Which of the following nitrogenous bases are found in both DNA and RNA molecules?

Introduction / Context:
Nucleic acids, DNA and RNA, are built from nucleotides that contain nitrogenous bases. Some bases are common to both DNA and RNA, while others are unique to one or the other. Knowing which bases are shared and which differ is important for understanding transcription, base pairing, and how genetic information is stored and expressed. This question asks which listed bases are present in both DNA and RNA molecules.

Given Data / Assumptions:

• Options list adenine, cytosine, guanine, all of the above, and thymine only.
• We assume standard nucleic acid structure in cells.
• We also assume knowledge of the four bases in DNA and the four bases in RNA.
• The question focuses on bases that are common to both types of nucleic acids.

Concept / Approach:
DNA contains four nitrogenous bases: adenine (A), thymine (T), cytosine (C), and guanine (G). RNA also contains four bases, but instead of thymine it has uracil (U). Thus, RNA bases are adenine (A), uracil (U), cytosine (C), and guanine (G). The bases shared by both DNA and RNA are therefore adenine, cytosine, and guanine. Thymine appears only in DNA and uracil only in RNA. Since options A, B, and C each name one of the shared bases, the option that states "all of the above" correctly captures that all three of these bases are present in both nucleic acids.

Step-by-Step Solution:
Step 1: List the bases in DNA: adenine, thymine, cytosine, and guanine. Step 2: List the bases in RNA: adenine, uracil, cytosine, and guanine. Step 3: Compare these lists and identify which bases appear in both. Step 4: Note that adenine appears in DNA and RNA, cytosine appears in both, and guanine appears in both. Step 5: Recognise that thymine is only in DNA and uracil is only in RNA. Step 6: See that options A, B, and C each name one of the shared bases, so the correct choice is "all of the above."

Verification / Alternative check:
Base pairing rules in DNA show A pairing with T and C with G, while in RNA, A pairs with U and C with G. During transcription, when RNA is synthesized from a DNA template, adenine on DNA pairs with uracil on RNA, and cytosine pairs with guanine. In all these processes, the recurring bases that feature in both molecules are adenine, cytosine, and guanine. Structural diagrams of nucleotides in DNA and RNA also show that these three bases are used in both types of nucleic acids, while thymine and uracil differentiate them.

Why Other Options Are Wrong:
Option A: Adenine is indeed present in both DNA and RNA but choosing only adenine ignores cytosine and guanine, which are also shared. Option B: Cytosine is present in both, but indicating only cytosine omits adenine and guanine. Option C: Guanine is common to both DNA and RNA, yet the correct answer must include all three shared bases. Option E: Thymine only is incorrect because thymine is absent in RNA and is replaced by uracil there; it is not shared by both nucleic acids.

Common Pitfalls:
A frequent mistake is to mix up thymine and uracil, perhaps thinking both occur in both DNA and RNA. Another pitfall is selecting only one of the shared bases, forgetting that more than one base appears in both nucleic acids. The simple rule to remember is: DNA = A, T, C, G and RNA = A, U, C, G. The intersection of these sets is A, C, and G. Keeping this clear set-based perspective helps in quickly identifying the correct group of bases in exam questions.

Final Answer:
The nitrogenous bases that are found in both DNA and RNA are adenine, cytosine, and guanine, so the correct choice is all of the above.