The Nature of Viruses

Vir engages be sub-cellular agents of infection that moldiness put on the cellular machinery of bacteria, plants or animals in ordinate to re bugger off. Composed of a private beach of inherited tangible ( deoxyribonucleic acid or ribonucleic acid) incased in a protein capsid, a computer virus is as well sm every to be seen by standard light microscopy indeed, closely be less(prenominal) than one ordinal the size of a bacterium. Specific proteins on the viral capsid attach to receptors on the phalanx cell this attachment carry out is inhering to viral infectivity and informs why viruses whitethorn scarce infect the cells of veritable species or whitethorn only infect certain cells or tissues at bottom a abandoned swarm species. speckle the infecting virus triggers an insubordinate solvent in the soldiery, more or less atomic number 18 competent-bodied of suppressing that response by infecting and cleanup position cells that halt immunity (e. g. HIV attacks lymphocytes). In addition, magic spell most infected cells atomic number 18 done for(p) by viral replication, any(prenominal) viruses go into a latent chassis within cells, reactivating in the future to produce degenerative or relapsing infections.M whatever viruses use particular proposition carriers (known as vectors) such as mosquitoes, ticks, bats and rodents that transmit the virus to a susceptible multitude term another(prenominal)s ar spread between individuals via decline wholesaler or through and through respiratory, enteric or sexual secretions. Of special match is the concomitant that alterations within the viral genome whitethorn resign viruses to skip from one legion (e. g. birds, swine, monkeys) to almost other (e. g. homos), unleashing pandemics.Mevery familiar human infections atomic number 18 produced by viruses these include the harsh cold, flu, mononucleosis, herpes virus virus infections (including shingles), viral hepatitis (A, B, C and others), HIV, viral gastroenteritis, conjunctivitis, viral pneumonia, encephalitis, viral meningitis and viral infections of the heart, including pericarditis and myocarditis. While viruses do non respond to antibiotics, specific antiviral agent agent agents whitethorn sustain (though non cure) chronic disease (such as HIV, Hepatitis B and Hepatitis C) or may modify the severeness of ague infection (as in influenza and herpes infections).However, in most viral infections, sermon is, for now, purely symptomatic and supportive. On the other hand, vaccines be capable of pr tear downting slightly viral infections (e. g. herpes simplex, measles, mumps, rubella, varicella, Hepatitis B) or reducing the sourness of an intense infection (e. g. influenza). Beyond the slap-up or chronic illness that they produce, round viral infections (such and Hepatitis C and certain strains of herpes simplex) atomic number 18 known to be precursors of malignancy. Finally, um teen researchers suspect that viruses play a intention in the pathogenesis of chronic illnesses such as multiple sclerosis and autoimmune disorders. ttp//naturesblog. blogspot. com/2013/01/the-nature-of-viruses. hypertext mark-up language The Nature of Viruses Viruses exist in deuce different states, the extracellular infectious molecule or virion and the intracellular state consisting of viral nucleic back breaker. The capsid may be a polyhedron or a helix, or a combination of some(prenominal) (in some phages). Viruses atomic number 18 infective micro beings that show some(prenominal) differences from car park microbial cells. 1. Size. The size throw away of viruses is from about 20 to 300 nm. On the whole, viruses atomic number 18 much smaller than bacteria. close to animal viruses and all plant viruses and phages argon invisible under the light microscope. 2. ingenuous construction. Viruses cook very simple structures. The simplest viruses are nucleoprotein ingredi ents consisting of genetic material ( deoxyribonucleic acid or ribonucleic acid) border by a protein capsid. In this observe they differ from typical cells which arc do up) of proteins, carbohydrates, lipids and nuc1eicacids. The more complex viruses contain lipids and carbohydrates in addition to proteins and nucleic acids, e. g. the enveloped viruses 3. Absence of cellular structure.Viruses do not book any cytoplasm, and thitherfrom cytoplasmic organelles like mitochondria, Golgi complexes, lysosomes, ribosomes, etc. , are absent. They do not allow any limiting cell membrane. They utilize the ribosomes of the legion cell for protein synthesis during reproduction. 4. No self-sufficient metabolism. Viruses cannot multiply outside a nutrition cell. No virus has been cultivated in a cell-free medium. Viruses do not micturate an independent metabolism. They are metabolically soundless outside the boniface cell because they do not posses enzyme systems and protein synthesis m achinery.viral nucleic acid replicates by utilizing the protein synthesis machinery of the military. It codes for the synthesis of a limited number of viral proteins, including the subunits or capsomeres of the capsid, the tail protein and some enzymes relate Viruses receive only one nucleic acid, either desoxyribonucleic acid or RNA. Typical cells bear both DNA and RNA. Genomes of certain with the synthesis or the release of virions. 5. Nucleic acids. RNA viruses can be transcribed into complementary DNA strands in the infected waiter cells, e. g. Rous Sarcoma Virus (RSV). such RNA viruses are and so alike called RNA-DNA viruses. 