- This article is about the shape. See helix (disambiguation) for other meanings.
A helix (pl: helices), from the Greek word έλικας/έλιξ, is a twisted shape like a spring, screw or a spiral staircase. Helixes are important in biology, as DNA is helical and many proteins have helical substructures, known as alpha helices.
Right-handed and left-handed helices can be distinguished from each other. If you move along a helix in the direction of your right hand's thumb, and the helix turns in the direction of your right hand's fingers, then it's a right-handed helix, otherwise a left-handed one. Another way to visualize this distinction: picture the helix vertical; if the front strands move from the lower left to the upper right, then it is a right-handed helix. Note that handedness (or chirality) is a property of the helix, not of the perspective: you can turn a right-handed helix around and it's still right-handed.
Most screws are right-handed helices. The alpha helix in biology as well as the A and B forms of DNA are also right-handed helices. The Z form of DNA is left-handed.
The pitch of a helix is the length of one complete helix turn, measured along the helix axis.
In cylindrical coordinates (r, θ, h), the same helix is described by:
- r = 1
- θ = h
Except for rotations, translations, and changes of scale, all right-handed helices are equivalent to the helix defined above. The equivalent left-handed helix can be constructed in a number of ways, the simplest being to negate either the x, y or z component.
A double helix typically consists geometrically of two congruent helixes with the same axis, differing by a translation along the axis, which may or may not be half-way.
A conic helix may be defined as a spiral on a conic surface, with the distance to the apex an exponential function of the angle indicating direction from the axis.