In the standard Watson Crick base pairing of nucleic acids, with which nitrogenous base does guanine (G) form a complementary pair in a DNA double helix?

Introduction / Context:
The structure of DNA depends on specific base pairing between nitrogenous bases. Understanding which bases pair together is fundamental to genetics, replication, and transcription. This question asks which base pairs with guanine in the DNA double helix, an idea first clearly described by Watson and Crick as part of the double stranded DNA model.

Given Data / Assumptions:

- The molecule in question is DNA, not RNA. - The nitrogenous base guanine (G) is a purine. - Answer choices include cytosine, adenine, thymine, and uracil. - We assume standard pairing rules in normal, non damaged DNA.

Concept / Approach:
DNA base pairing is governed by hydrogen bonding and the requirement that a purine always pairs with a pyrimidine to maintain a constant helix width. In DNA, adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). Guanine and cytosine form three hydrogen bonds, making this pair slightly stronger and more stable than the adenine thymine pair. Uracil is found in RNA instead of thymine and does not normally appear in DNA.

Step-by-Step Solution:
1. Recall that DNA contains four major bases: adenine, thymine, cytosine, and guanine. 2. Guanine is a purine with a two ring structure. 3. To maintain the geometry of the DNA helix, guanine must pair with a pyrimidine, which has a single ring structure. 4. The pyrimidines in DNA are cytosine and thymine. 5. In the correct base pairing scheme, guanine forms three hydrogen bonds specifically with cytosine. 6. Adenine pairs with thymine via two hydrogen bonds and does not normally pair with guanine. 7. Uracil replaces thymine in RNA and is therefore not the standard DNA partner for guanine.

Verification / Alternative check:
A quick check is to remember common abbreviations: G C and A T pairs in DNA. Textbooks often present this as a fundamental rule for replication and transcription. Experimental evidence such as Chargaff rules and X ray diffraction data support this pairing scheme. Regions of DNA rich in G C pairs are known to have slightly higher melting temperatures due to the extra hydrogen bond, which further confirms the special guanine cytosine pairing.

Why Other Options Are Wrong:
- Adenine: In DNA, adenine pairs with thymine, not guanine. Pairing guanine with adenine would disrupt helix geometry. - Thymine: Thymine pairs with adenine, not with guanine, under normal circumstances in DNA. - Uracil: Uracil is commonly found in RNA and pairs with adenine in RNA; it is generally absent from normal DNA and does not pair with guanine in the DNA double helix.

Common Pitfalls:
Learners sometimes confuse RNA and DNA base pairing and may think of adenine pairing with uracil. Others may not clearly remember which base pairs with which. A useful memory aid is that the letters A and T are straight line dominated, while G and C both have curves, helping to link A T and G C visually. Another pitfall is misremembering the number of hydrogen bonds, but focusing on the purine pyrimidine pairing pattern helps avoid errors.

Final Answer:
In DNA, guanine (G) pairs specifically with cytosine (C).