Restriction enzymes

Options

A. are endonucleases which cleave DNA at specific sites
B. make DNA complementary to an existing DNA or RNA
C. cut or join DNA fragments
D. are required in vectorless direct gene transfer

Answer

### Step-by-Step Solution:

1. **Definition of Restriction Enzymes**:
- Restriction enzymes, also known as restriction endonucleases, are enzymes that cleave DNA at specific sites. They are crucial tools in molecular biology for manipulating DNA.

2. **Function of Restriction Enzymes**:
- These enzymes cut the DNA double helix into fragments at specific sequences, which allows for the analysis and modification of DNA. They can create sticky or blunt ends, which are useful for ligating DNA fragments together.

3. **Example of Restriction Enzymes**:
- A common example of a restriction enzyme is found in *Escherichia coli* (E. coli). This bacterium produces various restriction enzymes that recognize specific nucleotide sequences in DNA.

4. **Mechanism of Action**:
- Restriction enzymes recognize specific palindromic sequences in the DNA and cleave the DNA at these sites. The resulting fragments can have sticky ends (overhangs) or blunt ends, depending on the enzyme used.

5. **Importance in Biotechnology**:
- Restriction enzymes are essential in various biotechnological applications, including cloning, gene transfer, and DNA analysis. They are often used in conjunction with ligases to join DNA fragments together.

6. **Restriction Modification System**:
- The restriction enzymes are part of a system known as the restriction modification system, which protects bacterial cells from foreign DNA (like that from viruses) by cutting it.