DNA Composition—Nitrogenous Bases How many different bases occur in DNA, and which are they?

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:

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