Question

Melting temperature of DNA is the temperature at which

• A. 100 % of double stranded DNA is changed to single stranded
• B. 30 % of double stranded DNA is changed to single stranded
• C. 50 % of double stranded DNA is changed to single stranded
• D. 80 % of double stranded DNA is changed to single stranded

Hint:

A commonly used formula for estimating the \( T_m \) of short primers is:
\[ T_m = 2 \times (A + T) + 4 \times (G + C) \]
This "Wallace Rule" highlights that G and C contribute twice as much to the stability as A and T.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question defines the term "Melting Temperature" (\( T_m \)) of DNA, which is a critical parameter in molecular techniques like PCR (Polymerase Chain Reaction) and DNA hybridization.
Detailed Explanation:

• DNA Denaturation:
When DNA is heated, the hydrogen bonds between the complementary base pairs (A-T and G-C) break, causing the double helix to separate into two single strands. This process is called melting or denaturation.

• Definition of \( T_m \):
The melting temperature (\( T_m \)) is specifically defined as the temperature at which exactly \( 50\% \) of a specific DNA sample has transitioned from the double-stranded (dsDNA) state to the single-stranded (ssDNA) state.
It represents the midpoint of the thermal denaturation curve.

• Factors Affecting \( T_m \):
GC Content: G-C pairs have three hydrogen bonds, while A-T pairs have two. Therefore, DNA with a higher GC content requires more energy to denature and has a higher \( T_m \).
DNA Length: Longer fragments generally have higher \( T_m \).
Salt Concentration: High ionic strength (e.g., \( Na^+ \) ions) stabilizes the double helix by neutralizing the negative charges on the phosphate backbone, increasing the \( T_m \).

Step 3: Final Answer:
The melting temperature is the temperature where \( 50\% \) of the DNA is denatured.