Superhelix
Relaxing
Protein
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Swiveling
Protein
As the replication fork moves down the DNA double helix the parental strands wind. The extent of untwisting is one turn for every 10 nucleotides laid down. In circular/DNA untwisting imposes a strain. This strain can be relieved to some extent by super twisting in the un replicated part of DNA.
However, there is a limit to super twisting, and unless the super twists are unwound the replication fork will not be able to move further along the DNA. The strain imposed by unwinding is relieved by the action of the super helix relaxing protein. This protein has nicking closing enzymatic activity.
