Uncoating: The viral capsid is removed and degraded by viral enzymes or host enzymes releasing the viral genomic nucleic acid. Replication: After the viral genome has been uncoated, transcription or translation of the viral genome is initiated. It is this stage of viral replication that differs greatly between DNA and RNA viruses and viruses with opposite nucleic acid polarity. This process culminates in the de novo synthesis of viral proteins and genome.
Assembly: After de novo synthesis of viral genome and proteins, which can be post-transrciptionally modified, viral proteins are packaged with newly replicated viral genome into new virions that are ready for release from the host cell. This process can also be referred to as maturation. Virion release: There are two methods of viral release: lysis or budding.
Lysis results in the death of an infected host cell, these types of viruses are referred to as cytolytic. An example is variola major also known as smallpox. Enveloped viruses, such as influenza A virus, are typically released from the host cell by budding. It is this process that results in the acquisition of the viral phospholipid envelope.
These types of virus do not usually kill the infected cell and are termed cytopathic viruses. Residual viral proteins that remain within the cytoplasm of the host cell can be processed and presented at the cell surface on MHC class-I molecules, where they are recognised by T cells.
For example, Ebola virus spreads from contact with infected blood, feces, or vomit. Unlike many other viruses, scientists think Ebola cannot spread through the air after people with the virus cough or sneeze. Still other viruses travel through an intermediary, like a mosquito, which then infects people by biting them. One example of these so-called mosquito-born diseases is dengue, which causes a potentially deadly flu-like infection.
The risk of dengue has risen in recent years, currently threatening roughly half of the global population, according to the World Health Organization. Other notorious mosquito-born diseases include Zika, Chikungunya, and West Nile.
Some scientists believe that viruses were fairly late to the evolutionary game, forming as remnants from cells that had somehow lost the ability to replicate. But other experts suggest that viruses could predate Earth's most ancient critters. The giant viruses have a surprising amount of independence compared to their tiny counterparts, so could have provided the building blocks of the diversity of life we know today.
By one hypothesis , the first complex life originated from a cell enveloping a virus or, alternatively, a failed viral takeover. Either way, the virus became a permanent cellular resident, forming the first nucleus. However, scientists can't even agree on whether viruses are truly alive.
To be considered a living thing, an organism must be able to grow, reproduce, and generate energy on its own. Some researchers also suggest that living things must be able to respond to stimuli and evolve over time. Viruses can't generate their own energy, and though they can reproduce and even evolve with the assistance of a host, those functions are impossible for one of the tiny entities out on its own.
Instead, Albert Erives of the University of Iowa suggests that viruses are more like vines wrapping around the many branches of the tree of life.
They can access and infect critters on each branch, racing to the top to evolve as their hapless hosts morph over time. All rights reserved. Bacteriophage viruses infect and replicate within bacteria, essentially taking them over.
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A rock is not alive. A metabolically active sack, devoid of genetic material and the potential for propagation, is also not alive. A bacterium, though, is alive. Although it is a single cell, it can generate energy and the molecules needed to sustain itself, and it can reproduce.
But what about a seed? A seed might not be considered alive. Yet it has a potential for life, and it may be destroyed. In this regard, viruses resemble seeds more than they do live cells. They have a certain potential, which can be snuffed out, but they do not attain the more autonomous state of life. Another way to think about life is as an emergent property of a collection of certain nonliving things. Both life and consciousness are examples of emergent complex systems.
They each require a critical level of complexity or interaction to achieve their respective states. A neuron by itself, or even in a network of nerves, is not conscious—whole brain complexity is needed. The enucleated cell is akin to the state of being braindead, in that it lacks a full critical complexity. A virus, too, fails to reach a critical complexity. So life itself is an emergent, complex state, but it is made from the same fundamental, physical building blocks that constitute a virus.
Approached from this perspective, viruses, though not fully alive, may be thought of as being more than inert matter: they verge on life. In fact, in October, French researchers announced fi ndings that illustrate afresh just how close some viruses might come. Didier Raoult and his colleagues at the University of the Mediterranean in Marseille announced that they had sequenced the genome of the largest known virus, Mimivirus, which was discovered in The virus, about the same size as a small bacterium, infects amoebae.
Sequence analysis of the virus revealed numerous genes previously thought to exist only in cellular organisms. Some of these genes are involved in making the proteins encoded by the viral DNA and may make it easier for Mimivirus to co-opt host cell replication systems. Impact on Evolution Debates over whether to label viruses as living lead naturally to another question: Is pondering the status of viruses as living or nonliving more than a philosophical exercise, the basis of a lively and heated rhetorical debate but with little real consequence?
I think the issue is important, because how scientists regard this question infl uences their thinking about the mechanisms of evolution. Viruses have their own, ancient evolutionary history, dating to the very origin of cellular life. For example, some viral- repair enzymes—which excise and resynthesize damaged DNA, mend oxygen radical damage, and so on— are unique to certain viruses and have existed almost unchanged probably for billions of years.
Nevertheless, most evolutionary biologists hold that because viruses are not alive, they are unworthy of serious consideration when trying to understand evolution. They also look on viruses as coming from host genes that somehow escaped the host and acquired a protein coat. In this view, viruses are fugitive host genes that have degenerated into parasites. And with viruses thus dismissed from the web of life, important contributions they may have made to the origin of species and the maintenance of life may go unrecognized.
Indeed, only four of the 1, pages of the volume The Encyclopedia of Evolution are devoted to viruses. Of course, evolutionary biologists do not deny that viruses have had some role in evolution. But by viewing viruses as inanimate, these investigators place them in the same category of infl uences as, say, climate change.
Such external infl uences select among individuals having varied, genetically controlled traits; those individuals most able to survive and thrive when faced with these challenges go on to reproduce most successfully and hence spread their genes to future generations.
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