Avirus is a tinyparasite.[1] Virusesreproduce inside other living things; they are not free-living. A virus makescopies of itself inside another organism'scells. When virusesinfect livingorganisms, they can causedisease. There are many types ofdiseases caused by viruses, such aspolio,ebola andhepatitis. Viruses outnumber all other forms of life on the planet by a long way.[1]
A virus has a simple structure. It has just aprotein coat which covers a strand ofnucleic acid. Usually the nucleic acid isRNA; sometimes it isDNA. Viruses are much smaller than most kinds ofbacteria, and can only be seen under anelectron microscope. Viruses were not visible until the invention of the electron microscope.
Viruses reproduce by getting their nucleic acid strand into aprokaryote oreukaryotecell. TheRNA orDNA strand then takes over the cell machinery to make copies of itself and the protein coat. The cell then bursts open, spreading the newly created viruses. All viruses reproduce this way, and there are no free-living viruses.[2][3] Viruses are everywhere in the environment, and all organisms can be infected by them.[4][5][6]
Witheukaryotic cells, the virus protein coat is able to enter the target cells by certaincell membrane receptors. With prokaryotebacteria cells, thebacteriophage physically injects the nucleic acid strand into the host cell.
When the host cell has finished making more viruses, it undergoeslysis, or breaks apart. The viruses are released and are then able to infect other cells. Viruses can remain "silent" (inactive) for a long time, and will infect cells when the time and conditions are right.
Some special viruses are worth noting.Bacteriophages have evolved to enterbacterial cells, which have a different type of cell wall from eukaryote cell membranes.Envelope viruses, when they reproduce, cover themselves with a modified form of the host cell membrane, thus gaining an outerlipid layer that helps entry. Some of the most difficult viruses to combat, likeinfluenza andHIV, use this method.
Viral infections in animals trigger animmune response which usually destroys the infecting virus.Vaccines can also produce immune responses. They give an artificiallyacquired immunity to the specific viral infection. However, some viruses (including those causingAIDS and viralhepatitis) escape from these immune responses and cause chronic infections.Antibiotics have no effect on viruses, but there are some other drugs which can be used against viruses.Virology is the study of viruses.
There are manygenomic structures in viruses. As a group they are more diverse than plants, animals, archaea, or bacteria. There are millions of different types of viruses,[4] but only about 7,000 of them have been described in detail.[2]49
A virus has either RNA or DNA genes and so is called an RNA virus or a DNA virus.The vast majority of viruses have RNA genomes. Plant viruses usually have single-stranded RNA genomes and bacteriophages usually have double-stranded DNA genomes.[7]96/99
Fewer than 7,000 types have been described in detail. but there are doubtlessly many more to be discovered.[8][9]
Viral populations do not grow through cell division, because they do not have cells. Instead, they use the machinery and metabolism of a host cell to produce many copies of themselves, and theyassemble (put together) in the cell.
Thelife cycle of viruses differs greatly between species but there are sixbasic stages in the life cycle of viruses:[7]75/91
Attachment is a binding between viralcapsid proteins and specificreceptors on the host cellular surface.
Penetration follows attachment: Virions (single virus particles) enter the host cell by receptor-mediatedendocytosis or fusion with the lipid bilayer. This is called viral entry. The infection of plant and fungal cells is different from that of animal cells. Plants have a rigid cell wall made ofcellulose, and fungi one ofchitin. This means most viruses can only get inside these cells by force.[2]70 An example would be: a virus travels on aninsectvector which feeds on plant sap. The damage done to cell walls would let the virus get in. Bacteria, like plants, have strong cell walls that a virus must get through to infect the cell. However, bacterial cell walls are much thinner than plant cell walls, and some viruses have mechanisms that inject their genome into the bacterial cell, while the viral capsid remains outside.[2]71
Uncoating is how the viral capsid is removed: This may be by viralenzymes or host enzymes or by simple dissociation; the end-result is the releasing of the viralnucleic acid.
Replication of viruses is multiplying the genome. This usually means production of viralmessenger RNA (mRNA) from "early" genes. This may be followed, for complex viruses, by one or more further rounds of mRNA synthesis: "late" gene expression is of structural or virion proteins.
After the self-assembly of the virus particles, some modification of the proteins often occurs. In viruses such asHIV, this modification (sometimes called maturation) occursafter the virus has been released from the host cell.[10]
Viruses can bereleased from the host cell bylysis, a process that kills the cell by bursting its membrane and cell wall. This is a feature of many bacterial and some animal viruses. In some viruses the viral genome is put bygeneticrecombination into a specific place in the host's chromosome. The viral genome is then known as a "provirus" or, in the case of bacteriophages a "prophage".[3]60 Whenever the host divides, the viral genome is also replicated. The viral genome is mostly silent within the host; however, at some point, the provirus or prophage may give rise to active virus, which may lyse the host cells.[2]chapter 15 Enveloped viruses (e.g. HIV) typically are released from the host cell after the virus acquires its envelope. The envelope is a modified piece of the host's plasma membrane.[2]185/7
The genetic material within virus particles, and the method by which the material is replicated, varies considerably between different types of viruses.
