The Building Blocks Of Viruses
It is hard for scientists to know exactly when viruses first emerged, but we do know that viruses originated at least as early as the first cells, around 4 billion years ago. Some even think viruses showed up before there was cellular life. Either way, viruses have had a really long time to get it right. As it turns out, they are efficient molecular systems with a metastable structure . When you get down to brass tacks, the basic functions of a virus are to protect its nucleic acid genome, and be able to attach and enter a host cell. Here, we will discuss the building blocks of viruses that help them do this job.
Rabies Virus: Can We Treat The Untreatable
Every year, an estimated 59,000 people die from rabies. Usually contracted following a bite from an infected animal, rabies is almost always fatal in people who have not been vaccinated. Once symptoms of appear, there is little-to-no hope for the infected individual, with no treatment options currently available.
Note # 9 Properties Of Viruses:
The properties of the plant viruses are conveniently divided into the following categories:
i. Host Range:
Some viruses like beet curly top, cucumber mosaic, and tobacco mosaic virus have wide host range and host plants may fall within widely different families. Others, have extremely restricted host range, for example corn mosaic virus. Host specificity is a genetic character of the virus and is determined by its nucleic acid.
In some cases virus is actively present in the host plant without causing obvious effect. The absence of distinctive symptoms is due to masking effects of unfavourable environmental conditions. Symptoms appear when conditions become favourable.
A virus showing the phenomenon of masking is a masked virus and the host plant is the masked carrier. Again the presence of a virus in the host with the total absence of visible symptoms over the entire range of environment to which the host is exposed is designated as a latency of a virus and such a virus is a latent virus.
A latent virus never induces symptoms or makes its presence known in a host over the entire range of environmental conditions. Host plants which harbour the virus but remain symptomless throughout the entire range of environmental conditions are the symptomless carriers.
ii. Physical Properties of Viruses:
Those most extensively used are:
The thermal inactivation point, it is the constant temperature at which a virus extract is completely inactivated when exposed for 10 minutes
RNA:
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Materials Science And Nanotechnology
Current trends in nanotechnology promise to make much more versatile use of viruses. From the viewpoint of a materials scientist, viruses can be regarded as organic nanoparticles.Their surface carries specific tools that enable them to cross the barriers of their host cells. The size and shape of viruses and the number and nature of the functional groups on their surface are precisely defined. As such, viruses are commonly used in materials science as scaffolds for covalently linked surface modifications. A particular quality of viruses is that they can be tailored by directed evolution. The powerful techniques developed by life sciences are becoming the basis of engineering approaches towards nanomaterials, opening a wide range of applications far beyond biology and medicine.
Because of their size, shape, and well-defined chemical structures, viruses have been used as templates for organising materials on the nanoscale. Recent examples include work at the Naval Research Laboratory in Washington, D.C., using Cowpea mosaic virus particles to amplify signals in DNA microarray based sensors. In this application, the virus particles separate the fluorescentdyes used for signalling to prevent the formation of non-fluorescent dimers that act as quenchers. Another example is the use of CPMV as a nanoscale breadboard for molecular electronics.
Why Dont They Fit The Bill
In order to replicate, viruses must first hijack the reproductive equipment of a host cell, redirecting it to photocopy the genetic code of the virus and seal it inside a newly formed container, known as the capsid. Without a host cell, they simply cant replicate.
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They fail the second question for the same reason. Unlike other living organisms that can self-divide, splitting a single cell into two, viruses must assemble themselves by taking control of the host cell, which manufactures and assembles the viral components.
Finally, a virus isnt considered living because it doesnt need to consume energy to survive, nor is it able to regulate its own temperature. Unlike living organisms that meet their energy needs by metabolic processes that supply energy-rich units of adenosine triphosphate , the energy currency of life, viruses can survive on nothing. In theory, a virus can drift around indefinitely until it contacts the right kind of cell for it to bind to and infect, thus creating more copies itself.
Thats three strikes against, but
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Virus Anatomy And Structure
A virus particle, also known as a virion, is essentially nucleic acid enclosed within a protein shell or coat. Viruses are extremely small, approximately 20 – 400 nanometers in diameter. The largest virus, known as the Mimivirus, can measure up to 500 nanometers in diameter. By comparison, a human red blood cell is around 6,000 to 8,000 nanometers in diameter.
In addition to varying sizes, viruses also have a variety of shapes. Similar to bacteria, some viruses have spherical or rod shapes. Other viruses are icosahedral or helical shaped. Viral shape is determined by the protein coat that encases and protects the viral genome.