6. Crystallization. some(prenominal) of the smaller viruses can be crystallized, and therefrom behave like chemicals. 7. No growth and division. Viruses do not have the power of growth and division. A in full material bodyed virus does not cast up in, size by addition of overbold molecules. The virus itself cannot divide. Only its genetic mate rial (RNA or DNA) is capable of reproduction and that too only in a host cell. It will thus be seen that viruses do not show all the characteristics of typical living organisms.They, however, possess two funda moral characteristics of living systems. Firstly, they contain nucleic acid as their genetic material. The nucleic acid contains instructions for the structure and function of the virus. Secondly, they can reproduce themselves, even if only by using the host cells synthesis machinery. Viral genomes The nucleic acid comprising the genome may be single-stranded or double-stranded, & in a linear, circular or segmental configuration. Single-stranded virus genomes may be positive (+) brain, i. e. of the equivalent polarity (nucleotide sequence) as mRNA negative (-)sense Ambisense a garland of the two. N/B. Virus genomes string in size from approximately 3,200 nucleotides (nt) to approximately 1. 2 million base pairs Unlike the genomes of all cells, which are composed of DNA, virus genomes may contain their genetic information encoded in either DNA or RNA. Since viruses are obligate intracellular parasites only able to replicate inside the appropriate host cells, the genome mustiness contain information encoded in a form which can be recognized & decoded by the accompaniment grammatical case of cell parasitized.Thus, the genetic code assiduous by the virus must play off or at least be recognized by the host organism. Similarly, the control signals which direct the expression of virus genes must be appropriate to the host. Many of the DNA viruses of eukaryotes closely resemble their host cells in terms of the biology of their genomes Some DNA virus genomes are complexed with cellular histones to form a chromatin-like structure inside the virus particle. http//expertscolumn. com/content/nature-viruses http//www. mcb. uct. ac. za/tutorial/virorig. html Viral phylogenyViral evolution is a subfield of evolutionary biology and virology that is specifica lly concerned with the evolution of viruses. Many viruses, in particular RNA viruses, have short genesis times and relatively high variant rates (on the order of one tear mutation or more per genome per roofy of replication for RNA viruses). This elevated mutation rate, when combined with natural selection, allows viruses to quickly correct to intensifys in their host environment. Viral evolution is an important aspect of the epidemiology of viral diseases such as influenza (influenza virus), AIDS (HIV), and hepatitis (e. . HCV). It also causes problems in the training of successful vaccines and antiviral drugs, as resistant mutations often bug out within weeks or months after the fount of the treatment.One of the main theoretical models to look at viral evolution is the quasispecies model, as the viral quasispecies. Origins Viruses are old-fashioned. Studies at the molecular aim have revealed relationships between viruses infecting organisms from each of the iii domai ns of life, and viral proteins that pre-date the divergence of life and thus the last universal common ancestor. 1 This indicates that viruses emerged early in the evolution of life and existed forwards modern cells. 2 There are third classical hypotheses on the origins of viruses Viruses may have once been small cells that parasitised larger cells (the rottenness hypothesis34 or reduction hypothesis5) some viruses may have evolved from bits of DNA or RNA that escaped from the genes of a larger organism (the vagrancy hypothesis6 or escape hypothesis) or viruses could have evolved from complex molecules of protein and nucleic acid at the equal time as cells first appeared on earth (the virus-first hypothesis). 5None of these hypotheses was fully veritable the regressive hypothesis did not relieve why even the smallest of cellular parasites do not resemble viruses in any way. The escape hypothesis did not explain the complex capsids and other structures on virus particles. The virus-first hypothesis was quickly dismissed because it contravened the comment of viruses, in that they require host cells. 5 Virologists are, however, number 1 to reconsider and re-evaluate all three hypotheses. 78 http//en. wikipedia. org/wiki/Viral_evolution developing Time-line of paleoviruses in the human lineage9Viruses do not form fossils in the traditional sense, because they are much smaller than the grains of sedimentary rocks that fossilise plants and animals. However, the genomes of some(prenominal) organism contain endogenic viral elements (EVEs). These DNA sequences are the remnants of ancient virus genes and genomes that ancestrally invaded the host germline. For example, the genomes of most craniate species contain hundreds to thousands of sequences derived from ancient retroviruses. These sequences are a valuable source of retrospective deduction about the evolutionary history of viruses, and have given birth to the science of paleovirology. 9 The evolutionar y history of viruses can to some period be inferred from analysis of contemporary viral genomes.The mutation rates for many viruses have been measured, and application of a molecular quantify allows dates of divergence to be inferred. 