When a virus enters a cell, its top priority is to replicate its genome rapidly, creating new viruses to infect elsewhere. The host has the opposite priority: to destroy the virus, ideally before it can do any harm. Biology has evolved diverse tactics for viral recognition and destruction, but they share a common theme—sensing that an invading viral genome is present in a cell.
This poses a general problem across all of life, including our own: How does a cell recognize invading DNA as viral when, to a cell, DNA is just DNA—the same chemical with the same shape, whatever its source? Different branches of life have answered this in strikingly different ways, and bacteria, as it turns out, found quite unexpected solutions.
Two solutions to an old problem
Recognizing viruses is an especially pressing problem for bacteria, which are outnumbered by viruses and under daily attack. From the 1950s–1970s, scientists realized that bacteria can chemically mark their own DNA at specific sequences, creating an ID tag for "self." A virus entering without the same ID is recognized as "non-self" and destroyed. Discovery of these "restriction-modification" systems revealed how viral infections are repelled. It also revolutionized molecular biology by giving rise to molecular cloning, which has altered society over the past few decades.
