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There
are several enzymes involved.
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1. Topoisomerase is responsible for
initiation of the unwinding of the
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DNA.
The tension holding the helix in its coiled and supercoiled
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structure
can be broken by nicking a single strand of DNA. Try this
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with
string. Twist two strings together, holding both the top and the
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bottom.
If you cut only one of the two strings, the tension of the
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twisting is
released and the strings untwist.
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2. Helicase accomplishes unwinding of
the original double strand,
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once
supercoiling has been eliminated by the topoisomerase. The two
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strands
very much want to bind together because of their hydrogen
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bonding
affinity for each other, so the helicase activity requires energy
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(in
the form of ATP ) to break the strands apart.
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