Which combination of nitrogenous bases is correctly found in RNA molecules but not in DNA?

Introduction / Context:
This question checks your knowledge of the nitrogenous bases present in nucleic acids, specifically the difference between DNA and RNA. Correctly identifying which base is unique to RNA and which is unique to DNA is a common theme in exams and is essential for understanding transcription and translation.

Given Data / Assumptions:
- The four main nitrogenous bases in DNA are adenine, guanine, cytosine and thymine.- The four main nitrogenous bases in RNA are adenine, guanine, cytosine and uracil.- The options list different combinations of these bases.- You must select the combination that correctly represents the bases found in RNA molecules.

Concept / Approach:
DNA and RNA share three bases: adenine, guanine and cytosine. The key difference is that DNA uses thymine where RNA uses uracil. This means RNA contains uracil instead of thymine, so any correct description of RNA bases must include uracil and exclude thymine when listing all four. The strategy is therefore to find the option that lists adenine, guanine, cytosine and uracil together as the bases present in RNA.

Step-by-Step Solution:
Step 1: Recall that DNA bases are adenine, guanine, cytosine and thymine.Step 2: Recall that RNA bases are adenine, guanine, cytosine and uracil.Step 3: Compare each option to see whether it correctly lists the four RNA bases.Step 4: Identify that option B lists adenine, guanine, cytosine and uracil, which matches the RNA composition.Step 5: Confirm that no other option accurately includes uracil as the replacement for thymine in RNA.

Verification / Alternative check:
You can verify this by thinking of base pairing during transcription, where adenine in DNA pairs with uracil in RNA, showing that uracil is present in RNA.DNA never contains uracil in its normal state, and RNA normally does not contain thymine.This confirms the unique presence of uracil in RNA and supports option B as the correct choice.

Why Other Options Are Wrong:
Option A is wrong because it lists thymine instead of uracil, which describes DNA rather than RNA.Option C is wrong because it omits adenine and still incorrectly includes thymine, so it does not represent full RNA composition.Option D is wrong because it mixes uracil and thymine together and omits guanine and cytosine, which is not correct for RNA.Option E is wrong because it only lists adenine and thymine, which does not describe either complete DNA or RNA base sets.

Common Pitfalls:
Students often remember that uracil is associated with RNA but forget that adenine, guanine and cytosine are also present.Another pitfall is to confuse which base is unique to RNA and which to DNA, leading to reversed answers.A simple memory aid is to note that T is for thymine in DNA, while U is for uracil in RNA.

Final Answer:
The nitrogenous bases found in RNA are adenine, guanine, cytosine and uracil.