In the structure of DNA, cytosine always pairs with which of the following bases?

• A. adenine
• B. guanine
• C. thymine
• D. uracil

Answer: (B)

Verified step by step guidance

1. Recall the base pairing rules in DNA, which are based on complementary hydrogen bonding between nitrogenous bases.
2. Identify the four bases in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G).
3. Remember that adenine pairs with thymine, and cytosine pairs with guanine.
4. Understand that uracil is found in RNA, not DNA, so it does not pair with cytosine in DNA.
5. Therefore, cytosine always pairs with guanine in the DNA double helix structure.