In molecular biology, what is the enzyme called that recognizes specific foreign DNA sequences and cleaves the DNA at or near those sites?

Introduction / Context:
Restriction endonucleases underpin recombinant DNA technology, cloning, and genomic mapping. They evolved as bacterial defense mechanisms against foreign DNA such as bacteriophages, providing sequence-specific cleavage.

Given Data / Assumptions:

• The enzyme must recognize a DNA sequence motif.
• It must introduce a cut (double-strand break or nick) at or near that motif.
• Context implies defense or laboratory usage.

Concept / Approach:
Restriction enzymes (e.g., EcoRI, HindIII) recognize palindromic sequences and cleave DNA to produce sticky or blunt ends. In bacteria, methylation systems protect host DNA from self-cleavage. Other enzymes listed do not perform sequence-specific restriction: transposase moves DNA elements, helicase unwinds DNA, and reverse transcriptase copies RNA into DNA.

Step-by-Step Solution:
Identify the requirement: sequence-specific recognition and cleavage.Match to restriction endonucleases from bacteria.Exclude enzymes with unrelated functions (helicase, transposase, reverse transcriptase).Select restriction endonuclease.

Verification / Alternative check:
Restriction–modification systems pair endonucleases with methyltransferases; this pairing is well documented in microbial genetics.

Why Other Options Are Wrong:
Transposase: catalyzes transposition; not a defense nuclease against foreign DNA sequences.Helicase: unwinds duplex DNA; no site-specific cutting.Reverse transcriptase: synthesizes DNA from RNA; no sequence-specific DNA cutting.

Common Pitfalls:
Confusing general nucleases with sequence-specific restriction enzymes; only the latter recognize defined motifs.

Final Answer:
Restriction endonuclease