Viruses are not living things. Viruses are complicated assemblies of molecules, including proteins, nucleic acids, lipids, and carbohydrates, but on their own they can do nothing until they enter a living cell. Without cells, viruses would not be able to multiply. Therefore, viruses are not living things. When a virus encounters a cell, a series of chemical reactions occur that lead to the production of new viruses. These steps are completely passive, that is, they are predefined by the nature of the molecules that comprise the virus particle.
Not everyone, though, necessarily agrees with this conclusion. There is much debate among virologists about this question. Three main hypotheses have been articulated: 1. The progressive, or escape, hypothesis states that viruses arose from genetic elements that gained the ability to move between cells; 2. Figure 3 Figure Detail Figure 2 Figure Detail According to this hypothesis, viruses originated through a progressive process. Mobile genetic elements, pieces of genetic material capable of moving within a genome , gained the ability to exit one cell and enter another.
To conceptualize this transformation , let's examine the replication of retroviruses, the family of viruses to which HIV belongs. Retroviruses have a single-stranded RNA genome. When the virus enters a host cell, a viral enzyme , reverse transcriptase , converts that single-stranded RNA into double-stranded DNA. This viral DNA then migrates to the nucleus of the host cell.
Another viral enzyme, integrase , inserts the newly formed viral DNA into the host cell's genome. Viral genes can then be transcribed and translated. Progeny viruses assemble and exit the cell to begin the process again Figure 2. This process very closely mirrors the movement of an important, though somewhat unusual, component of most eukaryotic genomes: retrotransposons.
Like retroviruses, certain classes of retrotransposons, the viral-like retrotransposons, encode a reverse transcriptase and, often, an integrase.
With these enzymes, these elements can be transcribed into RNA, reverse-transcribed into DNA, and then integrated into a new location within the genome Figure 3. We can speculate that the acquisition of a few structural proteins could allow the element to exit a cell and enter a new cell, thereby becoming an infectious agent. Indeed, the genetic structures of retroviruses and viral-like retrotransposons show remarkable similarities.
In contrast to the progressive process just described, viruses may have originated via a regressive, or reductive, process. Microbiologists generally agree that certain bacteria that are obligate intracellular parasites, like Chlamydia and Rickettsia species , evolved from free-living ancestors. Indeed, genomic studies indicate that the mitochondria of eukaryotic cells and Rickettsia prowazekii may share a common, free-living ancestor Andersson et al. It follows, then, that existing viruses may have evolved from more complex, possibly free-living organisms that lost genetic information over time, as they adopted a parasitic approach to replication.
These viruses, which include smallpox virus and the recently discovered giant of all viruses, Mimivirus, are much bigger than most viruses La Scola et al. A typical brick-shaped poxvirus, for instance, may be nm wide and nm long. About twice that size, Mimivirus exhibits a total diameter of roughly nm Xiao et al.
Conversely, spherically shaped influenza virus particles may be only 80 nm in diameter, and poliovirus particles have a diameter of only 30 nm, roughly 10, times smaller than a grain of salt.
Again, poxvirus genomes often approach , base pairs, and Mimivirus has a genome of 1. In addition to their large size, the NCLDVs exhibit greater complexity than other viruses have and depend less on their host for replication than do other viruses. Poxvirus particles, for instance, include a large number of viral enzymes and related factors that allow the virus to produce functional messenger RNA within the host cell cytoplasm.
Because of the size and complexity of NCLDVs, some virologists have hypothesized that these viruses may be descendants of more complex ancestors. According to proponents of this hypothesis, autonomous organisms initially developed a symbiotic relationship.
Over time, the relationship turned parasitic, as one organism became more and more dependent on the other. As the once free-living parasite became more dependent on the host, it lost previously essential genes. Eventually it was unable to replicate independently, becoming an obligate intracellular parasite, a virus. Analysis of the giant Mimivirus may support this hypothesis. This virus contains a relatively large repertoire of putative genes associated with translation — genes that may be remnants of a previously complete translation system.
Interestingly, Mimivirus does not differ appreciably from parasitic bacteria, such as Rickettsia prowazekii Raoult et al.
Figure 4 The progressive and regressive hypotheses both assume that cells existed before viruses. What if viruses existed first? Recently, several investigators proposed that viruses may have been the first replicating entities. Koonin and Martin postulated that viruses existed in a precellular world as self-replicating units. Over time these units, they argue, became more organized and more complex. Eventually, enzymes for the synthesis of membranes and cell walls evolved, resulting in the formation of cells.
Viruses, then, may have existed before bacteria, archaea , or eukaryotes Figure 4; Prangishvili et al. We also know that some RNA molecules, ribozymes, exhibit enzymatic properties; they can catalyze chemical reactions. Perhaps, simple replicating RNA molecules, existing before the first cell formed, developed the ability to infect the first cells. Villarreal and DeFilippis and Bell described models explaining this proposal.
Perhaps, both groups postulate, the current nucleus in eukaryotic cells arose from an endosymbiotic-like event in which a complex, enveloped DNA virus became a permanent resident of an emerging eukaryotic cell. Where viruses came from is not a simple question to answer.
But this has also helped scientists to draw parallels between viruses and other forms of life. One theory on their origin is that viruses evolved from cells then branched out and evolved separately, backing the notion that they are indeed alive.
Studying the shapes of their proteins , for example, has shown that viruses share certain protein structures — and therefore properties — with organisms from all branches of the tree of life. There are variations to this theory, such as the idea that viruses might have come from circular pieces of DNA called plasmids in archaeans , and that giant viruses might be the remnants of extinct domains of life.
Ultimately, science may never agree on whether viruses are alive or not. The answer is not as straightforward as you may think. This is because viruses do not have the tools to replicate their genetic material themselves. More recently, scientists have discovered a new type of virus, called a mimivirus.
These viruses do contain the tools for making a copy of its DNA. This suggests that certain types of viruses may actually be living. Viruses only become active when they come into contact with a host cell. Image by CarlosRoBe. Living things use energy. Outside of a host cell, viruses do not use any energy. They only become active when they come into contact with a host cell. Because they do not use their own energy, some scientists do not consider them alive.
This is a bit of an odd distinction though, because some bacteria rely on energy from their host, and yet they are considered alive. These types of bacteria are called obligate intracellular parasites. Living things respond to their environment. Whether or not viruses really respond to the environment is a subject of debate. They interact with the cells they infect, but most of this is simply based on virus anatomy. For example, they bind to receptors on cells, inject their genetic material into the cell, and can evolve over time within an organism.
Living cells and organisms also usually have these interactions. Cells bind to other cells, organisms pass genetic material, and they evolve over time, but these actions are much more active in most organisms.
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