Note # 6 Nomenclature Classification And Identification Of Viruses:
In spite of continuous efforts made by Johnson , Smith , Fawcett , Holmes , Valleau , Lwoff, Home and Tournier , Pereira , Tourinier , Hansen , Thornberry , Gibbs , Martyn , Harrison and many others, there is no final agreement about nomenclature and classification of viruses as because viruses are genetically variable and new strains differ in host range, virulence and other characteristics arising from different arrangements of the nucleotides in the nucleic acid molecule.
A specific virus exhibits fixed characteristic properties. These include the size, structure, and chemical composition of the virus particles, the host range, the tissue specificity, and the nature of the infection caused. When the properties of a large number of different viruses are examined, it is found that they fall into groups, each characterized by the possession of a number of properties in common.
The major groups of viruses may broadly be separated on the basis of characters like:
Type of nucleic acid present in the virus particle, nature of host and disease induced, properties of virus particles , and other related characters. Although nothing is known about the origin and relationships of the viruses, it is tempting to imagine that these groups are natural ones, each of which unites a series of virus that are genetically related to one another.
The most widely used taxonomic criteria for viruses depend upon the structure of a virus itself.
Four major criteria are used:
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The Virus That Learns
If you dont have an immune system, you dont last long in this parasite-riddled world. Your body receives a steady stream of invadersviruses, bacteria, and other pathogenswhich it has to recognize and fight. In many cases, its a brutal battle with an ultimate goal of eradication. In other cases, the immune system simply keeps strangers in check, preventing them from spreading. As many as a third of all humans have cysts in their brains containing a single-celled parasite called Toxoplasma. As long as the parasite stays in its cyst, the immune system lets it be. If Toxoplasma breaks out and starts to multiply, however, the immune system picks off the new cells. And if people lose their immune systemdue to HIV infection, for exampleToxoplasma runs rampant and causes devastating brain damage.
The cells and molecules we use to recognize these invaders are unquestionably amazing. Whats perhaps most amazing is that the immune system can learn. When a new pathogen turns up, our immune cells undergo a kind of interior version of natural selection. Over the course of several cell divisions, new variants emerge that do a better and better job of recognizing the newcomer. Our bodies can then mount a powerful, focused attack on, say, a particular strain of the flu. And once the immune system learns how to recognize that new enemy, it can store that memory away, enabling it to attack the same pathogen years later.
But one virusdubbed ICP1was ominpresent.
Lower life indeed.
Different Hosts And Their Viruses
Viruses are often very specific as to which hosts and which cells within the host they will infect. This feature of a virus makes it specific to one or a few species of life on earth. So many different types of viruses exist that nearly every living organism has its own set of viruses that try to infect its cells. Even the smallest and simplest of cells, prokaryotic bacteria, may be attacked by specific types of viruses.
Bacteriophage: This transmission electron micrograph shows bacteriophages attached to a bacterial cell.
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Note # 14 Distinction Between Virus And Cellular Organism:
Distinction between Viruses and Cellular Organisms may be Summarized as Follows:
The only unit of viral structure, the virion has quite different properties from the unit of structure of an organism, the cell.
The virion contains only one kind of nucleic acid, either ribo- or deoxyribonucleic acid. The cell always contains both.
The organic constituents in the virion are nucleic acid and protein. The cell contains, in addition to nucleic acids and proteins, many other organic constituents.
Although the virion may contain one or a few enzymes, its enzymatic complement is insufficient to reproduce another virion. The cell always contains a very elaborate complement of enzymes suitable for the reproduction of the cell.
The virion never arises directly from a pre-existing virion. The cell always arises directly from a pre-existing cell.
The virus is always reproduced exclusively from its genetic material. The cell is reproduced from the integrated sum of all its constituent parts.
Growth of the virus involves the independent synthesis of its nucleic acid and protein, which are assembled into organized structures after the completion of their synthesis. Whereas growth of the cell consists of the increase in the amount of all its constituent parts, during which the individuality of the whole is continuously maintained. Cellular growth culminates in an increase in cell number by a process of fission.
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Note # 1 Origin Of Viruses:
Broadly speaking, only three general hypotheses of the origin of viruses are taken into consideration.
The ancestors of viruses were at one time cellular organisms. As a result of parasitic existence in other cells, they gradually lost more and more of their own cellular machinery until they eventually became reduced to their present form.
The ancestors of viruses were once free-living pre-cellular forms of life, which managed to survive after the evolutionary emergence of cellular organisms only by becoming parasitic on them.
The viruses have not evolved from organisms, either pre-cellular or cellular, but have arisen from detached fragments of the genetic material of cellular organisms. These genetic fragments, as a result of detachment from the rest of the genetic system, acquired the ability to multiply more rapidly than the other constituents of the cell, and their unregulated growth caused disease and death of the cell.
Liberated after cell death, the genetic fragments were able to ensure their own perpetuation by entering adjacent healthy cells and again multiply there.
