Which nitrogenous base is present in RNA but is not found in the normal structure of DNA molecules?

Introduction / Context:
DNA and RNA are the two main nucleic acids in living organisms. Both are polymers of nucleotides, and each nucleotide contains a sugar, a phosphate group, and a nitrogenous base. While they share several bases, there is a key difference in one of the bases used in RNA compared with DNA. This difference is often tested in basic genetics and molecular biology questions. Here, you are asked to identify the base that is found in RNA but not in normal DNA.

Given Data / Assumptions:

• We are comparing the nitrogenous bases of RNA and DNA.
• The options include uracil, thymine, adenine, guanine, and cytosine.
• We assume typical DNA and RNA molecules in living cells, without considering rare modifications.
• Only one base is unique to RNA in standard textbook descriptions.

Concept / Approach:
DNA contains the bases adenine (A), guanine (G), cytosine (C), and thymine (T). RNA, on the other hand, contains adenine, guanine, cytosine, and uracil (U). In RNA, uracil takes the place of thymine and pairs with adenine during base pairing. Therefore, uracil is characteristic of RNA and is not present in normal DNA, while thymine is characteristic of DNA and is typically absent from RNA. Adenine, guanine, and cytosine are common to both nucleic acids.

Step-by-Step Solution:
Step 1: List the four standard bases found in DNA: adenine, guanine, cytosine, and thymine. Step 2: List the four standard bases found in RNA: adenine, guanine, cytosine, and uracil. Step 3: Compare these lists to find the base that appears only in RNA and not in DNA. Step 4: Notice that thymine appears only in DNA and uracil appears only in RNA under normal conditions. Step 5: Adenine, guanine, and cytosine appear in both nucleic acids. Step 6: Therefore, the base that is present in RNA but not in DNA is uracil.

Verification / Alternative check:
Standard diagrams of DNA and RNA structures in textbooks clearly show thymine paired with adenine in DNA and uracil paired with adenine in RNA. Descriptions of transcription explain that when RNA is synthesised from a DNA template, uracil is incorporated opposite adenine instead of thymine. This consistent information across sources confirms that uracil is unique to RNA in typical examples.

Why Other Options Are Wrong:
Thymine: Found in DNA, not in normal RNA; it is essentially the mirror case of uracil. Adenine: Present in both DNA and RNA. Guanine: Present in both DNA and RNA. Cytosine: Present in both DNA and RNA.

Common Pitfalls:
Some learners memorise the base names but forget which nucleic acid uses which base. A helpful memory trick is that DNA uses T for thymine, and RNA uses U for uracil. Thinking of U as the “replacement” for thymine in RNA makes it easy to recall that uracil is unique to RNA.

Final Answer:
The correct answer is Uracil.