What is a characteristic of viruses?

Viruses are unique biological entities that differ significantly from living cells in several ways. One of their key characteristics is that they can be composed of either DNA or RNA, but not both simultaneously. This property is fundamental to their structure and function. The genetic material of a virus—either DNA (which can be double-stranded or single-stranded) or RNA (which can also be single-stranded or double-stranded)—encodes the instructions necessary for the virus to replicate and infect host cells.

This characteristic highlights the simplicity of viruses compared to cellular life forms that typically contain both DNA and RNA in various forms within their structures. Additionally, viruses do not possess the cellular machinery and organelles necessary for metabolic functions, which sets them apart further from living organisms. Instead, they rely entirely on hijacking the host's cellular machinery for replication and propagation. Consequently, the composition of their genetic material as either DNA or RNA is a defining feature of viral biology.