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JEE Main · 2021 · 25 Jul 2021 · Shift 2 · Biomolecules

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Which one of the following is correct structure for cytosine?
  1. (A)(see structure)
  2. (B)(see structure)
  3. (C)(see structure)
  4. (D)(see structure)
    ✓ Correct

Answer: (D)

Solution

Cytosine is one of the two pyrimidine bases of DNA. Its six-membered ring carries nitrogen at positions 1 and 3, a carbonyl group at C-2 lying between those two nitrogen atoms, and an amino group at C-4, while N-1 bears the hydrogen through which the base is linked to the sugar in a nucleoside. A ring with a methyl group in place of that amino group is not a nucleic acid base at all, and a ring with two carbonyl groups and a methyl group at C-5 is thymine. Cytosine is therefore 4-aminopyrimidin-2(1H)-one, with at C-4, at C-2 and hydrogen on N-1.

JEE Main · 2021 · 25 Jul 2021 · Shift 2 · Single correct option

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