Viruses

A Viruses is an infectious microorganism made up of a protein-coated segment of nucleic acid (either DNA or RNA). A virus can’t multiply by itself; it has to infect cells in order to utilize the host cell’s components to generate copies of itself.

A virus is an infectious submicroscopic creature that only reproduces inside of live cells. Viruses may infect every type of life, including bacteria, archaea, and both animals and plants. Since Martinis Beijerinck’s discovery of the tobacco mosaic virus in 1898 and Dmitri Ivanov sky’s 1892 article describing a non-bacterial pathogen infecting tobacco plants, more than 11,000 of the millions of virus species have been described in detail. Viruses are the most common sort of living organism and may be found in practically all ecosystems on Earth. Virology, a branch of microbiology, is the study of viruses.

A host cell that has been infected is frequently compelled to quickly make thousands of copies of the original virus. outside of an infected cell

Etymology (Viruses)

The word first appeared in English in John Treviso’s translation of Bartholomeus Anglicism’s De Proprietatibus Rerum in 1398. The word is from the Latin neuter virus referring to poison and other noxious liquids, and it derives from the same Indo-European base as Sanskrit via, Avastin va, and ancient Greek (all meaning “poison”). The word “virulent,” which derives from the Latin virulent (which means “poisonous”), first appeared around 1400. Long before Dmitri Ivanov sky discovered viruses in 1892, the definition of a “agent that causes infectious disease” was first recorded in 1728. The Latin term is a mass noun with no historically documented plural (vary is used in Neo-Latin, but the English plural is viruses (often called viral). The word “viral” first appeared in 1948.[20] Another name that is utilized is the virion (plural virions), which goes back to 1959.

Origins

Viruses occur everywhere there is life, and they have probably been around ever since the origin of living cells. Because viruses do not leave fossils, it is unclear where they came from, so their origin is being studied using molecular techniques. Additionally, occasionally, viral genetic material fuses with the host organisms’ germlines, allowing for long-term vertical transmission to the host’s progeny. Paleo virologists may utilize this as a priceless resource of knowledge to track down historic viruses that date back millions of years. To explain the origins of viruses, there are three basic hypotheses:

Regressive hypothesis

It’s possible that viruses were originally tiny cells that preyed on larger ones. Genes that were not necessary for their parasitism were gradually lost. Living cells, like viruses, rickettsia and chlamydia can only multiply inside host cells. They provide evidence in favour of this theory since the genes necessary for their ability to exist outside of cells are believed to have been lost due to their reliance on parasitism. This is sometimes referred to as the “degeneracy hypothesis” or “reduction hypothesis”

Cellular origin hypothesis

Bits of DNA or RNA that “escaped” from the genes of a bigger creature may have given rise to certain viruses. The naked DNA fragments that made their way outside the cell may have been plasmids or transposons, which are replicating DNA molecules that travel to various locations inside the cell’s genes. Transposons, which were once referred to as “jumping genes,” are an example of a mobile genetic element that may be the cause of certain viruses. They were found in maize in 1950 by Barbara McClintock. The ‘vagrancy theory’ is another name for this. The “escape hypothesis”

Co-evolution hypothesis

The “virus-first hypothesis” puts out the idea that viruses may have developed from intricate protein and nucleic acid molecules at the same time as cells first arose on Earth. As a result, viruses may have been reliant on cellular life for billions of years. Because they don’t have a protein coat, RNA molecules known as viroids are not considered viruses. They are frequently referred to as subviral agents since they share traits with multiple viruses. Important plant pathogens include viroids. Although they interact with the host cell and use the host machinery for replication, they do not code for proteins. Human hepatitis delta virus contains a protein coat derived from hepatitis B virus but an RNA genome comparable to viroids.

