Supercoiling (from super- and coil = winding) refers to an additional spatial twisting of the helical double-stranded DNA.
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Supercoiling
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For example, the helical DNA in prokaryotic cell nuclei is twisted by enzymes (e.g. bacterial topoisomerases (gyrases) into simple "supercoils" tangled structures (double strand, deoxyribonucleic acids). They thus resemble a curled telephone cord. In a relaxed DNA, the two nucleotide strands wind around the helical axis approximately every 10.5 base pairs. Such a twist is called a "twist".
If these supercoils are twisted around themselves again, the DNA chains are tightly packed in the nucleus. Their space requirement decreases dramatically. Since the total length of the DNA of a cell is often a thousand times the diameter of the cell, supercoiling is essential for the functioning of prokaryotic and eukaryotic cells.
Depending on the direction in which supercoiling takes place, a distinction is made between:
- positive supercoiling (left-handed or counterclockwise) and
- negative supercoiling (right-handed or clockwise).
Topoisomerases are responsible for the enzymatic control of supercoiling.