Restriction enzymes

Options

A. Are endonucleases which cleave DNA at specific sites
B. MakeDNA complementary to an existingDNA or RNA
C. Cut or join DNA fragments
D. Are required in vector-less direct gene transfer

Answer

### Step-by-Step Text Solution

1. **Definition of Restriction Enzymes**:
- Restriction enzymes, also known as restriction endonucleases, are enzymes that cut DNA at specific sequences.

2. **Understanding the Term 'Endonucleases'**:
- The term 'endonuclease' can be broken down into two parts:
- "Endo" means 'inside' or 'within'.
- "Nuclease" refers to an enzyme that cuts nucleic acids (DNA or RNA).
- Therefore, endonucleases are enzymes that cut nucleic acids from within the strand.

3. **Functionality of Restriction Enzymes**:
- These enzymes recognize specific sequences in the DNA and cleave the DNA at those sites. This is crucial for various biotechnological applications, including cloning and genetic engineering.

4. **Analyzing the Options**:
- **Option 1**: "Are endonucleases which cleave DNA at specific sites."
- This is correct as it directly describes the function of restriction enzymes.
- **Option 2**: "Make DNA complementary to an existing DNA or RNA."
- This is incorrect. This process involves other mechanisms such as transcription or reverse transcription.
- **Option 3**: "Cut or join DNA fragments."
- This is partially correct. While they do cut DNA, they do not join fragments; ligase enzymes are responsible for joining DNA fragments.
- **Option 4**: "Are required in vectorless direct gene transfer."
- This is incorrect as vectorless gene transfer does not involve restriction enzymes, which typically require vectors for gene transfer.

5. **Conclusion**:
- Based on the analysis, the correct answer is that restriction enzymes are endonucleases that cleave DNA at specific sites.