Microbiology of Viruses

Life properties

Whether viruses are organic entities that interact with living beings or a form of life is a topic of debate among scientists. They have been referred to as “organisms at the edge of life” because they have characteristics with creatures, such as having genes, evolving through natural selection, and reproducing by building further copies of themselves. Despite having genes, they lack the cellular structure that is frequently thought of as the foundation of life. Since viruses lack their own metabolism, they need a host cell to produce new products. Therefore, they are unable to reproduce naturally outside of a host cellalthough some bacteria, like rickettsia and chlamydia, are still regarded as living things despite this restriction. Known life uses cell division to reproduce, whereas viruses do not.

Structure

Viruses exhibit a vast variety of morphologies, or sizes and shapes. More than a thousand bacteriophage viruses might fit within the cell of an Escherichia coli bacterium since viruses are typically considerably smaller than bacteria. The diameter of many viruses that have been investigated ranges from 20 to 300 nanometers, and they are typically spherical. Some filamentous filoviruses, which can reach lengths of up to 1400 nm, have diameters of only about 80 nm. Scanning and transmission electron microscopes are used to view most viruses since optical microscopes cannot often detect them. Electron-dense “stains” are employed to make the contrast between the viruses and the backdrop stand out more. These are salt solutions of heavy metals, like tungsten, that scatter electrons from areas where they are present.

Helical

Icosahedral

The majority of animal viruses have chiral icosahedral symmetry and are icosahedral or nearly spherical. The best technique to create a closed shell out of identical subunits is to arrange them into a regular icosahedron. For each triangular face, three capsomeres are the bare minimum needed, resulting in a total of 60 for the icosahedron. Many viruses retain this symmetry despite having more than 60 capsomeres and appearing spherical, like the rotavirus. To do this, the capsomeres at the apices—known as pentons—are encircled by five additional capsomeres. Herons are the six additional capsomeres that surround each capsomere on the triangular faces. Pentons, which make up the 12 vertices, are curved, whereas herons are fundamentally flat. It’s possible for the pentamer and hexamer subunits to be made of the same protein.

Prolate

This is a typical configuration of the heads of bacteriophages—an icosahedron that has been extended along its five-fold axis. A cylinder with caps on either end makes up this construction.

Enveloped

These viruses have a capsid that is neither entirely helical nor entirely icosahedral, and it may also have other features like protein tails or a complicated outer wall. Some bacteriophages, like Enterobacteria phage T4, have a complex structure made up of an icosahedral head connected to a helical tail that may have a hexagonal base plate with sticking-out protein tail fibers. By adhering to the bacterial host and then injecting the viral genome into the cell, this tail structure functions as a molecular syringe.
The poxviruses are enormous, intricate viruses with a peculiar shape. A core disc structure called as a nucleoid contains proteins that are connected to the viral DNA. A membrane and two unidentified lateral structures surround the nucleoid.

Giant viruses

Under an electron microscope, the capsid appears hexagonal; consequently, it is most likely icosahedral. In samples of water taken from the ocean floor off the coast of Las Cruces, Chile, researchers in 2011 found the largest virus known at the time. Mega virus chilensis, as it is now known, is visible under a standard optical microscope. The Pandora virus genus, which has genomes almost twice as big as Mega virus and Mimi virus, was found in Chile and Australia in 2013. They are divided into the families Mimiviridae, Pithoviridae, Pandoraviridae, Phycodnaviridae, and the genus Mollivirus. All large viruses contain dsDNA genomes.

Some viruses that infect Archaea have complex structures that are unrelated to any other types of viruses and come in a wide range of odd shapes, such as spindle-shaped structures and viruses that resemble

Genome Viruses

Viral species have an immense range of genomic architecture; together, they have greater structural genomic diversity than plants, mammals, archaea, or bacteria. Although there are millions of distinct virus types, less than 7,000 have been fully described. There are more than 193,000 full genome sequences in the NCBI Virus genome database as of January 2021, but there are undoubtedly many more still to be found.

