Nucleic acid basics – In double-stranded DNA, guanine (G) base-pairs with which base?

Introduction:
Correct base pairing in DNA ensures accurate replication and transcription. Recognizing the standard Watson–Crick partners is a foundational competency in genetics and molecular biology courses.

Given Data / Assumptions:

• DNA uses the bases A, T, G, and C.
• Base pairing relies on specific hydrogen bonding patterns.
• Uracil replaces thymine only in RNA.

Concept / Approach:
Guanine pairs with cytosine via three hydrogen bonds in canonical B-DNA, contributing to the higher thermal stability of GC-rich sequences compared with AT-rich sequences. This stability is reflected in higher melting temperatures for GC-rich duplexes and is exploited in PCR primer design and genome analysis.

Step-by-Step Solution:

Verification / Alternative check:
Empirical nearest-neighbor thermodynamics show increased duplex stability with GC content, confirming the stronger pairing of G–C relative to A–T pairs.

Why Other Options Are Wrong:

• Adenine/thymine: pair with each other, not with guanine.
• Uracil: present in RNA, not DNA.
• Hypoxanthine: appears in certain wobble or deamination contexts, not standard DNA pairing.

Common Pitfalls:
Mixing RNA and DNA base-pair rules or assuming identical stability for all pairs.

Final Answer:
Cytosine (C)