Final answer:
Bacteria and Archaea defend against bacteriophage infection using the restriction modification system. This system utilizes restriction enzymes to cut foreign DNA and methylation enzymes to protect host DNA. This selective destruction of non-methylated, foreign DNA prevents productive phage infections.
Step-by-step explanation:
Mechanism of Host Defense Against Bacteriophage Infection
When a bacteriophage injects its DNA into a host cell, the cellular defense mechanisms are not guaranteed to allow a productive infection. Bacteria and Archaea have evolved systems to protect themselves against the invasion of foreign DNA, such as that from bacteriophages. One of the primary defense mechanisms is the restriction modification system. In this system, restriction enzymes recognize and cleave foreign DNA at specific sequences, thereby destroying the invading viral genome before it can take over the host machinery for replication.
Restriction enzymes are very specific; they cut double-stranded DNA at recognition sites that are typically 4-8 nucleotides long. Each bacterial species may have its own set of restriction enzymes that recognize different sequences. What prevents these enzymes from degrading the bacterium's own DNA is the addition of methyl groups to the DNA at the recognition sites by methylation enzymes. This process is called modification. The host DNA is modified, or "marked", by methylation, which signals the restriction enzymes to leave this DNA intact while targeting only unmethylated, foreign DNA for destruction.
The restriction enzymes serve as a molecular defense against bacteriophages by recognizing and cutting the bacteriophages' DNA, thereby inactivating the phage and preventing a productive infection. This process is part of a bacterial cell's immune-like response, allowing them to survive in environments where bacteriophages are present.