Question

Guanine and Cytosine base pairs are united by

• A. 2 Covalent bonds
• B. 3 Hydrogen bonds
• C. 3 Covalent bonds
• D. 2 hydrogen bonds

Hint:

DNA Base Pairing: $A=T$ (2 Hydrogen bonds), $G\equiv C$ (3 Hydrogen bonds).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Watson-Crick complementary base pairing in the DNA double helix is stabilized by non-covalent hydrogen bonding between nitrogenous purines and pyrimidines.
Key Formula or Approach:
\[ \text{Adenine } (\text{A}) = \text{Thymine } (\text{T}) \quad \text{[2 Hydrogen Bonds]} \]
\[ \text{Guanine } (\text{G}) \equiv \text{Cytosine } (\text{C}) \quad \text{[3 Hydrogen Bonds]} \]

Step 2: Detailed Explanation:
In the B-DNA double-helical structure:
- Adenine pairs with Thymine via two hydrogen bonds ($A = T$).
- Guanine pairs with Cytosine via three hydrogen bonds ($G \equiv C$).
The triple hydrogen bond between Guanine and Cytosine imparts higher thermal stability and higher melting temperature ($T_m$) to GC-rich DNA regions.

Step 3: Final Answer:
Thus, Guanine and Cytosine base pairs are united by 3 Hydrogen bonds, matching option (B).