A single strand of nucleic acid tagged with a radioactive molecule is called:

Options

A. selectable marker
B. plasmid
C. probe
D. vector

Answer

To answer the question "A single strand of nucleic acid tagged with a radioactive molecule is called:", we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Components**:
- We need to identify what a single strand of nucleic acid is. Nucleic acids include DNA and RNA, which can exist in single-stranded forms.

2. **Identifying the Tag**:
- The question specifies that this nucleic acid is tagged with a radioactive molecule. Radioactive tagging is often used in molecular biology for detection purposes.

3. **Defining the Term**:
- In molecular biology, a single strand of nucleic acid (either DNA or RNA) that is labeled with a radioactive molecule is referred to as a "probe." Probes are used to detect specific sequences of nucleic acids.

4. **Understanding the Function of Probes**:
- Probes are designed to be complementary to the target sequence of interest. When the probe binds to the target sequence, it can be detected due to its radioactive label, allowing researchers to locate specific genes or sequences within a sample.

5. **Conclusion**:
- Therefore, the answer to the question is that a single strand of nucleic acid tagged with a radioactive molecule is called a "probe."

### Final Answer:
A single strand of nucleic acid tagged with a radioactive molecule is called a **probe**.