10 Viruses evolve through changes in their DNA (or RNA), some quite rapidly, and the best adapted mutants quickly amount their less fit counterparts. In this sense their evolution is Darwinian, just like that of their host organisms. 11 The way viruses reproduce in their host cells makes them particularly susceptible to the genetic changes that attend to to drive their evolution. 12 The RNA viruses are in particular prone to mutations. 13 In host cells there are mechanisms for correcting mistakes when DNA replicates and these kick in whenever cells divide. 13 These important mechanisms prevent potentially lethal mutations from being passed on to offspring. But these mechanisms do not work for RNA and when an RNA virus replicates in its host cell , changes in their genes are occasionally introduced in error, some of which are lethal. One virus particle can produce millions of take viruses in just one cycle of replication, therefore the production of a few bomb viruses is not a problem.Most mutations are silent and do not matter in any obvious changes to the progeny viruses, but others confer advantages that increase the fittingness of the viruses in the environment. These could be changes to the virus particles that inter them so they are not set by the cells of the immune system or changes that make antiviral drugs less effective. both(prenominal) of these changes occur frequently with HIV. 14 Phylogenetic manoeuvre showing the relationships of morbilliviruses of different species15 Many viruses (for example, influenza A virus) can shuffle their genes with other viruses when two similar strains infect the same cell.This phenomenon is called genetic shift, and is often the cause of youthful and more virulent strains appearing. Other viruses change more slowly as mutations in their genes gradually accumulate over time, a process known as genetic drift. 16 Through these mechanisms new viruses are everlastingly emerging and present a go on challenge to attempts to control the diseases they cause. 1718 Most species of viruses are now known to have common ancestors, and although the virus first hypothesis has so far to gain full acceptance, there is undersize doubt that the thousands of species of modern viruses have evolved from less numerous ancient ones. 19 The morbilliviruses, for example, are a group of closely related, but transparent viruses that infect a broad range of animals.The group includes measles virus, which infects humans and order Primates canine distemper virus, which infects many animals including dogs, cats, bears, weasels and hyaenas rinderpest, which infects kine and buffalo and other viruses of seals, porpoises and dolphins. 20 Although it not doable to prove which of these rapidly evolving viruses is the earliest, for such a closely related group of viruses to be found in such versatile hosts suggests a possible ancient common ancestor. 21 The Nature of Viruses Viruses are sub-cellular agents of infection that must utilize the cellular machinery of bacteria, plants or animals in order to reproduce. Composed of a single strand of genetic material (DNA or RNA) encased in a protein capsid, a virus is too small to be seen by standard light microscopy indeed, most are less than one hundredth the size of a bacterium.Specific proteins on the viral capsid attach to receptors on the host cell this attachment process is essential to viral infectivity and explains why viruses may only infect the cells of certain species or may only infect certain cells or tissues within a given host species. While the infecting virus triggers an immune response in the host, some are capable of suppressing that response by infecting and killing cells that control immunity (e. g. HIV attacks lymphocytes).In addition, while most infected cells are destroyed by viral replication, some viruses enter a latent phase within cells, reactivating in the future to produce chronic or relapsing infections. Many viruses use specific carriers (known as vectors) such as mosquitoes, ticks, bats and rodents that transmit the virus to a susceptible host while others are spread between individuals via blood contact or through respiratory, intestinal or sexual secretions.Of special concern is the fact that mutations within the viral genome may allow viruses to skip from one host (e. g. birds, swine, monkeys) to another (e. g. humans), unleashing pandemics. Many common human infections are produced by viruses these include the common cold, influenza, mononucleosis, herpes infections (including shingles), viral hepatitis (A, B, C and others), HIV, viral gastroenteritis, conjunctivitis, viral pneumonia, encephalitis, viral meningitis and viral infections of the heart, incl uding pericarditis and myocarditis.While viruses do not respond to antibiotics, specific antiviral agents may control (though not cure) chronic disease (such as HIV, Hepatitis B and Hepatitis C) or may modify the hardness of acute infection (as in influenza and herpes infections). However, in most viral infections, treatment is, for now, purely symptomatic and supportive. On the other hand, vaccines are capable of preventing some viral infections (e. g. erpes simplex, measles, mumps, rubella, varicella, Hepatitis B) or reducing the severity of an acute infection (e. g. influenza). Beyond the acute or chronic illness that they produce, some viral infections (such and Hepatitis C and certain strains of herpes simplex) are known to be precursors of malignancy. Finally, many researchers suspect that viruses play a determination in the pathogenesis of chronic illnesses such as multiple sclerosis and autoimmune disorders. development of viruses (images)