Double helices
What is a double helix?
A double helix consists of two identical cylindrical helices twisting around each other.- Both helices share the same cylinder and have the same handedness (both left-handed or both right-handed).
- One helix can be obtained by rotating the other around the cylinder’s axis. They also partially overlap when translated along the cylinder’s axis.
- In a typical double helix, the two helices are offset by a \(180^\circ\) rotation, corresponding to a translation of half the pitch.
Where can we find double helices?
Double helices appear in nature, engineering, and everyday objects:- The most common form of DNA molecule (B-DNA) has the shape of a double helix. Its two right-handed helices are offset by \(180^\circ\) and are quite stretched (namely, the slope is large): the cylinder radius is around 1 nanometer while the pitch is around 3,4 nanometers.
Picture reference, CC-BY-SA-3.0, cropped
- Double helical staircases or ramps often use one helix for ascending and the other for descending. These designs are space-efficient compared to two separate staircases.
Picture reference, CC-BY-SA-1.0
- A bolt and nut system forms a double helix, where the helix in the nut slides along the helix on the bolt.
Picture reference, CC-BY-SA-3.0
Triple helices and more
It’s possible to intertwine three or more identical helices: Triple staircases exist in some architectural designs and some twisted marshmallows consist of four intertwined helices.
