Question

What type of nitrogenous base is Uracil and what is its structure?

• A. Purine base \(\rightarrow\) 2 rings, structure similar to adenine
• B. Pyrimidine base \(\rightarrow\) 1 ring, 2 keto groups at C2 & C4; structure similar to thymine but without \(-CH_{3}\) at C5
• C. Purine base \(\rightarrow\) 1 ring, 3 keto groups
• D. Pyrimidine base \(\rightarrow\) 2 rings, 1 keto group

Hint:

Remember the nitrogenous bases: \[ \boxed{ \begin{aligned} \text{Purines} &: A,\;G\\ \text{Pyrimidines} &: C,\;T,\;U \end{aligned} } \] Uracil is present only in RNA and is structurally similar to thymine, except that it lacks the methyl (\(-CH_3\)) group at C5.

Answer 1

The Correct Option is B

Concept: Nitrogenous bases present in nucleic acids are classified into two groups:

• Purines: Adenine (A) and Guanine (G) — contain two fused rings.

• Pyrimidines: Cytosine (C), Thymine (T) and Uracil (U) — contain a single six-membered ring.
Uracil is present in RNA and differs from thymine by the absence of a methyl group at the fifth carbon atom.

Step 1: Identify the class of Uracil.
Uracil belongs to the pyrimidine family of nitrogenous bases. Hence, \[ \boxed{\text{Uracil is a pyrimidine base}.} \] It contains \[ \boxed{\text{one six-membered ring}.} \]

Step 2: Study its structure.
Uracil possesses

• Two keto (\(=O\)) groups at carbon atoms C2 and C4.

• No methyl group at the C5 position.
Thymine has a methyl group at C5, whereas Uracil does not. Thus, \[ \boxed{\text{Uracil is similar to thymine except for the absence of }-CH_3.} \]

Step 3: Choose the correct option.
Therefore, \[ \boxed{\text{Uracil is a pyrimidine base having one ring and two keto groups.}} \] Hence, \[ \boxed{\textbf{Option (B)}} \] is the correct answer.