superhelical

Superhelical is an adjective used to describe macromolecules that have adopted a higher-order coiling as a result of torsional stress. It most often refers to DNA, which can form a superhelix or supercoil when the molecule is overwound or underwound relative to its relaxed state. The term is central to DNA topology and the regulation of genetic processes.

Topological framework for DNA superhelicity involves the linking number (Lk), twist (Tw), and writhe (Wr). The linking number is the total number of times the two DNA strands cross in a closed molecule and is conserved under normal conditions. Changes in twist—the helical turns of the strands around the axis—and writhe—the three-dimensional coiling of the DNA axis—satisfy Lk = Tw + Wr. Supercoiling arises when Lk deviates from its relaxed value (Lk0), giving the molecule a superhelical density that can be negative or positive depending on the direction of the twist.

Structural forms and biological significance: Supercoiled DNA commonly adopts plectonemic geometries, where intertwined helical segments loop around each other, or toroidal geometries, where the helix wraps around a central axis. The sign and degree of supercoiling influence DNA accessibility, transcription, replication, recombination, and chromosomal compaction. Negative supercoiling generally promotes strand separation and transcription initiation, while positive supercoiling can hinder these processes.

Regulation and measurement: Cellular enzymes called topoisomerases modulate DNA supercoiling by cutting and resealing strands to relax or introduce twists. Experimental and computational methods quantify Lk, Tw, and Wr to characterize topological states in plasmids and chromosomes. The term superhelical is also used more broadly for other macromolecules that form a superhelix, but it is most commonly applied to DNA topology.