Question

How does ethidium bromide effect the melting temperature of DNA?

Answer 1

The amine (-NH₂) groups an EtBr molecule forms hydrogen bonds with phosphate groups of the DNA backbone of ds DNA molecule with which it is chelated. The number of H-bonds between EtBr and DNA molecule depends on the relative proportion of EtBr molecule per unit quantity of DNA- greater is the concentration of EtBr, greater is the number of additional H-bonds. Inter-chelation also reduces the density of DNA molecule to some extent by distorting the helix a bit.

However, the overall melting point of the ds DNA molecule increases due to relatively higher proportion of H-bonds in the EtBr bound DNA molecules.