When double-stranded DNA is heated at neutral pH, which change does not occur?

A) The absorption of ultraviolet (260 nm) light increases.

B) The covalent N-glycosidic bond between the base and the pentose breaks.

C) The helical structure unwinds.

D) The hydrogen bonds between A and T break.

E) The viscosity of the solution decreases.

B) The covalent N-glycosidic bond between the base and the pentose breaks

Explanation:

When forces that bind 2 DNA strands are broken, these strands finally split. This process is called denaturalization

The main forces that remain both DNA strands together are the hydrogen bonds (physical forces), between pair of bases (Adenine with Thymine, and Guanine with Cytosine)

Denaturalization may occur by different factor: Ph alteration, salt addition, temperature (heating), etc

When heating a DNA solution, denaturalization process may be followed using UV absorption (normally, at wavelength of 260 nm) : when DNA (double strand) has its regular structure, UV light absorption is low, as the nitrogen bases are stacked like a pile of coins, for which structure absorbs less light

When DNA structure is denaturalized, these nitrogen bases are exposed, and UV absorption increases

Also, DNA denaturalization may be followed by viscosity: for a double strand DNA solution, viscosity is high due to double chain stiffness but when strands are denaturalized, simple strands solution are less viscous (so viscosity decreases over time, when heating the solution)

Lastly, when solution is heated, only physical forces are broken (hydrogen bonds) and not chemical bonds (such as covalent bonds), for which extreme conditions are needed