DNA Structure and Replication

ID #2229

So if the Tm of a certain molecule of DNA has a higher salt concentration does that mean it's harder to break or easier to break? So does it have a higher Tm or lower Tm?

A higher melting temperature means that more energy needs to be added to disrupt half of the DNA base pairs present in duplex. So if the melting temperature is higher, then the DNA duplex is more stable than a DNA duplex with a lower melting temperature. 

Lets compare one dsDNA duplex in the absence or presence of salt. You can imagine two strands of DNA with negatively-charged phosphates on their DNA backbones. Phosphates of both strands would repel each other in the absence of salt. This doesn't mean that the helix would melt completely without salt, but it does mean that you wouldn't need as much energy to melt the strands. With salt present, the positively charged portion of salt would interact with the negatively charged phosphates of the DNA backbone on both strands, and reduce that repulsive force between two strands. Thus, a greater salt concentration in the solution means the DNA duplex is more stable meaning a higher melting temperature is required to melt the DNA duplex. 

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