In genetic engineering, the antibiotics are used

Options

A. as selectable markers
B. to select healthy vectors
C. as sequences from where replication starts
D. to keep the culture free of infection

Answer

**Step-by-Step Solution:**

1. **Understanding the Role of Antibiotics in Genetic Engineering:**
- Antibiotics are chemical substances produced by certain bacteria and fungi that can kill or inhibit the growth of other microorganisms, particularly pathogenic bacteria.

2. **Evaluating the Options:**
- The question provides several options regarding the use of antibiotics in genetic engineering. We need to analyze each option carefully.

3. **Option 1: Used as a Selectable Marker**
- This option is incorrect. While antibiotic resistance genes can act as selectable markers in genetic engineering, the antibiotics themselves are not used directly as selectable markers. Instead, the genes that confer resistance to antibiotics are utilized.

4. **Option 2: To Select Healthy Vectors**
- This option is also incorrect. The term "healthy vectors" is not a standard term in genetic engineering. There is no established method to classify vectors as healthy or unhealthy.

5. **Option 3: As Sequences from Where the Replication Starts**
- This option is incorrect as well. The origin of replication (ORI) is a specific sequence in DNA where replication begins, but it is not related to the use of antibiotics.

6. **Option 4: To Keep Cell Culture Free from Infection**
- This option is correct. Antibiotics are commonly used in cell culture to prevent contamination by other microorganisms, ensuring that the cultures remain sterile and free from infection.

7. **Conclusion:**
- The correct answer to the question is that antibiotics are used in genetic engineering to keep cell cultures free from infection.