The presence or absence of hydroxyl group on which carbon atom of sugar differentiates RNA and DNA ?

Biomolecules PYQ — JEE Main 2011 · Shift 0 · Q11 with Solution

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Correct answer: A

Identify the sugars present in RNA and DNA
- RNA contains ribose.
- DNA contains 2-deoxyribose.
Compare ribose and deoxyribose

The key structural difference is at the $2'$ carbon atom:
- In ribose (RNA), the $2'$ carbon has an $-OH$ group.
- In 2-deoxyribose (DNA), the $2'$ carbon has $-H$ instead of $-OH$ .
Conclusion

Therefore, RNA and DNA are differentiated by the presence or absence of the hydroxyl group on the 2nd carbon atom.
Evaluate options
- A: 2nd $ightarrow$ Correct
- B: 3rd $ightarrow$ Incorrect
- C: 4th $ightarrow$ Incorrect
- D: 1st $ightarrow$ Incorrect

Hence, the correct answer is A.