Originally passed from cell to cell in the form of nucleic acid, they eventually acquired the capacity to direct the simultaneous synthesis of the infected cell of a special protein, which served to enclose the nucleic acid fragments, and thus made their transfer from cell to cell a much less hazardous operation.
The above hypotheses have not yet been supported by factual information.
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How Do Viruses Spread
Once a person is infected with a virus, their body becomes a reservoir of virus particles which can be released in bodily fluids such as by coughing and sneezing or by shedding skin or in some cases even touching surfaces.
The virus particles may then either end up on a new potential host or an inanimate object. These contaminated objects are known as fomites, and can play an important role in the spread of disease.
Note # 5 Virus Induced Symptoms:
Effects of Viruses on Plants:
Viruses are similar to obligate parasites in that they cannot be grown on non-living media. They are intimately associated with the host cell and few kill the infected plant although some cause severe distortion and dwarfing.
The changes brought about by viruses are treated as symptoms which may be:
Morphological changes or external symptoms,
Histological and cytological or internal symptoms, and
Metabolic changes but these are all correlated.
The symptoms of the majority of plant virus diseases are most conspicuous on plants making rapid growth. Plants that are almost mature at the time of infection usually do not develop symptoms on any part except on new growths.
Viruses being infectious induce a variety of symptoms covering a wide range of host reactions.
Some of the symptoms frequently encountered are mentioned below:
I. External Symptoms:
The most common symptom in the green tissues of higher plants is the alteration in the normal development of chlorophyll chlorosis. This may be accompanied with various other malformation. Besides this, necrosis of tissue and dwarfing, distortion of a particular organ or the entire plant are also common symptoms. The external symptoms may be primary or localized and systemic.
The primary or initial symptom is a local reaction at the actual site of inoculation consisting of spots or rings of various types.
Some of the external symptoms are described below:
Chlorosis:
Mosaic:
Mosaic mottling:
Necrosis:
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How Does Virus Work
Viruses contain only one kind of nucleic acid as the hereditary material. The DNA viruses have single DNA molecule which may be either linear or circular in shape, mostly, the linear DNA molecule is double stranded . The circular DNA molecule may be either single stranded or double stranded as in most animal viruses. The RNA viruses are the only biological systems known in which RNA is the genetic material mostly, the RNA is present in its usual single stranded form but some viruses such as Retrovirus have a core of double stranded RNA similar in properties to DNA.
Classification Based On The Replication Properties And Site Of Replication
Here, viruses invade into the host cell, where it replicates and assembly within the cell organelles.
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Interesting Facts About Viruses
- Viruses are not classified in any of the five kingdoms of living things. This means that they are not bacteria, fungi, protists, plants or animals.
- Most viruses are so small that they cannot be seen under a light microscope.
- The word virus comes from the Latin word virulentus which meansa poisoned wound or full of poison.
- Viruses can sometimes attack and kill bacteria .
- The first human virus discovered was the yellow fever virus in 1901 by Walter Reed.
- A virus that contains RNA instead of DNA is sometimes called a retrovirus.
- There are two main types of reproductive cycles for viruses: the lytic cycle and the lysogenic cycle.
- Diseases caused by a virus with a lytic cycle show symptoms much faster than viruses with a lysogenic cycle.
Structure And Function Of Viruses
Viruses are tiny and smaller in its size, ranging between 30-50 nm. They usually lack a cell wall but are surrounded by a protective protein coating called the capsid. It can be seen as a genetic element and is characterized by the combined evolution of the virus and the host. They contain either RNA or DNA as the genetic material
Viruses mainly depend on a host to deliver the complex metabolic machinery of prokaryotic or eukaryotic cells for propagation. The main task of the virus is to carry its DNA or RNA genome to the host cell, which then can be transcribed by the host cell. The viral genome is packed in a capsulated symmetric protein. The protein associated with nucleic acid produces the nucleocapsid with the genome.
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S Of Virus Infections
A virus must take over a cell to replicate. The viral replication cycle can produce dramatic biochemical and structural changes in the host cell, which may cause cell damage. These changes, called cytopathic effects, can change cell functions or even destroy the cell. Some infected cells, such as those infected by the common cold virus , die through lysis or apoptosis , releasing all the progeny virions at once. The symptoms of viral diseases result from the immune response to the virus, which attempts to control and eliminate the virus from the body, and from cell damage caused by the virus. Many animal viruses, such as HIV , leave the infected cells of the immune system by a process known as budding, where virions leave the cell individually. During the budding process, the cell does not undergo lysis and is not immediately killed. However, the damage to the cells that HIV infects may make it impossible for the cells to function as mediators of immunity, even though the cells remain alive for a period of time. Most productive viral infections follow similar steps in the virus replication cycle: attachment, penetration, uncoating, replication, assembly, and release.