Final answer:
The P1 virus is a bacteriophage that infects bacteria and carries genes 'doc' and 'phd' that function as a toxin-antidote system. The 'doc' gene acts as a toxin, while the 'phd' gene provides the antidote, ensuring bacterial cells maintain the plasmid to avoid death.
Step-by-step explanation:
The P1 virus is a bacteriophage virus that infects bacteria, particularly E. coli. The 'doc' gene in P1's genome is part of a system that ensures the propagation of the virus within the bacterial population. It is coupled with another gene, 'phd', which encodes for an antidote to the 'doc' gene product. This antidote is crucial for the infected bacteria to survive. If a bacterial cell which was carrying P1 loses the plasmid due to improper plasmid segregation during cell division, the remaining 'doc' gene product, which is a toxin, will not be neutralized due to the absence of the 'phd' gene antidote, leading to the death of the daughter cell that lacks the P1 plasmid. This system creates a situation where bacteria are 'addicted' to carrying the P1 plasmid to avoid being killed by the unneutralized toxin.
Therefore, to answer the original questions - P1 is a bacteriophage that infects bacteria (a). The 'doc' gene in P1's genome functions as a toxin, part of a toxin-antidote system, ensuring the plasmid's maintenance in bacterial populations (b). The 'phd' gene encodes an antidote that neutralizes the 'doc' gene's toxin to prevent the cell from dying, hence it is characteristically an antidote to the 'doc' toxin (c). Lastly, without the P1 plasmid, a daughter bacterial cell would succumb to the toxin effects from the 'doc' gene and die, as it lacks the protective 'phd' antidote (d).