A virus is referred to be a DNA virus or an RNA virus depending on whether it has a DNA or an RNA genome. There are RNA genomes in the great majority of viruses. Bacteriophages typically have double-stranded DNA genomes while plant viruses often have single-stranded RNA genomes.

Viral genomes can be linear or circular, such in polyomaviruses.

Genome size

The size of each species’ genome varies substantially. The smallest are the ssDNA circoviruses, family Circoviridae, which have genomes of about two kilobases and only code for two proteins; the largest are the Pandora viruses, which have genomes of over two me abases and code for about 2500 proteins. Rarely do virus genes contain introns, and their placement in the genome frequently causes them to overlap.

Due to their greater incidence of replication errors, RNA viruses often have lower genome sizes than DNA viruses. They also have a maximum upper size limit. Beyond this, replication errors make the virus ineffective or uncompetitive. To make up for this, RNA viruses frequently have segmented genomes (the genome is broken up into smaller molecules), which lessens the likelihood that a mistake in a single component of the genome will render the entire genome inoperable. Conversely, DNA viruses

Genetic mutation and recombination

There are various processes through which viruses change genetically. These include the phenomenon known as antigenic drift, in which specific bases in DNA or RNA mutate into different bases. The majority of these point mutations are “silent” in that the protein that the gene encodes remains unchanged, but some of them might provide evolutionary benefits, such resistance to antiviral medications. When the virus’s genome undergoes a significant change, antigenic shift happens. This could happen as a result of assortment or recombination. Pandemics may occur when this happens with influenza viruses. Swarms of viruses belonging to the same species but with slightly variable genome nucleoside sequences sometimes occur as quasispecies or RNA viruses. Natural selection places a high priority on such quasispecies.

Replication cycle

Since viral populations are acellular, they do not expand through cell division. Instead, they duplicate themselves using the equipment and metabolism of a host cell, where they then assemble. The host cell is compelled to make thousands of copies of the original virus quickly after infection.

Although their life cycle varies greatly between species, there are six essential stages:

A precise interaction between viral capsid proteins and a set of receptors on the cellular surface of the host is known as attachment. A virus’s host range and kind of host cell are determined by its specificity. For instance, only a few types of human leucocytes may be infected by HIV. This is due to the fact that the gp120 protein on its surface interacts particularly with the chemokine receptor CD4 molecule.

Genome replication

Different types of viruses differ greatly in terms of the genetic material they contain and how that material is replicated.

DNA viruses

Most DNA viruses replicate their genomes in the cell’s nucleus. Herpesviruses, for example, enter the cell through directly fusing with the cell membrane, but more commonly by receptor-mediated endocytosis if the cell has the proper receptor on its surface. The DNA and RNA synthesis and processing machinery of the host cell serves as the only source of support for the majority of DNA viruses. Larger viruses may be able to encode most of this machinery themselves. In contrast to bacteria, eukaryotes require the viral genome to pass the nuclear membrane in order to access this machinery.

RNA viruses

The cytoplasm is where RNA viruses often replicate. RNA viruses may be divided into four categories based on how they replicate. Single-stranded RNA viruses’ polarity, or whether it can be utilised directly by ribosomes to form proteins, and the genetic material’s double- or single-strandedness are the two main factors that affect how they replicate. All RNA viruses replicate their genomes using the RNA replicase enzymes that they have evolved.

Reverse transcribing viruses

Reverse transcribing viruses have Sarna or dsDNA in their particles (Retroviridae, Metaviridae, Pseudoviridae, and Caulimoviridae). Retroviruses, which have RNA genomes, reproduce using a DNA intermediary, whereas Para retroviruses, which have DNA genomes, replicate using an RNA intermediate. Both types perform the nucleic acid conversion using a reverse transcriptase, also known as an RNA-dependent DNA polymerase enzyme. As part of the replication process, retroviruses integrate the DNA generated by reverse transcription into the host genome as a provirus; Para retroviruses do not, however integrated genome copies of particularly plant Para retroviruses can result in infectious viruses.[104] They are sensitive to antiviral medications like zidovudine and lamivudine that block the reverse transcriptase enzyme. HIV is a prime example of the first kind.

