68. Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason
R
Assertion A: Heat and extremes of pH cause denaturation, or melting, of double- helical DNA
Reason R: Breaking of covalent bonds of DNA causes unwinding of the double helix
In the light of the above statements, choose the most appropriate answer from the options given
below
1. Both A and R are correct and R is the correct explanation of A
2. Both A and R are correct but R is NOT the correct explanation of A
3. A is correct but R is not correct
4. A is not correct but R is correct

 

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Detailed Explanation:

Assertion A is Correct:

Heat and extremes of pH do denature DNA by disrupting the hydrogen bonds between base pairs. This leads to unwinding or melting of the double helix into single strands.

• This process is reversible (renaturation can occur).

• It’s the hydrogen bonds (non-covalent) that break—not the covalent phosphodiester backbone.

Reason R is Incorrect:

The statement that “breaking of covalent bonds of DNA causes unwinding” is false.

• The covalent bonds (phosphodiester bonds between nucleotides) maintain the integrity of each strand and are not broken during denaturation.

• Denaturation only affects non-covalent interactions, mainly hydrogen bonds and base-stacking interactions.

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Summary:

• Denaturation = breaking hydrogen bonds (not covalent bonds)

• Covalent bonds remain intact during heat or pH-induced DNA denaturation