EcoRI is..........

Options

A. A restriction enzyme
B. A plasmid
C. Used to join two DNA fragments
D. The abbreviation for bacterium Escherichia coli.

Answer

**Step-by-Step Solution:**

1. **Identify EcoRI**: EcoRI is a restriction enzyme, which is a type of protein that can cut DNA at specific sequences.

2. **Source of EcoRI**: This enzyme is isolated from the bacterium Escherichia coli, specifically from the strain RY13.

3. **Naming Convention**: The name EcoRI is derived from its source and characteristics:
- "Eco" stands for Escherichia coli (the bacterium from which it is derived).
- "R" stands for the specific strain (RY13).
- "I" indicates that it is the first enzyme isolated from this strain.

4. **Function of EcoRI**: The primary function of EcoRI is to cleave (cut) DNA at specific recognition sites, which is crucial in molecular biology for cloning and DNA manipulation.

5. **Distinction from Other Terms**: EcoRI is not a plasmid; plasmids are extrachromosomal DNA molecules that can replicate independently of chromosomal DNA. EcoRI is also not used for joining DNA fragments; that function is performed by ligase enzymes.