DNA supercoil
![Supercoiled structure of circular DNA molecules with low writhe. The helical nature of the DNA duplex is omitted for clarity.](/Images/godic/202502/14/Circular_DNA_Supercoiling3652.png")
![Supercoiled structure of linear DNA molecules with constrained ends. The helical nature of the DNA duplex is omitted for clarity.](/Images/godic/202502/14/Linear_DNA_Supercoiling3652.png")
![Drawing showing the difference between a circular DNA chromosome (a plasmid) with a secondary helical twist only, and one containing an additional tertiary superhelical twist superimposed on the secondary helical winding.](/Images/godic/202502/14/Helix_vs_superhelix3652.jpg")
![Figure showing the various conformational changes which are observed in circular DNA at different pH. At a pH of about 12 (alkaline), there is a dip in the sedimentation coefficient, followed by a relentless increase up to a pH of about 13, at which pH the structure converts into the mysterious](/Images/godic/202502/14/Circular_DNA_s_vs_pH3652.jpg")
DNA supercoiling refers to the over- or under-winding of a DNA strand, and is an expression of the strain on that strand. Supercoiling is important in a number of biological processes, such as compacting DNA, and by regulating access to the genetic code, DNA supercoiling strongly affects DNA metabolism and possibly gene expression. Additionally, certain enzymes such as topoisomerases are able to change DNA topology to facilitate functions such as DNA replication or transcription. Mathematical expressions are used to describe supercoiling by comparing different coiled states to relaxed B-form DNA.