science and nanotechnology.

Etymology: the term derives from the Greek helix, meaning spiral.