Why does high salt favour DNA strand hybriisation?

[Dr Who]

I have often wondered why higher NaCl concentrations favour 
DNA strand hybridisation, e.g. in a Southern blot. I have 
never seen anyone try to explain why. My guess is that Na+ 
ions could shield the -ve charges of the phosphate backbone, 
which might otherwise tend to cause the 2 DNA strands to 
repel each other.

Anyone out there have any ideas?



Regards,
Steve.

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