"""DNA double helix: two backbones winding round an axis, base pairs between.
Three full turns at ten base pairs a turn. The backbones are tubes, each lit
by the same light as the rest of the kit and each in its own colour; the
rungs are the base pairs, every base in its own colour so the sequence reads
down the ladder: A with T, G with C, and each rung split at the middle where
the two bases meet.
The backbones run half a turn apart rather than the real major and minor
grooves' uneven split, which keeps every rung crossing the axis; at this size
the uneven version reads as a drawing mistake rather than as biology.
The catalog's `pitch` toggle loosens the twist, which is a different body
rather than a layer, so it ships no PNG. The loop is the default orbit spin:
the helix turning about its long axis under a fixed camera and the camera
turning round it are the same eight seconds.
The full Manim source and the rendered video come with a plan.
