DNA base-pairing specificity — In nucleic acids, guanine (G) forms specific hydrogen bonds with which base under standard Watson–Crick pairing rules?

Introduction / Context:
Accurate information flow in cells depends on complementary base pairing. In DNA (and standard RNA contexts), each base has a preferred partner that stabilizes the double helix or local duplex structures via hydrogen bonds. Recognizing these pairs is essential for understanding replication, transcription, PCR, and sequencing.

Given Data / Assumptions:

• Watson–Crick pairing rules apply.
• We are considering the canonical pairing at physiological conditions.
• Unusual pairings (wobble, Hoogsteen) are not the default in this question.

Concept / Approach:
Guanine pairs specifically with cytosine via three hydrogen bonds, making G≡C pairs more stable than A=T pairs in DNA. This stability influences melting temperature and GC content’s effect on duplex behavior. Guanine does not pair with itself in standard duplex DNA, and thymine pairs with adenine, not guanine.

Step-by-Step Solution:

Verification / Alternative check:
Thermal denaturation experiments show higher melting temperatures in GC-rich duplexes due to the third hydrogen bond and better stacking interactions.

Why Other Options Are Wrong:

• Guanine with guanine: non-canonical and destabilizing.
• Thymine: pairs with adenine in DNA.
• None of these: incorrect because cytosine is correct.

Common Pitfalls:
Assuming pairing rules change in RNA: while U replaces T, G still pairs canonically with C.

Final Answer:
cytosine