There are 5 kingdoms; Kingdom Animalia, Kingdom Plantae, Kingdom Fungi, Kingdom Minera, and Kingdom Protistae. Why isn’t there another one for viruses, since they fit in neither of these kingdoms, due to the lack of DNA?
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Viruses aren’t assigned a kingdom because they can’t live without a host cell. Same as prions I believe.
Would they be in the same category as mitochondria and chloroplasts, then?
I believe the major reason is because viruses aren’t considered alive. You can’t assign a non-living thing to an animal kingdom. If you do, where do you draw the line? Are self-replicating proteins animals as well?
On top of that, there are various definitions of life and they all have trouble with weird edge cases like viruses, self replicating RNA, some crystals etc. I prefer the consensus list definition. If it’s on the list it’s alive. If not, it’s dead. Want something in the list? Convince the scientific community that your new edge cases is alive.
I thought something that self-replicates is alive. Viruses aren’t alive because they can’t replicate by themselves, they need another living thing to inhabit.
It’s just one of several criteria, and not everyone agrees about which ones count.
They didn’t evolve from the Last Universal Common Ancestor the same way living organisms did. I think the current theory (or one of them) is that they were mechanisms for horizontal gene transfer that ran amok.
Viruses have DNA. What they lack is the ability to self-replicate. They need a host cell for that, and thus miss one of the key criteria we use to define “life”.
