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In every cell, DNA supercoiling is a precisely regulated process that influences many
aspects of DNA metabolism. Not surprisingly, there are enzymes in every cell whose sole
purpose is to underwind and/or relax DNA. The enzymes that increase or decrease the extent
of DNA underwinding are called topoisomerases, and the property of DNA
they affect is the linking number. These enzymes play an especially important role in
processes such as replication and DNA packaging. There are two classes of topoisomerases.
Type 1 topoisomerases act by transiently breaking one of the two DNA strands, rotating one
of the ends about the unbroken strand, and rejoining the broken ends; they change Lk
in increments of 1. Type 2 topoisomerases break both DNA strands and change Lk in
increments of 2.
Lehninger-Nelson-Cox: Principles of Biochemistry, 805.o.
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