The evolutionary background of viruses represents afascinating, albeit murky, topic for virologists and cell biologists. Becauseof the great diversity among viruses, biologists have struggled with how toclassify this entities and also how come relate them to the typical tree oflife. They might represent genetic elements that got the ability to movebetween cells. They might represent formerly free-living biology that came to be parasites.They may be the precursors the life together we understand it.
We know that viruses are quite diverse. Unlike every otherbiological entities, part viruses, favor poliovirus, have RNA genomes and also some,like herpesvirus, have actually DNA genomes. Further, some viruses (like influenza virus)have single-stranded genomes, while others (like smallpox) have actually double-strandedgenomes. Their structures and replication strategies space equally diverse.Viruses, do, however, share a couple of features: First, they typically are quitesmall, with a diameter of much less than 200 nanometers (nm). Second, they canreplicate just within a host cell. Third, no well-known virus includes ribosomes, anecessary component of a cell"s protein-making translational machinery.
To consider this question, we need to have actually a goodunderstanding that what we median by "life." Although particular definitions mayvary, biologists typically agree that all living organisms exhibit number of keyproperties: They deserve to grow, reproduce, keep an internal homeostasis, respondto stimuli, and also carry out assorted metabolic processes. In addition, populationsof life organisms evolve end time.
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Do viruses conform to this criteria? Yes and no. Us probablyall realize that viruses blee in some way. We can become infected v asmall variety of virus corpuscle — by inhaling corpuscle expelled when anotherperson coughs, for instance — and then come to be sick several days later as theviruses replicate within our bodies. Likewise we probably all realize thatviruses evolve over time. We require to gain a flu vaccine yearly primarilybecause the influenza virus changes, or evolves, indigenous one year come the next(Nelson & Holmes 2007).
Viruses carry out not, however, lug out metabolic processes.Most notably, viruses differ from life organisms in that they can not generateATP. Viruses additionally do not possess the vital machinery because that translation, asmentioned above. They carry out not own ribosomes and cannot separately formproteins from molecules of messenger RNA. Since of these limitations, virusescan replicate only within a living organize cell. Therefore, viruses room obligateintracellular parasites. Follow to a stringent definition of life, castle arenonliving. Not everyone, though, necessarily agrees with this conclusion. Perhapsviruses stand for a different kind of organism on the tree of life — the capsid-encodingorganisms, or CEOs (Figure 1; Raoult & Forterre 2008).
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There is much debate amongst virologists around thisquestion. Three key hypotheses have actually been articulated: 1. The progressive, orescape, hypothesis claims that viruses arose from genetic aspects that gainedthe capacity to move in between cells; 2. The regressive, or reduction, hypothesisasserts that viruses space remnants of to move organisms; and also 3. Thevirus-first hypothesis says that viruses predate or coevolved v theircurrent moving hosts.