Viruses are small infectious agents that replicate only inside the living cells of an organism. They can infect all forms of life, from animals and plants to microorganisms.

DNA or RNA:
- Viruses can either have DNA or RNA as their genetic material.
- DNA viruses contain double-stranded DNA while RNA viruses contain single-stranded RNA.
- The genetic material of viruses is surrounded by a protein coat called a capsid.

Prokaryotic nucleus:
- Viruses do not have a prokaryotic or eukaryotic nucleus because they are not living organisms.
- They lack the cellular machinery required for metabolic processes and depend on the host cell for replication.

Single chromosome:
- Viruses do not have a chromosome in the traditional sense because they lack the cellular machinery required for cell division.
- Instead, they use the host cell's machinery to replicate their genetic material.

DNA enclosed in a protein coat:
- The genetic material of viruses is enclosed in a protein coat called a capsid.
- Some viruses also have an outer envelope made up of lipids, proteins, and glycoproteins.
- The capsid and envelope protect the genetic material of the virus and aid in its attachment to host cells.

Conclusion:
In summary, viruses have either DNA or RNA as their genetic material, lack a prokaryotic or eukaryotic nucleus, do not have a traditional chromosome, and have their genetic material enclosed in a protein coat.