Introduction to viruses: Difference between revisions

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; Regressive theory : Viruses may have once been small cells that [[parasitism|parasitised]] larger cells. Eventually, the genes they no longer needed for a parasitic way of life were lost. The bacteria ''[[Rickettsia]]'' and ''[[Chlamydia (bacterium)|Chlamydia]]'' are living cells that, like viruses, can reproduce only inside host cells. This lends credence to this theory, as their dependence on being parasites may have led to the loss of the genes that once allowed them to live on their own.<ref>{{harvnb|Collier|Balows|Sussman|1998|p=11}}</ref>

; Cellular origin theory : Some viruses may have evolved from bits of DNA or RNA that "escaped" from the genes of a larger organism. The escaped DNA could have come from [[plasmid]]s—pieces of DNA that can move between cells—while others may have evolved from bacteriacells.<ref>{{harvnb|Collier|Balows|Sussman|1998|pp=11–12}}</ref>

; Coevolution theory : Viruses may have evolved from complex molecules of protein and DNA at the same time as cells first appeared on earth, and would have depended on cellular life for many millions of years.<ref name =Wessner>{{cite journal | vauthors = Wessner DR | year = 2010 | title = The Origins of Viruses | journal = Nature Education | volume = 3 | issue = 9| page = 37 }}</ref>