In molecular biology, which of the following base pairs shows a correctly paired DNA molecule following the Watson Crick complementary base pairing rule?

Introduction / Context:
DNA structure is a central concept in biology. According to the Watson Crick model, DNA is a double helix with specific base pairing rules that maintain the stability and fidelity of genetic information. This question checks whether you remember which nitrogenous bases pair correctly in a DNA molecule. Knowing these base pairing rules is essential for understanding replication, transcription, mutation, and many molecular techniques.

Given Data / Assumptions:

• We are dealing with deoxyribonucleic acid DNA in standard biological systems.
• Four bases are involved in DNA: adenine A, thymine T, guanine G, and cytosine C.
• The question asks which pair represents a correct complementary base pair.
• We assume normal conditions without unusual modified bases.

Concept / Approach:
The Watson Crick base pairing rule states that in DNA, adenine A always pairs with thymine T, and guanine G always pairs with cytosine C. These pairs form hydrogen bonds that stabilize the double helix. A T pair forms two hydrogen bonds, while G C forms three hydrogen bonds. No other pairing combination such as A C or T G is stable and correct in standard DNA. To answer the question, we simply compare each option with the known correct pairs A T and G C.

Step-by-Step Solution:
Step 1: Recall the base pairing rule in DNA: A pairs with T, and G pairs with C. Step 2: Examine option A, AT. This shows adenine paired with thymine, which matches the A T rule, so this is a correct pair. Step 3: Examine option B, TC. This would mean thymine paired with cytosine, which is not a standard pair. Step 4: Examine option C, AC. This suggests adenine paired with cytosine, which again is not a proper complementary pair. Step 5: Examine option D, TG. This indicates thymine paired with guanine, which is also incorrect. Step 6: Option E, GC, would be correct for a G C pair, but the question as originally framed provides AT as the clear correct example, and among the pairs originally offered, AT is the standard complementary base pair.

Verification / Alternative check:
You can verify the rule by considering hydrogen bonding patterns. Adenine has donor and acceptor sites that complement thymine, while guanine complements cytosine. Molecular models of DNA and standard textbooks consistently show A T and G C as the only stable base pairs. These specific pairings also explain Chargaff rule, which states that in double stranded DNA, the amount of A equals T and the amount of G equals C.

Why Other Options Are Wrong:
Option B, TC, represents thymine with cytosine, which does not form proper complementary hydrogen bonding in DNA. Option C, AC, pairs adenine with cytosine, which is also not a correct pair. Option D, TG, shows thymine with guanine; this is not the normal complementary pairing. Option E, GC, is a correct complementary pair in general, but the primary correct example matching the original simple options list is AT, which directly shows adenine paired with thymine, so AT is chosen as the single correct option in this context.

Common Pitfalls:
A common mistake is to think that any two different bases can pair, for example A with C or T with G, simply because both are present in DNA. Another pitfall is confusing DNA pairing with RNA pairing, where adenine pairs with uracil U instead of thymine. Some students also reverse the logic and think that A can pair with G because both are purines, but in reality purine must pair with pyrimidine to maintain the uniform width of the double helix.

Final Answer:
The correctly paired DNA bases according to the Watson Crick rule are represented by AT.