Everybody knows what a double helix looks like. It’s the familiar typical shape of DNA:
Here’s some real DNA, seen under a microscope:
So we have a mental model for the double helix.But what’s a single helix? First of all a single helix is just a helix; the adjective single isn’t neccessary when talking about just one helix.
A helix is just what you might think it is: a twisted shape that curls like a spring. The shape of a sprial staircase, or a curly ringlet of human hair.
It’s a shape familar to anyone who’s seen a pea plant, sweet pea, wild cucumber, or similar climbing vine.
Helix is a Greek word, so technically the correct plural is helices, not helixes.
Helices can be right- or left-handed, a quality called chirality. If the helix’s curve moves from the lower left to the upper right, it’s a right-handed helix. If it moves from the lower right to the upper left (like the vine-helix pictured above), it’s a left-handed helix. Scroll up to the helices in the DNA model — both are right-handed.
…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.