Which of the following statements correctly describes purines and pyrimidines in DNA molecules?

• A. Purines are double-ringed bases (adenine and guanine), while pyrimidines are single-ringed bases (cytosine and thymine).
• B. Both purines and pyrimidines are double-ringed structures found only in RNA.
• C. Purines are single-ringed bases (cytosine and thymine), while pyrimidines are double-ringed bases (adenine and guanine).
• D. Purines and pyrimidines are both single-ringed structures that pair with each other randomly.

Answer: (A)

Verified step by step guidance

1. Step 1: Understand the basic structure of nitrogenous bases in DNA. DNA bases are divided into two categories based on their ring structure: purines and pyrimidines.
2. Step 2: Recall that purines have a double-ring structure, which means they consist of two fused rings. The purines in DNA are adenine (A) and guanine (G).
3. Step 3: Recall that pyrimidines have a single-ring structure, meaning they consist of only one ring. The pyrimidines in DNA are cytosine (C) and thymine (T).
4. Step 4: Recognize that purines always pair with pyrimidines in DNA to maintain a consistent width of the DNA double helix. Specifically, adenine pairs with thymine, and guanine pairs with cytosine.
5. Step 5: Evaluate the given statements by matching them with these facts: purines are double-ringed bases (adenine and guanine), and pyrimidines are single-ringed bases (cytosine and thymine). This will help identify the correct description.