Restriction enzymes were first discovered with the observation that
a. DNA is restricted to the nucleus.
b. Bacteriophage DNA is destroyed in a host cell.
c. Foreign DNA is kept out of a cell.
d. Foreign DNA is restricted to the cytoplasm.
e. All of the above.

Verified step by step guidance

1. Understand the historical context: Restriction enzymes were discovered through studies involving bacteriophages (viruses that infect bacteria) and their interaction with bacterial host cells.
2. Recall that the key observation was that bacteriophage DNA was selectively destroyed when introduced into certain bacterial cells, which led to the concept of restriction-modification systems.
3. Recognize that this destruction of foreign DNA (such as bacteriophage DNA) is a defense mechanism by bacteria to protect themselves from viral infection.
4. Evaluate each option in the question: option (b) states that bacteriophage DNA is destroyed in a host cell, which aligns with the original discovery of restriction enzymes.
5. Conclude that the correct understanding is that restriction enzymes were first discovered because bacteriophage DNA was destroyed in the host bacterial cell, corresponding to option (b).

Key Concepts

Restriction Enzymes

Restriction enzymes are proteins produced by bacteria that cut DNA at specific sequences. They serve as a defense mechanism by recognizing and cleaving foreign DNA, such as that from invading bacteriophages, thereby protecting the bacterial genome.

Bacteriophage Infection and Host Defense

Bacteriophages are viruses that infect bacteria by injecting their DNA. Bacteria defend themselves by destroying this foreign DNA using restriction enzymes, preventing the phage from hijacking the bacterial machinery for replication.

Foreign DNA Recognition and Degradation

Cells can distinguish between their own DNA and foreign DNA based on specific markers like methylation patterns. Restriction enzymes target unmethylated foreign DNA for cleavage, thereby restricting the presence and replication of foreign genetic material within the host.