Out of following isomeric forms of uracil, which one is present in RNA?

Solution

Chemistry ma’am · 3 steps

Uracil is drawn in many tautomers, but in RNA it sits in the diketo form with two C=O and two N–H.

1. Same molecule, moved hydrogens. All four structures are tautomers of uracil. They differ only in where an H sits: on a ring N, giving N-H next to C=O (the lactam, or keto form), or on the O, giving C-OH next to C=N (the lactim, or enol form).

   Option	At C2	At C4	Form
   (a)	C-OH	C-OH	di-lactim (both enol)
   (b)	C=O, N1–H	C-OH	mixed
   (c)	C-OH	C=O, N3–H	mixed
   (d)	C=O, N1–H	C=O, N3–H	di-lactam (diketo)

2. Why the diketo form. The amide-like N-H/C=O arrangement is much more stable, so uracil exists almost entirely as the diketo tautomer. In RNA, N1 bonds to C1′ of ribose, and the N3–H and C4=O form the two hydrogen bonds with adenine. Only the diketo form has all of these groups in place.
3. So. The form present in RNA is (d), pyrimidine-2,4(1H,3H)-dione, with both carbonyls and both ring N–H groups.

The idea to keep. For bases in DNA and RNA (uracil, thymine, guanine, cytosine), always pick the lactam form: C=O next to N–H. That is the stable tautomer, and it gives the correct base pairing with adenine or its partner.

Answer from the NTA final answer key. Solution by PrepWiser.