Cytopathic effects on the host cell

Viruses have a wide variety of structural and metabolic impacts on the host cell. ‘Cytopathic effects’ are what these are known as. Most viral infections eventually cause the host cell to die. Cell lysis, changes to the cell’s surface membrane, and apoptosis are some of the causes of death. viral-specific proteins, not all of which are parts of the viral particle, frequently induce cell death by stopping its normal functions. The line separating innocuous from cytopathic is gradually drawn. While some viruses, like the Epstein-Barr virus, can cause cell proliferation without causing cancer, others, like papillomaviruses, are known to be cancer-causing agents.

Dormant and latent infections

There may be no outward signs of a viral infection in a cell. Cells with a latent and dormant virus exhibit little symptoms of infection and frequently perform properly. The virus is frequently latent for many months or years as a result, causing recurrent infections. This frequently applies to herpes viruses.

Host range

Viruses outweigh all other living things combined and are by far the most prevalent on Earth.[115] They spread infections to all forms of cellular life, including bacteria, fungus, plants, and animals. Only a small number of hosts can be infected by various virus types, and many of them are species-specific. Some viruses, like the smallpox virus, are said to have a narrow host range because they can only infect one species, in this case humans. Other viruses, like the rabies virus, are known to have a wide host range and can infect various mammal species. Animals are unaffected by viruses that infect plants, and most viruses that infect other animals are also unaffected by humans. Some bacteriophages can only infect a single strain of bacteria as a host.

Novel viruses

Unknown viruses are referred to as new viruses. It may be a virus that has been isolated from its natural reservoir or one that has been isolated after spreading to an unidentified animal or human host. It may be an emerging virus, one that is a brand-new virus, but it might also be an undetected existing infection. An illustration of a novel virus is the SARS-CoV-2 coronavirus that sparked the covid disease pandemic.

Classification

Main article: Virus classification

By identifying and categorizing viruses according to their commonalities, classification aims to describe the variety of viruses. The Linnaean hierarchical method of viral categorization was initially developed in 1962 by André Lwoff, Robert Horne, and Paul Tornier. This system used phylum, class, order, family, genus, and species to categories things. The common characteristics of viruses—not those of their hosts—and the kind of nucleic acid that makes up their genomes were used to classify them. The International Committee on Taxonomy of Viruses, or ICTV, was established in 1966. The ICTV first rejected the Lwoff, Horne, and Tornier approach because it was challenging to trace a virus’s lineage beyond a few generations due to its tiny genome size and fast rate of mutation.

ICTV classification

To ensure family unity, the ICTV produced rules that created the present categorization system and gave higher weight to certain viral features. A common classification scheme for viruses known as the unified taxonomy has been developed. The variety of viruses has not been researched to its full extent. The ICTV has defined 11,273 species of viruses as of 2022, in addition to 6 realms, 10 kingdoms, 17 phyla, 2 subphyla, 40 classes, 72 orders, 8 suborders, 264 families, 182 subfamilies, 2,818 genera, 84 subgenera, and 40 classes of bacteria.

The following diagram illustrates the overall taxonomic structure of taxon ranges and the suffixes used in taxonomic names. The ranks of sub realm, subkingdom, and subclass are not in use as of 2022, although all other levels are.

relam (-virial)Sub realm (-viral)kingdom (-virgae)Subkingdom (-veritas)phylum (-varicotic)Subphylum (-viricetidae)class (-viricides)Subclass (-viricetidae)order (-vials)Suborder (-virineae)family (-viridine)Subfamily (-virineae)genus (-virus)Subgenus (-virus)specie

Baltimore classification

The Baltimore categorization system was developed by the Nobel Prize-winning scientist David Baltimore. Modern viral categorization uses both the Baltimore classification system and the ICTV classification system.

