Which one of the following is not uniform in all DNA molecules?

Options

A. The arrangement of sugar-phosphate groups
B. The pattern of the helix structure
C. The pairing of adenine with thymine and cytosine with guanine
D. The sequence of nitrogenous bases on a strand of DNA

Answer

To determine which aspect of DNA molecules is not uniform across all DNA, we can analyze the components of DNA and their characteristics. Here’s a step-by-step breakdown of the solution:

### Step 1: Understand the Structure of DNA
DNA (deoxyribonucleic acid) is composed of two strands that form a double helix. Each strand is made up of a backbone of sugar and phosphate groups, and attached to this backbone are nitrogenous bases.

### Step 2: Identify the Components of DNA
The main components of DNA include:
- **Sugar-phosphate backbone**: This is uniform across all DNA molecules.
- **Nitrogenous bases**: These include adenine (A), thymine (T), cytosine (C), and guanine (G). The pairing of these bases is also uniform (A pairs with T, and C pairs with G).

### Step 3: Analyze the Options
Given the question asks which aspect is not uniform, we need to evaluate the following:
- **Arrangement of sugar-phosphate groups**: This is uniform in all DNA molecules.
- **Helical structure**: The helical structure of DNA is also uniform.
- **Base pairing**: The pairing of A with T and C with G is consistent across all DNA.
- **Sequence of nitrogen bases**: This varies from one DNA molecule to another, as different organisms and individuals have different genetic information.

### Step 4: Conclusion
The only aspect that is not uniform in all DNA molecules is the **sequence of nitrogen bases**. This sequence can differ significantly between different organisms and even between individuals of the same species.

### Final Answer
The correct answer is: **The sequence of nitrogen bases in a DNA strand.**

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