खालीलपैकी कोणता नायट्रोजन बेस हा आर.एन.ए. मध्ये नसून फक्त डी.एन.ए. मध्येच आढळतो ?

The correct answer is Option A (Thymine).

Key Points:
• Core Concept
• Nucleic acids (DNA and RNA) are composed of purine and pyrimidine nitrogenous bases bonded to a pentose sugar-phosphate backbone.
• Why Correct
• DNA contains four bases: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T).
• RNA contains four bases: Adenine (A), Guanine (G), Cytosine (C), and Uracil (U).
• Therefore, Thymine is the pyrimidine nitrogen base that is present in DNA but absent in RNA (where it is replaced by Uracil).
• Key Details
• Chemically, Thymine is 5-methyluracil; methylation of uracil provides DNA with greater photochemical stability against enzymatic degradation.
• Exam Traps
• Read the question carefully: "absent in RNA but present in DNA" points to Thymine. If asked "present in RNA but absent in DNA", the answer would be Uracil.
• Other Options
• Uracil (Option 2) is present in RNA and absent in DNA.
• Adenine (Option 3) and Guanine (Option 4) are purines present in both DNA and RNA.
• Key Takeaway: Thymine is present only in DNA (replaced by Uracil in RNA), while Adenine, Guanine, and Cytosine are common to both.