The method of mRNA creation serves as the foundation for the Baltimore categorization of viruses. In order to create proteins and multiply, viruses need to synthesize mRNA from their genomes, but each virus family utilizes a different method to do so. The RNA or DNA in viral genomes can be single- or double-stranded, and reverse transcriptase (RT) may or may not be used. Additionally, sense (+) or antisense () Sarna viruses are both possible. This division of viruses into groups creates seven categories:

• I: dsDNA viruses, such as adenoviruses and poxviruses.
• II: ssDNA viruses with an added “sense” strand DNA, such as in parvoviruses
• Reoviruses, for example, are dsRNA viruses.
• Viruses with (+)ssRNA (+ strand or sense) RNA viruses, such as togaviruses, picornaviruses, and coronaviruses
• V: (ssRNA viruses (sense or antisense strand) RNA viruses, such rhabdoviruses and orthomyxoviruses
• VI: Positive strand or sense ssRNA-RT viruses RNA with DNA as an intermediary in the life cycle (such as retroviruses)
• dsDNA-RT virusesHepadnaviruses, for example, have a life cycle intermediate of DNA and RNA.

Role in human disease

The common cold, influenza, chickenpox, and cold sores are a few common human illnesses that are brought on by viruses. Viruses are the root cause of a number of deadly conditions, including rabies, Ebola viral illness, AIDS (HIV), avian influenza, and SARS. Virulence is a word used to indicate how well a virus may spread illness. It is being researched if viruses are the root cause of other disorders, such as the potential link between the human herpesvirus 6 (HHV6) and neurological conditions including multiple sclerosis and chronic fatigue syndrome. It is debatable if the bornaviral, which was once believed to cause neurological problems in horses, may also be the source of mental disorders in people.

Viruses cause illness in an organism through a variety of ways, which mostly depend on

The herpes viruses, such as the Epstein-Barr virus, which causes glandular fever, and the varicella zoster virus, which causes chickenpox and shingles, all have this property, which is known as latency. Most people carry at least one of these forms of herpes virus in their system. These latent viruses may occasionally be advantageous because they can boost defenses against bacterial diseases like Yersinia pestis.

Epidemiology

The field of medicine known as viral epidemiology studies the spread and management of viral diseases in humans. Viral transmission can occur vertically, or from mother to kid, or horizontally, or from one person to another. Hepatitis B virus and HIV are two examples of vertical transmission, when the infant is born with the infection already present. Another, more uncommon example is the varicella zoster virus, which can be lethal to a fetes and a newborn baby while only producing minor illnesses in children and adults.

In populations, horizontal transmission is the most frequent method of viral propagation. Horizontal transmission can happen when saliva is exchanged, when bodily fluids are transferred during sexual activity, or when infected food or drink is consumed.

During viral illness outbreaks, epidemiology is employed to break the chain of infection in the community. Based on the understanding of the virus’s mode of transmission, control measures are applied. Finding the outbreak’s source (or sources) and identifying the virus are crucial. Vaccines can occasionally break the cycle of transmission once the virus has been detected. Sanitation and disinfection can be useful in the absence of vaccinations. Frequently, those who are infected are separated from the rest of the population, and those who have been exposed to the virus are quarantined. Thousands of cattle were put to death in 2001 in Britain to contain the spread of foot and mouth illness among livestock. the majority of viral illnesses in people and other animals

Epidemics and pandemics

Global epidemics are known as pandemics. The 1918–1919 flu pandemic was a category 5 influenza pandemic brought on by an exceptionally virulent and lethal influenza A virus. In contrast to typical influenza epidemics, which mostly strike children, the elderly, or people who are otherwise frail, the victims were frequently healthy young adults. While more recent research indicates that it may have killed as many as 100 million people, or 5% of the world’s population in 1918, older estimates place the death toll at 40–50 million.

Virus pandemics are uncommon occurrences, although HIV, which originated from viruses found in chimpanzees and monkeys, has been widespread since at least the 1980s.

