Final answer:
Bacteriophages have two life cycles: the lytic cycle, which leads to host cell death, and the lysogenic cycle, where the phage's DNA integrates into the host's genome. The cycle chosen by the phage can be influenced by environmental stressors.
Step-by-step explanation:
The bacteriophages have two alternative life cycles called the lytic cycle and the lysogenic cycle. In the lytic cycle, a bacteriophage infects the bacterial cell, takes control of the cell machinery, replicates itself, and eventually causes the host cell to burst, releasing new phages. In contrast, the lysogenic cycle involves the integration of the phage's DNA into the host cell's genome, where it can remain dormant for an extended period until external stressors trigger it to revert to the lytic cycle.
DNA injection is a critical step in these cycles, where bacteriophages inject their DNA into bacteria, in contrast to animal viruses which may use endocytosis or membrane fusion. Virulent bacteriophages, such as the T4 phage, typically proceed through the lytic cycle, whereas temperate bacteriophages, like the λ (lambda) phage, can opt for the lysogenic cycle. Environmental factors may influence a bacteriophage to switch from a dormant lysogenic state to an active lytic phase.