Question

Which of the followings is not true about the DNA denaturation process?

• A. DNA denaturation causes decrease in UV absorption at all the wavelengths
• B. DNA denaturation is a cooperative process
• C. The melting temperature of the DNA decreases under alkaline conditions.
• D. The melting temperature of the DNA increases linearly with its GC content

Hint:

The "Hyperchromic Effect" refers to the increase in UV absorbance at $260\text{ nm}$ that occurs when double-stranded DNA denatures into single strands.
This absorbance change is used to monitor DNA melting and determine $\text{T}_m$.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
DNA denaturation (melting) is the process by which double-stranded DNA unwinds and separates into single-stranded polynucleotides by breaking the hydrogen bonds between complementary bases.

Step 2: Detailed Explanation:
Let us analyze the statements concerning DNA denaturation:
1. Statement A: DNA denaturation causes an increase in UV absorption at $260\text{ nm}$ (a phenomenon known as the hyperchromic effect), not a decrease.
In double-stranded DNA, the aromatic bases are stacked closely together, which shields them and reduces their ability to absorb UV light.
When DNA denatures into single strands, the base stacking is disrupted, exposing the bases and increasing their UV absorbance by approximately $30 - 40\%$.
Therefore, Statement A is incorrect.
2. Statement B: DNA denaturation is a cooperative process.
The disruption of the first few base pairs destabilizes adjacent base pairs, making them easier to melt. This results in a sharp, sigmoidal transition curve. This statement is correct.
3. Statement C: Under alkaline conditions (high pH), the nitrogenous bases deprotonate, disrupting the hydrogen bonds that stabilize the double helix.
This decreases the melting temperature ($\text{T}_m$) of the DNA. This statement is correct.
4. Statement D: Guanine-Cytosine ($\text{G-C}$) base pairs are held together by three hydrogen bonds, whereas Adenine-Thymine ($\text{A-T}$) pairs are held by two.
Higher $\text{G-C}$ content requires more thermal energy to denature, increasing the melting temperature ($\text{T}_m$) linearly. This statement is correct.

Step 3: Final Answer:
The statement that is not true is that DNA denaturation causes a decrease in UV absorption, which corresponds to Option (A).