supercoiling

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supercoiling A form of DNA in which the double helix is further twisted about itself, forming a tightly coiled structure. This is the form generally adopted by DNA in nature, and enables it to condense sufficiently to be packaged into living cells (see chromatin). In negative supercoiling the DNA is twisted about an axis in a direction opposite to that of the clockwise turns of the (right-handed) double helix; this decreases the number of turns of one helix around the other. In positive supercoiling the twist of the supercoils is in the same direction as that of the double helix, which increases the number of turns of one helix around the other. Supercoiling must be temporarily removed when DNA replication takes place, and the degree of supercoiling can affect gene transcription. Changes in supercoiling are performed by topoisomerase enzymes.

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