DNA Composition—Nitrogenous Bases How many different bases occur in DNA, and which are they?
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AFive: adenine, guanine, thymine, tryptophan, and cytosine
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BFour: adenine, guanine, thymine, and cytosine
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CThree: adenine, guanine, and cytosine
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DTwo: adenine and cytosine
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EFour: adenine, guanine, uracil, and cytosine
Answer
Correct Answer: Four: adenine, guanine, thymine, and cytosine
Explanation
Introduction / Context:DNA stores genetic information using four nitrogenous bases attached to a deoxyribose-phosphate backbone. Recognizing the correct set distinguishes DNA from RNA and from amino acids (e.g., tryptophan is an amino acid, not a nucleobase).
Given Data / Assumptions:
- Pyrimidines in DNA: thymine (T), cytosine (C).
- Purines in DNA: adenine (A), guanine (G).
- RNA replaces thymine with uracil (U).
Concept / Approach:Identify the canonical DNA bases and exclude non-DNA bases and amino acids. Tryptophan is an amino acid; uracil belongs to RNA, not DNA.
Step-by-Step Solution:
Step 1: List DNA bases: A, G, T, C.Step 2: Exclude U (RNA-specific) and tryptophan (amino acid).Step 3: Select the option listing A, G, T, C.Verification / Alternative check:Chargaff’s observations and modern sequencing confirm the four-base composition of DNA.
Why Other Options Are Wrong:
- Includes tryptophan: not a nucleobase.
- Includes uracil: RNA base.
- Two or three bases: incomplete.
Common Pitfalls:Mixing RNA and DNA bases; confusing amino acids with nucleotides.
Final Answer:Four: adenine, guanine, thymine, and cytosine