Single ringed nitrogenous base.

Single ringed nitrogenous base.

• A. Purine
• B. Nucleotide
• C. Purine base
• D. Pyrimidine

Answer: (D)

Single-ringed nitrogenous bases belong to the pyrimidine family. The bases are split into two groups based on ring structure: purines have two rings, while pyrimidines have one. So a base with a single ring is a pyrimidine, with examples like cytosine, thymine, and uracil. These pyrimidines pair with purines to maintain the width of the DNA/RNA helix (A pairs with T or U, G pairs with C). The other options don’t fit because a nucleotide is the whole molecule (base plus sugar and phosphate), a purine base refers to the two-ringed family, and purine denotes the double-ringed group.

Single-ringed nitrogenous bases belong to the pyrimidine family. The bases are split into two groups based on ring structure: purines have two rings, while pyrimidines have one. So a base with a single ring is a pyrimidine, with examples like cytosine, thymine, and uracil. These pyrimidines pair with purines to maintain the width of the DNA/RNA helix (A pairs with T or U, G pairs with C). The other options don’t fit because a nucleotide is the whole molecule (base plus sugar and phosphate), a purine base refers to the two-ringed family, and purine denotes the double-ringed group.