How does helicase unwind DNA?

Answer by Patrick Reilly:

There are a number of different helicase families which use different techniques to split the double-helix of DNA.  For the sake of brevity, I'll only describe two of these techniques.

RecBCD:
Possibly my favorite of the helicases, RecBCD is a helicase found in Escherichia coli which is one of the most processive (i.e. it unwinds the longest stretches of DNA) and fastest known helicases.  One RecBCD unit can unwind up to 42 kilobases of DNA[1]!
As you have read elsewhere, many helicases require the energy stored in ATP in order to split the double-helix.  Well, RecBCD is one of those helicases.

But the awesome thing about RecBCD is that it doesn't even use ATP at the splitting site!  The RecB and RecD subunits act as motors, crawling along each ssDNA strand, thereby pulling the dsDNA through a tunnel with a little wedge in the middle.  The wedge splits the dsDNA into two ssDNA strands, each of which proceeds into its respective tunnel (the 5' and 3' tunnels) and out the other end of RecBCD.

So RecBCD is quite similar to this log splitter:
https://www.youtube.com/watch?v=EJKwbSVLbdw
The only difference is that the "log" (dsDNA) gets pulled toward the wedge, rather than pushed toward it.

California Lutheran University has a fantastic page on RecBCD:
RecBCD – DNA Complex

DnaB:
The next helicase on my list of favorites is DnaB, another helicase found in E. coli, but DnaB is a ring-shaped hexamer, whereas RecBCD is a trimer and isn't ring-shaped.

Incidentally, the ring shape is the cool part about DnaB!  The ring surrounds a single strand of ssDNA and the ATP-dependent motor domain of DnaB pulls the single strand through the ring.  The ring diameter is just large enough for one strand, but too small for dsDNA, so the dsDNA has to split, forcing the other strand of ssDNA to go outside of the ring in order to meet up with its partner ssDNA on the other side of DnaB.

Take a look at Figure 4, specifically at part A, from [2]:

See how DnaB squeezes one strand through, thereby forcing the dsDNA to split so that the other ssDNA strand can pass around DnaB?

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I'd love to get someone's suggestion for a good macroscopic/"real-life" analog of the strand exclusion mechanism.  If you've got one, I'd be glad to add it in here and give all due credit!

[1] Bianco, PR, Brewer, LR, Corzett, M, Balhorn, R, Yeh, Y, Kowalczykowski, SC and RJ Baskin (2001) Processive translocation and DNA unwinding by individual RecBCD enzyme molecules. Nature, 409: 374-378. doi: 10.1038/35053131
Page on nature.com (beware the paywall)

[2] Patel, SS and I Donmez (2006) Minireview: Mechanisms of Helicases. J. Biol. Chem., 281(27): 18265-18268. doi: 10.1074/JBC.R600008200
Mechanisms of Helicases (this one's free!)

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