In molecular biology, which nitrogenous bases normally pair together in the DNA double helix structure?

Introduction / Context:
Base pairing is a fundamental idea in molecular biology and genetics. Deoxyribonucleic acid, or DNA, stores genetic information in the form of specific sequences of four nitrogenous bases, and these bases pair in a very specific way in the double helix. Many general science and biology questions test whether a learner remembers which bases pair with which, because this underlies replication, transcription, and heredity.

Given Data / Assumptions:

• We are dealing with DNA, not ribonucleic acid (RNA).
• The options refer to nitrogenous bases adenine, thymine, guanine, cytosine, and uracil.
• Standard Watson and Crick base pairing rules are assumed.
• We need to identify which option correctly describes how bases pair in DNA.

Concept / Approach:
In DNA, the four bases are adenine (A), thymine (T), guanine (G), and cytosine (C). The base pairing rules state that A pairs with T and G pairs with C. These pairs are held together by hydrogen bonds, with A and T forming two hydrogen bonds and G and C forming three. Uracil (U) does not appear in DNA; it is found in RNA where it replaces thymine and pairs with adenine.

Step-by-Step Solution:
Step 1: Recall that in DNA the first base pair is adenine with thymine, so A with T is correct.Step 2: Recall that the second base pair is guanine with cytosine, so G with C is also correct.Step 3: Check each option. Option A states adenine with thymine in DNA, which matches the rule. Option B states guanine with cytosine, which also matches the rule.Step 4: Option C claims that both option A and option B are correct base pairing rules. Since both base pairs are indeed correct in DNA, option C summarises the correct information.Step 5: Options D and E propose guanine with uracil or adenine with cytosine in DNA, which contradict standard base pairing rules.

Verification / Alternative check:
Any standard biology textbook diagram of the DNA double helix shows the rungs of the ladder made up of A T and G C pairs. If the pairing were different, replication accuracy would be lost and the genetic code would not be stable. Molecular models, structural studies, and countless exam diagrams all reinforce A T and G C base pairing, confirming that the combined statement in option C is correct.

Why Other Options Are Wrong:
Option A is only partially correct because it gives one correct pair but ignores the second. Option B also gives a single correct pair but not the complete answer that the question seeks when it asks which bases pair together. Option D incorrectly links guanine with uracil in DNA, but uracil is not present in DNA at all. Option E suggests adenine with cytosine, which never occurs as a normal base pair in DNA. Therefore only the combined statement in option C fully represents the correct pairing rules for DNA.

Common Pitfalls:
Students sometimes mix up DNA and RNA and think uracil is present in both, or they assume any base can pair with any other. Another common mistake is to memorise only one base pair, usually adenine with thymine, and forget the guanine with cytosine pair. Exam questions may hide the correct answer inside a combined option like option C, so it is important to read all choices carefully.

Final Answer:
The correct description of DNA base pairing is given in Both option A and option B are correct base pairing rules because A pairs with T and G pairs with C in the DNA double helix.