In the structure of DNA, which two nitrogenous bases are classified as purines?

Introduction / Context:
DNA is made up of four main nitrogenous bases: adenine, thymine, guanine, and cytosine. These bases can be grouped into two families, purines and pyrimidines, based on their chemical structure. Knowing which bases belong to which family is essential for understanding base pairing rules and the geometry of the double helix. This question focuses on identifying the two purine bases in DNA.

Given Data / Assumptions:

• The question deals with DNA bases, not RNA bases.
• Options list different pairings of adenine, thymine, guanine, and uracil.
• We assume basic knowledge of the four DNA bases and that uracil is normally found in RNA instead of thymine.

Concept / Approach:
Purines are larger, double ring bases, while pyrimidines are smaller, single ring bases. In DNA, the purines are adenine and guanine. The pyrimidines in DNA are thymine and cytosine. Uracil is a pyrimidine found mainly in RNA, replacing thymine. Because a purine always pairs with a pyrimidine in the DNA double helix, adenine pairs with thymine, and guanine pairs with cytosine, maintaining a uniform width of the double helix.

Step-by-Step Solution:
Step 1: Recall that adenine and guanine are both purine bases with a double ring structure. Step 2: Remember that thymine and cytosine (and uracil in RNA) are pyrimidine bases with a single ring structure. Step 3: Note that uracil is not normally present in DNA; it appears in RNA instead. Step 4: Select the option that pairs adenine with guanine, which correctly lists the two purines in DNA.

Verification / Alternative check:
Chemical diagrams of bases show adenine and guanine each with two fused rings, confirming their classification as purines. Genetic code charts and structural diagrams of DNA list adenine and guanine together as purines and thymine and cytosine as pyrimidines. This distinction is widely used in explaining mutation types and base pairing, which supports the correctness of identifying adenine and guanine as the purines.

Why Other Options Are Wrong:

• Adenine and thymine: Adenine is a purine, but thymine is a pyrimidine; together they form a base pair but not a purine pair.
• Thymine and uracil: Both of these are pyrimidines; uracil is in RNA, not DNA.
• Guanine and uracil: Guanine is a purine, but uracil is a pyrimidine common in RNA, not DNA.

Common Pitfalls:
Students sometimes confuse base pairing with base classification, thinking that paired bases must belong to the same family. In fact, base pairing always involves one purine with one pyrimidine. A helpful memory trick is that the shorter names (A and G) are the purines, while the longer names (C, T, and U) are pyrimidines. Keeping these categories straight is useful for understanding DNA structure and mutation mechanisms.

Final Answer:
The two purines in DNA are adenine and guanine.