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I am reading through my textbook (Genetics, Brooker 4th edition) and the summary it gives seems imprecise for the functionality of helicase and topoisomerase.

The following is an exerpt:

At each replication fork, DNA helicase unwinds the DNA and topisomerase alleviates positive supercoiling.

Is it proper to say that helicase "unwinds" DNA? In other places in the book, it says the function of helicase is to separate the two strands of DNA, and topoisomerase "relieves the supercoiling" caused by the strand separation activity of helicase.

Pardon my ignorance, but it seems rather like helicase actually induces more winding, and topoisomerase (gyrase) actually unwinds DNA.

Is this just a case of poor wording, or does helicase actually somehow unwind (untwist) DNA? I am finding this particularly frustrating because virtually every resource I can find says that helicase unwinds DNA, but based on the mechanisms they offer, it definitely does not "unwind".

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  • $\begingroup$ It would be pretty difficult to separate the strands without unwinding them. As you pull the strands apart, the adjacent regions will either have to rotate (unwind) to accomodate this or break. $\endgroup$ – bpedit Oct 7 '16 at 23:32
  • $\begingroup$ Why wouldn't the replication complex just rotate (spiral) along the double helix? $\endgroup$ – Ryan Ward Oct 7 '16 at 23:38
  • $\begingroup$ The strands must be separated so the incoming nucleotides for the newly fabricating strand can be base-paired to the template strand. The other (antisense) strand must be moved out of the way for this to occur. $\endgroup$ – bpedit Oct 8 '16 at 3:29
  • $\begingroup$ I mean, I get that they're separated but how do they "unwind"? $\endgroup$ – Ryan Ward Oct 8 '16 at 3:30
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Got a short piece of three stranded rope handy? (I don't think they make two stranded rope.) Hold the rope firmly in each hand about 5 cm apart. Twist in the direction opposite the winding. The strands of the rope will separate in the region between your hands. The unwinding and the separation of the strands are, um, inseparable from one another. You can't separate the strands without unwinding.

The heilcase unwinding is merely straightening out the double helix in a very local area. This separates the strands such that one strand can serve as a template for transcription or replication.

But a consequence of the unwinding in the local region where the strands are being separated, is that other nearby regions get more tightly wound. This occurs because the DNA is far too long for the ends to spin around to accomodate this unwinding. These more tightly wound areas are what is refered to as supercoiling. The topoisomerase alleviates this stressful coiling. https://en.wikipedia.org/wiki/Topoisomerase

So, the answer is both helicase and topoisomerase result in the unwinding of DNA.

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