Which one of the following vectors is used to replace the defective gene in gene therapy ?

Options

A. Retrovirus
B. Cosmid
C. Ri plasmid
D. Ti plasmid

Answer

To solve the question regarding which vector is used to replace the defective gene in gene therapy, we can follow these steps:

### Step 1: Understand the Concept of Gene Therapy
Gene therapy involves introducing, removing, or altering genetic material within a person's cells to treat or prevent disease. The goal is to correct defective genes responsible for disease development.

**Hint:** Think about how genetic material can be altered or replaced in cells.

### Step 2: Identify the Role of Vectors in Gene Therapy
Vectors are vehicles used to deliver genetic material into cells. They are crucial in gene therapy as they help introduce the new or corrected gene into the target cells.

**Hint:** Consider what types of biological entities can carry genes into cells effectively.

### Step 3: Evaluate the Options Provided
The options given are:
- A) Retrovirus
- B) Cosmid
- C) Ri plasmid
- D) Ti plasmid

We need to analyze each option to determine which is most suitable for gene therapy.

**Hint:** Recall the characteristics of each vector type and their common applications.

### Step 4: Analyze Each Vector
- **Retrovirus:** These are RNA viruses that can integrate their genetic material into the host's DNA. This property makes them effective for gene therapy as they can permanently alter the genetic makeup of the target cells.

- **Cosmid:** These are hybrid vectors that can carry larger DNA fragments but are not typically used for gene therapy in humans.

- **Ri plasmid and Ti plasmid:** These are vectors used primarily in plant genetic engineering, not in human gene therapy.

**Hint:** Focus on which vector can integrate its genetic material into human cells effectively.

### Step 5: Conclusion
Based on the analysis, the most suitable vector for replacing a defective gene in gene therapy is the **retrovirus**. It is capable of integrating into the host genome, allowing for long-term expression of the introduced gene.

**Final Answer:** A) Retrovirus