Genetic engineering would not have been possible if one of these were absent

Options

A. DNA polymerase
B. Reverse transcriptase
C. DNA ligase
D. RNA synthetase

Answer

### Step-by-Step Solution:

1. **Understanding Genetic Engineering**: Genetic engineering involves manipulating an organism's DNA to achieve desired traits. This process requires specific enzymes to facilitate various reactions.

2. **Analyzing the Options**:
- **Option 1: DNA Polymerase**: This enzyme is crucial for DNA replication and is used in techniques like PCR (Polymerase Chain Reaction). While important, it is not the only enzyme necessary for genetic engineering.
- **Option 2: Reverse Transcriptase**: This enzyme synthesizes DNA from RNA. It is primarily used in retroviruses and is not essential for all genetic engineering processes.
- **Option 3: DNA Ligase**: This enzyme is responsible for joining two DNA fragments together. It is essential for creating recombinant DNA by ligating foreign DNA with vector DNA.
- **Option 4: RNA Synthetase**: This enzyme synthesizes RNA from a DNA template. While important for RNA synthesis, it is not directly involved in the core processes of genetic engineering.

3. **Identifying the Key Enzyme**: Among the options, DNA ligase stands out as the most critical enzyme for genetic engineering. It facilitates the ligation process, which is necessary for forming recombinant DNA.

4. **Conclusion**: Therefore, genetic engineering would not have been possible without DNA ligase, as it is essential for joining DNA fragments together.

### Final Answer:
**DNA Ligase** is the enzyme without which genetic engineering would not be possible.

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