If a radiolabel is used to tag a DNA molecule, the technique used to localise would be:

Options

A. X-ray crystallography
B. Autoradiography
C. Fluorescence microscopy
D. Electron microscop

Answer

To solve the question regarding the technique used to localize a DNA molecule tagged with a radiolabel, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Question**: The question asks for a technique to localize a DNA molecule that has been tagged with a radiolabel. This means we need to identify a method that can detect and visualize the presence of the radiolabel in the DNA.

2. **Analyze the Options**: The options provided are:
- A) X-ray crystallography
- B) Autoradiography
- C) Fluorescence microscopy
- D) Electron microscopy

3. **Evaluate Each Technique**:
- **X-ray Crystallography**: This technique is primarily used to determine the atomic structure of crystals. It does not specifically localize radiolabeled DNA.
- **Autoradiography**: This technique is specifically designed to visualize the distribution of radiolabeled molecules. It involves exposing a photographic film or a detector to a sample containing the radiolabel, allowing for the localization of the tagged DNA.
- **Fluorescence Microscopy**: This method uses fluorescent dyes to visualize structures in cells, but it does not directly relate to radiolabeling.
- **Electron Microscopy**: This technique provides high-resolution images of biological samples but is not specifically used for detecting radiolabels.

4. **Identify the Correct Answer**: Based on the analysis, the technique that is specifically used to localize a radiolabeled DNA molecule is **Autoradiography** (Option B).

5. **Conclusion**: Therefore, the correct answer to the question is **B) Autoradiography**.