What characterizes a retrovirus?

A retrovirus is primarily characterized by containing RNA as its genetic material. This type of virus employs a unique mechanism for replication that involves reverse transcription, where the viral RNA is converted back into DNA after entering a host cell. This DNA then integrates into the host's genome, allowing the virus to replicate alongside the host's cellular machinery.

The other options do not correctly depict retroviruses. For instance, a retrovirus does not contain DNA as its genetic information; instead, its structure is based on RNA. Moreover, while retroviruses contain protein, they are not solely made of protein; they also require RNA genomes for their replication and infection processes. Lastly, retroviruses are not a type of bacteria; they belong to a distinct category of pathogens, separate from prokaryotic cells commonly associated with bacterial species. Understanding these characteristics is essential in virology and helps differentiate retroviruses from other types of pathogens.