Final answer:
The statement that a bacteriophage injects nucleic acid into a bacterium is true. Bacteriophages use their tail structures to inject nucleic acid, leading to either the lytic or lysogenic cycle, with environmental stress typically initiating the lytic cycle from a dormant lysogenic state. The correct option is A.
Step-by-step explanation:
A bacteriophage infects a bacterial cell by injecting nucleic acid into the bacterium. This statement is TRUE. Bacteriophages, which are viruses that infect bacteria, attach to the bacterial cell and use complex tail structures to actively insert their nucleic acid into the host. The protein coat, however, remains outside.
In the lytic cycle, the bacteriophage commandeers the host's cellular machinery to replicate its nucleic acid and produce new phage particles, which ultimately are released into the environment when the bacterial cell lyses. Conversely, in the lysogenic cycle, the phage DNA gets incorporated into the host genome and can remain dormant as a prophage until certain triggers initiate the lytic cycle.
It's important to note that an environmental stressor typically causes the prophage to exit the genome and enter the lytic cycle, not initiate the lysogenic cycle as one might mistakenly assume.