Question

The base which is present in DNA but not in RNA is :

• A. Guanine
• B. Cytosine
• C. Thymine
• D. Adenine

Hint:

Mnemonic: DNA uses "T" for "Tall" (double helix), RNA uses "U" for "Understudy" (often single-stranded).

Answer 1

The Correct Option is C

Step 1: Sorting the five nitrogenous bases into families.
Nucleic acids use five bases in total. Two of them are purines, adenine and guanine, and three are pyrimidines, cytosine, thymine and uracil. Both DNA and RNA use adenine, guanine and cytosine without any difference.
Step 2: The one place DNA and RNA part ways.
DNA carries thymine as its fourth base, giving it the set A, G, C, T. RNA instead swaps thymine out for uracil, giving it A, G, C, U. Thymine differs from uracil only by an extra methyl group on the ring, but that small change is exactly what marks it as the DNA-only base.
Step 3: Matching against the options.
Guanine, cytosine and adenine all show up in both nucleic acids, so none of them can be the answer. Thymine, on the other hand, is found only in DNA and is replaced by uracil whenever RNA is built instead. \[ \boxed{\text{Thymine}} \]