The fact that a purine base always paired through hydrogen bonds with a pyrimidine base leads to, in the DNA double helix

Options

A. the antiparallel nature
B. the semiconservative nature
C. uniform width throughout DNA
D. uniform length in all DNA

Explanation

To solve the question regarding the pairing of purine and pyrimidine bases in the DNA double helix, we can break it down into the following steps:

### Step-by-Step Solution:

1. **Understanding Base Pairing**:
- In DNA, there are two types of nitrogenous bases: purines (adenine and guanine) and pyrimidines (cytosine and thymine).
- Purines have a double-ring structure, while pyrimidines have a single-ring structure.

2. **Hydrogen Bonding**:
- Purines always pair with pyrimidines through hydrogen bonds. Specifically, adenine pairs with thymine (A-T) via two hydrogen bonds, and guanine pairs with cytosine (G-C) via three hydrogen bonds.
- This specific pairing is crucial for the stability of the DNA structure.

3. **Constant Diameter of DNA**:
- The pairing of one purine with one pyrimidine ensures that the width of the DNA double helix remains constant.
- If two purines or two pyrimidines were to pair, the width of the helix would vary, leading to structural instability.

4. **Uniform Width**:
- Because of the consistent pairing of purines and pyrimidines, the DNA maintains a uniform diameter throughout its length.
- This uniformity is essential for the proper functioning of DNA, including replication and transcription.

5. **Conclusion**:
- The fact that a purine base always pairs with a pyrimidine base leads to a uniform width throughout the DNA double helix.

### Final Answer:
The correct conclusion is that the pairing of purines with pyrimidines leads to **uniform width throughout the DNA**.

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