Genetic engineering is possible because

Options

A. We can cut DNA at specific sites by restriction endonucleases
B. restriction endonucleases purified sites by restriction used in bacteria
C. the phenomenon of transduction in bacteria is well understood
D. we can see DNA by electron microscope

Explanation

### Step-by-Step Solution:

1. **Understanding Genetic Engineering**:
- Genetic engineering refers to the process of altering the DNA within an organism's genome. This can involve adding, removing, or modifying genetic material to achieve desired traits or characteristics.

2. **Role of Restriction Endonucleases**:
- A key tool in genetic engineering is the use of restriction endonucleases (also known as restriction enzymes). These enzymes can cut DNA at specific sequences, allowing scientists to isolate and manipulate genes of interest.

3. **Analyzing the Options**:
- **Option A**: "We can cut DNA at specific sites by restriction endonucleases."
- This option directly relates to the ability to manipulate DNA, making it a fundamental aspect of genetic engineering.
- **Option B**: "Restriction endonucleases purified sites by restriction used in bacteria."
- This statement is unclear and does not directly explain the process of genetic engineering.
- **Option C**: "The phenomena of transduction in bacteria is well understood."
- While transduction is important in genetics, it does not specifically relate to the techniques used in genetic engineering.
- **Option D**: "We can see DNA by electron microscope."
- Visualization of DNA does not contribute to the process of genetic engineering itself.

4. **Conclusion**:
- The correct answer is **Option A** because the ability to cut DNA at specific sites using restriction endonucleases is essential for the manipulation and recombination of DNA, which is the basis of genetic engineering.