In the structure of DNA, which option describes a correct complementary base pairing?

Introduction / Context:
The famous double helix structure of DNA depends on specific base pairing rules between the nitrogenous bases on the two strands. This question tests your understanding of which bases pair together in a standard DNA molecule. Knowing correct base pairing is essential for understanding replication, transcription, and mutation.

Given Data / Assumptions:

• The DNA bases involved are adenine, thymine, guanine, and cytosine.
• We assume standard Watson Crick base pairing in a normal, double stranded DNA molecule.
• The options offer various possible pairs between these bases.

Concept / Approach:
In DNA, the base pairing rules follow the principle that adenine pairs with thymine and guanine pairs with cytosine. Each pairing is stabilized by a specific number of hydrogen bonds: adenine and thymine share two hydrogen bonds, while guanine and cytosine share three. These rules ensure accurate replication and maintain a uniform width of the DNA double helix. To answer the question, you simply recall which pairings are correct and match them to the options given.

Step-by-Step Solution:
Step 1: Recall the base pairing rules in DNA: A pairs with T, and G pairs with C.Step 2: Evaluate option A, guanine pairs with cytosine. This directly matches the rule G C, so it is correct.Step 3: Evaluate option B, adenine pairs with cytosine. This combination does not occur in standard Watson Crick base pairing.Step 4: Evaluate option C, guanine pairs with thymine. This pairing is not a normal complementary pair in DNA.Step 5: Evaluate option D, thymine pairs with cytosine. This also does not match the standard rules.Step 6: Conclude that only option A correctly describes a complementary base pair in DNA.

Verification / Alternative check:
A quick way to verify is to visualize the DNA ladder model, where adenine and thymine always face each other forming two hydrogen bonds, and guanine and cytosine always face each other forming three hydrogen bonds. Any proposed pairing that does not follow A T or G C is inconsistent with the standard structure and would destabilize the double helix.

Why Other Options Are Wrong:
Option B is wrong because adenine does not pair with cytosine; this would break the hydrogen bonding pattern. Option C is wrong since guanine does not pair with thymine in a normal DNA helix. Option D is wrong because thymine does not pair with cytosine, which would again conflict with canonical base pairing rules.

Common Pitfalls:
Students sometimes mix up base pairing rules between DNA and RNA or remember only one pair and guess the other. Another pitfall is to think that any purine can pair with any pyrimidine, but in fact only specific pairs form stable hydrogen bonds in the DNA double helix. Repeated practice with diagrams and replication problems helps fix the correct A T and G C pairings in memory.

Final Answer:
Guanine pairs with cytosine.