[Answer] Why are viruses not included in the three domain classification system?

Answer: They are not composed of cells
Why are viruses not included in the three domain classification system?

Baltimore classification (first defined in 1971) is a classification system that places viruses into one of seven groups depending on a combination of their nucleic acid (DNA or RNA) strandedness (single-stranded or double-stranded) sense and method of replication. Named after David Baltimore a Nobel Prize-winning biologist these groups are designated by Roman numerals. Other classifications are determined by the disease caused by the virus or its morphology neither of which are satisfactory due t…

The three-domain system adds a level of classification (the domains) “above” the kingdoms present in the previously used five- or six-kingdom systems. This classification system recognizes the fundamental divide between the two prokaryotic groups insofar as Archaea appear to be more closely related to Eukaryotes than they are to other prokaryotes – bacteria-like organisms with no cell nucleus. The current system sorts the previously known kingdoms into these three domains: Archaea Bacteria and Eukarya.

Virus classification – Wikipedia

Virus classification – Wikipedia

Baltimore classification – Wikipedia

Virus classification – Wikipedia

This is an interesting position. Viruses aren’t considered a life-form therefore they do not belong to any of the three branches but viruses themselves are defined as non-life because they depend on a pre-existing living organism to maintain themselves. …

In general virus realms have no genetic relation to each other based on common descent in contrast to the three domains of cellular life—Archaea Bacteria and Eukarya—which share a common ancestor. Likewise viruses within each realm are not necessarily descended from a common ancestor since realms group viruses …

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