This false-coloured transmission electron micrograph depicts the ultrastructural details of an influenza virus particle, or “virion”. The influenza virus is a single-stranded RNA organism
RNA viruses
Replication usually takes place in thecytoplasm. RNA viruses can be placed into four different groups depending on their modes of replication. All RNA viruses use their ownRNA replicase enzymes to create copies of their genomes.[7]79
DNA viruses
The genome replication of most DNA viruses takes place in the cell'snucleus. Most DNA viruses are entirely dependent on the host cell's DNA and RNA synthesising machinery, and RNA processing machinery. Viruses with larger genomes may encode much of this machinery themselves. In eukaryotes the viral genome must cross the cell's nuclear membrane to access this machinery, while in bacteria it need only enter the cell.[3]54[7]78
Reverse transcribing viruses
Reverse transcribing viruses with RNA genomes (retroviruses) use a DNA intermediate to replicate. Those with DNA genomes (pararetroviruses) use an RNA intermediate during genome replication.[11] They are susceptible toantiviral drugs that inhibit thereverse transcriptaseenzyme. An example of the first type is HIV, which is a retrovirus. Examples of the second type are theHepadnaviridae, which includesHepatitis B virus.[7]88/9
The body's first line of defence against viruses is theinnate immune system. This has cells and other mechanisms which defend the host from any infection. The cells of the innate system recognise, and respond to, pathogens in a general way.[12]
RNA interference is an important innate defence against viruses.[13] Many viruses have a replication strategy that involves double-stranded RNA (dsRNA). When such a virus infects a cell, it releases its RNA molecule. A protein complex calleddicer sticks to it and chops the RNA into pieces. Then a biochemical pathway, called the RISC complex, starts up. This attacks the viral mRNA, and the cell survives the infection.
Tworotaviruses: the one on the right is coated with antibodies which stop it attaching to cells and infecting them
Rotaviruses avoid this by not uncoating fully inside the cell and by releasing newly produced mRNA through pores in the particle's inner capsid. The genomic dsRNA remains protected inside the core of the virion.[14][15]
The production ofinterferon is an important host defence mechanism. This is a hormone produced by the body when viruses are present. Its role in immunity is complex; it eventually stops the viruses from reproducing by killing the infected cell and its close neighbours.[16]
Vertebrates have a second, more specific, immune system. It is called theadaptive immune system. When it meets a virus, it produces specificantibodies that bind to the virus and render it non-infectious. Two types of antibodies are important.
The first, called IgM, is highly effective at neutralizing viruses but is produced by the cells of the immune system only for a few weeks. The second, called IgG, is produced indefinitely. The presence of IgM in the blood of the host is used to test for acute infection, whereas IgG indicates an infection sometime in the past.[17] IgG antibody is measured when tests forimmunity are carried out.[18]
Another vertebrate defence against viruses involves immune cells known asT cells. The body's cells constantly display short fragments of their proteins on the cell's surface, and, if a T cell recognises a suspicious viral fragment there, the host cell is destroyed bykiller T cells and the virus-specific T-cells proliferate. Cells such asmacrophages are specialists at this antigen presentation.[19]
Not all virus infections produce a protective immune response. These persistent viruses evade immune control by sequestration (hiding away);cytokine resistance; evadingnatural killer cell activity; escape fromapoptosis (cell death), and antigenic shift (changing surface proteins).[20]HIV evades the immune system by constantly changing theamino acid sequence of the proteins on the surface of the virion. Other viruses move alongnerves to places the immune system cannot reach.
Viruses do not belong to anygroup of living organisms, including any of thekingdoms ordomains. They are not seen as living organisms because they are not active until the point of infection. However, that is just a verbal point.
Their structure and mode of operation means they have evolved from other living things, and the loss of normal structure occurs in many endoparasites. The origins of viruses in theevolutionary history of life are unclear; some may haveevolved fromplasmids – pieces of DNA that can move between cells – while others may have evolved from bacteria. In evolution, viruses are an important means ofhorizontal gene transfer, which increasesgenetic diversity.[21]
A recent project discovered nearly 1500 newRNA viruses by sampling over 200invertebrate species. "The research team... extracted their RNA and, using next-generationsequencing, deciphered the sequence of a staggering sixtrillion letters present in the invertebrate RNA libraries".[22] The research showed that viruses changed bits and pieces of their RNA by a variety of genetic mechanisms. "The invertebrate virome [shows] remarkable genomic flexibility that includes frequentrecombination,lateral gene transfer among viruses and hosts, gene gain and loss, and complex genomic rearrangements".[23]
Viruses have been on this planet a long time. We now know that bacteria andarchaea had to deal with them first, before our type of cellular life evolved. Details of the defence mechanisms used by archaea and bacteria are discussed on the pageCRISPR, which briefly introduces the topic of early defences against viruses.
Viruses are everywhere in the sea. They may outnumber all other forms ofmarine life by at least anorder of magnitude. Through selectiveinfection, viruses influencenutrient cycling and evolution in the ocean.[24]
Viruses are used widely incell biology.[25]Geneticists often use viruses asvectors to introduce genes into cells that they are studying. This is useful for making the cell produce a foreign substance, or to study the effect of introducing a new gene into the genome. Eastern European scientists have usedphage therapy as an alternative to antibiotics for some time, and interest in this approach is increasing, because of the high level ofantibiotic resistance now found in some pathogenic bacteria.[26]
12345Collier, Leslie; Balows, Albert; Sussman, Max 1998.Topley and Wilson's Microbiology and microbial infections. 9th ed, vol 1,Virology. volume editors: Mahy, Brian and Collier, Leslie. Arnold.ISBN0-340-66316-2
↑Barman S, Ali A, Hui EK, Adhikary L, Nayak DP (2001). "Transport of viral proteins to the apical membranes and interaction of matrix protein with glycoproteins in the assembly of influenza viruses".Virus Res.77 (1):61–9.doi:10.1016/S0168-1702(01)00266-0.PMID11451488.{{cite journal}}: CS1 maint: multiple names: authors list (link)
↑Staginnus C, Richert-Pöggeler KR (2006). "Endogenous pararetroviruses: two-faced travelers in the plant genome".Trends in Plant Science.11 (10):485–91.doi:10.1016/j.tplants.2006.08.008.PMID16949329.