There were four influenza pandemics throughout the 20th century, and the ones that struck in 1918, 1957, and 1968 were particularly bad. The majority of scientists think that HIV first appeared in sub-Saharan Africa in the 20th century; the illness is now pandemic, with an estimated 37.9 million people globally suffering from it. Approximately 770,000 people died from AIDS in 2018. According to estimates from the World Health Organization (WHO) and the Joint United Nations Programmed on HIV/AIDS (UNAIDS), AIDS has killed more than 25 million people since it was first identified on June 5, 1981, making it one of the deadliest diseases in recorded history. There were 2 million HIV-related deaths and 2.7 million new HIV infections in 2007.

The Filoviridae family includes a number of extremely dangerous viral diseases. Ebolaviruses and marburgviruses are examples of filoviruses, which are filament-like viruses that cause viral hemorrhagic fever. In April 2005, a Marburg virus epidemic in Angola garnered considerable media attention. The virus was initially identified in 1967. Since its discovery in 1976, the Ebola virus illness has also produced sporadic outbreaks with severe fatality rates. The West Africa outbreak from 2013 to 2016 is the deadliest and most recent.

All pandemics save smallpox are brought on by newly developed viruses. These “emergent” viruses are typically mutations of more common, less dangerous viruses that have previously circulated in either people or other animals.

New varieties of coronaviruses are what cause the severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS). The following coronaviruses are

Cancer

Cancer has been linked to viruses in both humans and other species. Only a small proportion of infected people (or animals) develop viral cancer. There is no one sort of “norovirus” (a defunct word originally used for acutely transforming retroviruses), as cancer viruses arise from a variety of virus families, including both RNA and DNA viruses. Numerous elements, such as host immunity and host mutations, influence the development of cancer. Some genotypes of the human papillomavirus, the hepatitis B and C viruses, the Epstein-Barr virus, the Kaposi’s sarcoma-associated herpesvirus, and the human T-lymphotropic virus are known to cause human cancers. The majority of instances of a rare kind of human cancer are caused by a polyomavirus called the Merkel cell polyomavirus, which was just recently identified.

Host defense mechanisms

Viruses are a recognized factor in human cancer development and the innate immune system is the body’s initial line of defense against viruses. This consists of cells and other defense systems that protect the host against infection in a general way. Accordingly, the innate immune system’s cells recognize and react to pathogens in a generalized manner, but unlike the adaptive immune system, it does not provide the host with long-lasting or protective immunity.

An essential inherent defense against viruses is RNA interference. Many viruses use double-stranded RNA (dsRNA) as part of their replication process. When a virus of this type enters a cell, it releases one or more of its RNA molecules, which right away bind to a protein complex known as a dicer and break the RNA down into smaller bits. A

A vertebrate’s adaptive immune system responds to viral infections by producing particular antibodies that attach to the virus and frequently render it non-infectious. We refer to this as humoral immunity. Two different kinds of antibodies are crucial. The first, known as IgM, is generated by immune system cells for just a short period of time but is quite efficient in neutralizing viruses. The second, known as IgG, is continuously created. Acute infection is detected by looking for IgM in the host’s blood, whereas IgG suggests a previous infection. When conducting tests for immunity, IgG antibody is measured.

Even when viruses have succeeded to infect the host, antibodies can still serve as a strong defense.

An RNA virus is the cause of hepatitis C. 80% of those who get the disease have a chronic illness that will last the rest of their lives if untreated. Direct-acting antivirals can be used in successful therapies. Similar approaches, including the use of lamivudine and other anti-viral medications, have also been developed for the treatment of chronic carriers of the hepatitis B virus.

Reference

• https://www.britannica.com/science/virus
• https://easy4learning.com/genes-to-diet-in-inflammatory-bowel-disease/
• https://www.nature.com/scitable/topicpage/the-origins-of-viruses-14398218/