This virus contains both DNA and RNA

The correct option is Leuko virus.

Typically, a virus contains only one type of nucleic acid as its genetic material, which is either DNA or RNA, but never both. However, certain viruses exhibit exceptions or unique replication cycles. Among the given options, the Leuko virus (specifically leukoviruses or retroviruses) is known to contain both RNA and DNA at different stages of its life cycle, or carry specific nucleic acid configurations that associate both molecules within the viral replication process.

Let us analyze the options to understand why Leuko virus is the correct choice:

1. Polio virus: This is a single-stranded RNA (ssRNA) virus. It contains only RNA and lacks DNA.
2. Herpes virus: This is a double-stranded DNA (dsDNA) virus. It contains only DNA and lacks RNA within its mature virion structure.
3. Cyanophage: These are viruses that infect cyanobacteria. Most cyanophages are double-stranded DNA viruses and do not contain RNA as their genomic material.
4. Leuko virus: Leukoviruses (frequently associated with retroviruses like Rous sarcoma virus) are RNA viruses that replicate through a DNA intermediate using the enzyme reverse transcriptase. Because their replication cycle intimately involves both genomic RNA and synthesized complementary DNA (cDNA) within the host cell, and because viral particles can carry reverse transcriptase containing or associated with nucleic acids, they are classically characterized in biology questions as the group containing both DNA and RNA during their complete life/replication cycle.