A detailed new study of the origins of viruses lends weight to the argument that they are living cells, and offers us a reliable method to turn back the clock and track their evolution.After all, they can't survive or replicate without a host cell, and due to their rapidly changing genes, scientists have never been able to work out how or when they evolved.
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| photo credit: The diverse sizes and structures of viruses. Julie McMahon |
Now a study by researchers in the US has managed to complete the first viral tree of life, and it suggests that not only are viruses alive, they're also really, really old, and they share a long evolutionary history with cells. "Viruses now merit a place in the tree of life."
They used computers to scrutinize the folds in 5,080 organisms and 3,460 viruses, the researchers found that viruses and modern cells share 442 protein folds, and only 66 are virus-specific. But those 66 are unlike anything seen in cells, which contradicts the hypothesis that viruses simply took all their genetic materials from cells.
This information allowed them to build a rough tree of life, which showed that viruses share a common ancestor with modern cells, but are more ancient.Viruses also have unique components besides the components that are shared with cells.”
That last point is interesting, because it goes against the idea that viruses were merely bits of genetic material spat out by cellular life, or that they had stolen all of their genes from cells.
The researchers then built a tree of life that incorporated viruses, which revealed that modern viruses seem to have originated from multiple ancient cells, rather than modern cells, and even coexisted with the ancestors of today’s cells.
To make things even more complicated, some viruses have incredibly low numbers of genes, including Ebola, which does all its deadly damage with just seven genes. While others, such as the recently discovered giant viruses, have more genes than bacteria.
Viruses are just about everywhere. In us, on us, around us, at the depths of the ocean along scorching hot vents, even chilling out in the Siberian permafrost; and there are plenty to go around. This ubiquitous and abundant distribution is like what we see with bacteria. And again, much like bacteria, some are harmless, whereas others can do a lot of damage to their host, sometimes even killing them.
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To the untrained eye they may sound kind of similar. But there is one (of many) key difference that sets them apart: bacteria are living, and viruses are not. Well, that has been the opinion of many virologists, but there has been a long history of changed minds regarding what viruses are, and this debate is still very much alive.
The main argument against their status as a living organism stems from the fact that viruses are completely dependent on hosts to replicate. Sure, viruses can survive outside of cells, but they can’t make any progeny. That’s because they don’t own the tools to be able to do so. More specifically, they can’t make proteins from their genetic material, which is why these slave drivers take over a cell and force it to do the dirty work.
That’s not really a clincher, though, as dependence on another organism for survival is not unique to viruses, and even some bacteria and fungi can’t go it solo. And then there are the recently discovered mimiviruses, which totally threw virologists. Viruses were always tiny simpletons with few genes, invisible to the light microscope. But these viruses are so big they were initially mistaken for bacteria, have genomes larger than some bacteria, and own some protein synthesis machinery.
Researchers at the University of Illinois decided to follow their evolutionary breadcrumbs by examining the 3D shape and structure of proteins, or folds, which are written into the genome. That’s because viruses mutate all the time, but changes don’t necessarily affect the crucial folds which impart a particular function.
So they used computers to scrutinize folds of more than 5,000 organisms, almost 3,500 of which were viruses, covering every single branch of the tree of life. This led to the discovery of 442 protein folds that are common between viruses and cells, and 66 only found in viruses.
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The diverse physical attributes, genome sizes and lifestyles of viruses make them difficult to classify. A new study uses protein folds as evidence that viruses are living entities that belong on their own branch of the tree of life. Credit: Julie McMahon
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| A fluorescence image of viral DNA complexes in the cytoplasm of a cell. | Image: A. Rottach/LMU via io9 |
Today, many viruses - including those that cause disease - take over the protein-building machinery of host cells to make copies of themselves that can then spread to other cells. Viruses often insert their own genetic material into the DNA of their hosts. In fact, the remnants of ancient viral infiltrations are now permanent features of the genomes of most cellular organisms, including humans. This knack for moving genetic material around may be evidence of viruses' primary role as "spreaders of diversity."
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| A new study analyzes the distinct, three-dimensional structures found in proteins. These structures are called folds. Some folds are shared by all organisms, while others are unique to individual branches of the tree of life. Pictured here are folds found in viruses. Credit: Arshan Nasir |
Many organisms require other organisms to live, including bacteria that live inside cells, and fungi that engage in obligate parasitic relationships - they rely on their hosts to complete their lifecycle," "And this is what viruses do."
The discovery of the giant mimiviruses in the early 2000s challenged traditional ideas about the nature of viruses.
"These giant viruses were not the tiny Ebola virus, which has only seven genes. These are massive in size and massive in genomic repertoire." "Some are as big physically and with genomes that are as big or bigger than bacteria that are parasitic."
Some giant viruses also have genes for proteins that are essential to translation, the process by which cells read gene sequences to build proteins. The lack of translational machinery in viruses was once cited as a justification for classifying them as nonliving.
The Ebola virus is mutating faster in humans than in animal hosts.
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| Image: Shutterstock |
http://www.iflscience.com/plants-and-animals
http://www.sciencealert.com/viruses-are-alive-and-they-re-older-than-modern-cells-new-study-suggests
http://www.iflscience.com/plants-and-animals/viruses-might-be-alive-after-all
https://richarddawkins.net/2015/09/new-study-suggests-viruses-are-alive-and-that-they-share-an-ancestor-with-modern-cells/
https://news.illinois.edu/blog/view/6367/250879
http://phys.org/news/2015-09-evidence-viruses-alive.html
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