Which of the following nucleotides is not found in the DNA molecules?

Analyze the question: You're asked which nitrogenous base is NOT found in DNA. (Strictly, adenine, cytosine, etc. are nitrogenous bases, and a nucleotide is base + sugar + phosphate, but WASSCE often uses the terms loosely.)

Concept: DNA (deoxyribonucleic acid) contains four nitrogenous bases:

• Adenine (A)
• Thymine (T)
• Cytosine (C)
• Guanine (G)

RNA also has four bases, but uracil (U) replaces thymine.

Base pairing in DNA:

• Adenine pairs with Thymine (A=T, two hydrogen bonds)
• Guanine pairs with Cytosine (G≡C, three hydrogen bonds)

Solution:

• A. Adenine: found in DNA
• B. Cytosine: found in DNA
• C. Guanine: found in DNA
• D. Uracil: not found in DNA; it is found only in RNA

Tip: Remember "U is for Unique to RNA" or "RNA has U, DNA has T." Another trick: Thymine = in DNA, Uracil = in RNA (T comes in "DNA Tower," U in "RNA Under"). Questions asking which base is absent from DNA or RNA appear often, so know this swap well.