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Bacteriophages, or phages for short, are viruses that infect bacteria. These microscopic invaders have two main types of life cycles: the lytic cycle and the lysogenic cycle. Understanding these cycles helps to grasp how phages reproduce and interact with their bacterial hosts.
In the lytic cycle, phages take control over a bacterium’s cellular machinery to produce more phages. This eventually leads to the bacterium bursting open (lysis) and releasing new phage particles.
The lysogenic cycle, on the other hand, is where the phage DNA becomes integrated into the bacterial genome. This incorporated DNA, known as a prophage, can remain dormant for a long time before becoming active.

In the lytic cycle, a phage attaches to a bacterium and injects its DNA into the host cell. The phage DNA quickly takes over the cell machinery, directing it to produce phage components such as DNA and proteins.
These components are assembled into new phage particles. Eventually, the bacterial cell is filled with new phages and bursts open, releasing them to infect other bacteria.
This process is efficient and allows for rapid multiplication of phages. It’s important to note that the lytic cycle ends with cell lysis, which is a key distinguishing factor from the lysogenic cycle.

The lysogenic cycle starts similarly to the lytic cycle, with the phage attaching to a bacterium and injecting its DNA. However, instead of taking control of the host machinery immediately, the phage DNA gets integrated into the host’s genome. This integrated DNA is called a prophage.
The prophage can remain inactive for long periods, being replicated along with the host cell’s DNA whenever the bacterium divides.
Under certain conditions, the prophage might become active again, initiating the lytic cycle. This cycle is important for the long-term stability of the phage DNA within bacterial populations.

Environmental stressors such as UV radiation, chemicals, or unfavorable growth conditions can influence phage life cycles. They often trigger a prophage to exit the lysogenic cycle and enter the lytic cycle.
When a bacterium is stressed, the dormant prophage senses this and can reactivate. This reactivation causes the prophage to excise itself from the bacterial genome and begin producing new phages, leading to the lytic cycle and eventual cell lysis.
Hence, rather than initiating the lysogenic cycle, environmental stressors typically disrupt it, pushing the phage to opt for the productive lytic cycle.