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The double-helical configuration that DNA strands naturally reside in makes them difficult to separate, and yet they must be separated by helicase proteins if other enzymes are to transcribe the sequences that encode proteins, or if chromosomes are to be replicated. In so-called circular DNA, in which double helical DNA is bent around and joined in a circle, the two strands are topologically linked, or knotted. Otherwise identical loops of DNA having different numbers of twists are topoisomers, and cannot be interconverted by any process that does not involve the breaking of DNA strands. Topoisomerases catalyze and guide the unknotting of DNA.
The insertion of viral DNA into chromosomes and other forms of recombination can also require the action of topoisomerases.
There are three main types of topology; supercoiling, knotting and catenation. When outside of replication or transcription DNA needs to be kept as compact as possible and these three states help this cause. However when transcription or replication occur DNA needs to be free and these states seriously hinder the processes.
Topoisomerases can fix these topological problems and are separated into two types separated by the number of strands cut in one round of action:
Both type I and type II topoisomerases change the linking number of DNA.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Topoisomerase". A list of authors is available in Wikipedia.|