In DNA, adenine always pairs with –
**Core Concept**
Adenine (A) is a purine nucleotide base found in DNA, and it always pairs with a specific pyrimidine nucleotide base through hydrogen bonding, playing a crucial role in the structure and replication of DNA.
**Why the Correct Answer is Right**
Adenine pairs with thymine (T) due to the unique shape and chemical properties of these bases. Adenine has two nitrogenous bases, while thymine has a single nitrogenous base. This allows for a specific hydrogen bonding pattern between adenine and thymine, with two hydrogen bonds forming between the bases. This pairing is essential for the stability and replication of DNA.
**Why Each Wrong Option is Incorrect**
**Option A:** Guanine (G) - Guanine is another purine nucleotide base, but it pairs with cytosine (C), not adenine. Guanine and cytosine form three hydrogen bonds, which is different from the two hydrogen bonds between adenine and thymine.
**Option B:** Cytosine (C) - Cytosine is a pyrimidine nucleotide base that pairs with guanine (G), not adenine. This pairing is also stabilized by three hydrogen bonds.
**Option C:** Uracil (U) - Uracil is a pyrimidine nucleotide base found in RNA, but not in DNA. In RNA, uracil replaces thymine and pairs with adenine.
**Clinical Pearl / High-Yield Fact**
Remember the base pairing rules in DNA: Adenine (A) pairs with Thymine (T) and Guanine (G) pairs with Cytosine (C). This fundamental concept is crucial for understanding DNA structure, replication, and mutation.
**Correct Answer: D. Thymine**