EcoRI, a significant tool in rDNA technology is -

The correct option/answer is Enzyme.

Here is a step-by-step explanation of why this answer is correct:

1. Understanding EcoRI:
EcoRI (pronounced "eco-R-one") is a restriction endonuclease. Restriction endonucleases are a class of proteins known as restriction enzymes.

2. Function in rDNA Technology:
In recombinant DNA (rDNA) technology, EcoRI acts as "molecular scissors." It specifically recognizes a unique double-stranded DNA sequence (5'-GAATTC-3') and cleaves the phosphodiester backbone between the Guanine (G) and Adenine (A) bases. This cleavage generates complementary single-stranded ends, commonly referred to as "sticky ends," which facilitate the joining of different DNA fragments.

3. Breakdown of the Options:
• Bacteria: While EcoRI is isolated from a bacterium (specifically, the RY13 strain of Escherichia coli), the tool itself is a protein isolated from the organism, not the organism itself.
• Plasmid: A plasmid is a circular DNA molecule used as a vector to carry foreign genetic material, not a cutting tool.
• Enzyme: EcoRI is biologically classified as a restriction enzyme (specifically a restriction endonuclease) because it catalyzes the hydrolysis of nucleic acids.
• Purine: Purine is a type of nitrogenous base (like Adenine and Guanine) found in nucleotides, not a macromolecular tool.

Therefore, EcoRI is classified as an Enzyme.