126-421: See text Tectiviridae is a family of viruses with 10 species in five genera. Bacteria serve as natural hosts. Tectiviruses have no head-tail structure, but are capable of producing tail-like tubes of ~ 60×10 nm upon adsorption or after chloroform treatment. The name is derived from Latin tectus (meaning 'covered'). The virions of Tectiviridae species are non-enveloped, icosahedral and display
252-406: A host cell is often forced to rapidly produce thousands of copies of the original virus. When not inside an infected cell or in the process of infecting a cell, viruses exist in the form of independent viral particles, or virions , consisting of (i) genetic material , i.e., long molecules of DNA or RNA that encode the structure of the proteins by which the virus acts; (ii) a protein coat,
378-507: A predatory microorganism invaded a large prokaryote, probably an archaean , but instead of killing its prey, the attacker took up residence and evolved into mitochondria; one of these chimeras later tried to swallow a photosynthesizing cyanobacterium, but the victim survived inside the attacker and the new combination became the ancestor of plants ; and so on. After each endosymbiosis, the partners eventually eliminated unproductive duplication of genetic functions by re-arranging their genomes,
504-425: A virion , consists of nucleic acid surrounded by a protective coat of protein called a capsid . These are formed from protein subunits called capsomeres . Viruses can have a lipid "envelope" derived from the host cell membrane . The capsid is made from proteins encoded by the viral genome and its shape serves as the basis for morphological distinction. Virally-coded protein subunits will self-assemble to form
630-606: A basic optical microscope. In 2013, the Pandoravirus genus was discovered in Chile and Australia, and has genomes about twice as large as Megavirus and Mimivirus. All giant viruses have dsDNA genomes and they are classified into several families: Mimiviridae , Pithoviridae, Pandoraviridae , Phycodnaviridae , and the Mollivirus genus. Some viruses that infect Archaea have complex structures unrelated to any other form of virus, with
756-418: A capsid diameter of 400 nm. Protein filaments measuring 100 nm project from the surface. The capsid appears hexagonal under an electron microscope, therefore the capsid is probably icosahedral. In 2011, researchers discovered the largest then known virus in samples of water collected from the ocean floor off the coast of Las Cruces, Chile. Provisionally named Megavirus chilensis , it can be seen with
882-581: A capsid, in general requiring the presence of the virus genome. Complex viruses code for proteins that assist in the construction of their capsid. Proteins associated with nucleic acid are known as nucleoproteins , and the association of viral capsid proteins with viral nucleic acid is called a nucleocapsid. The capsid and entire virus structure can be mechanically (physically) probed through atomic force microscopy . In general, there are five main morphological virus types: The poxviruses are large, complex viruses that have an unusual morphology. The viral genome
1008-639: A cellular structure, which is often seen as the basic unit of life. Viruses do not have their own metabolism and require a host cell to make new products. They therefore cannot naturally reproduce outside a host cell —although some bacteria such as rickettsia and chlamydia are considered living organisms despite the same limitation. Accepted forms of life use cell division to reproduce, whereas viruses spontaneously assemble within cells. They differ from autonomous growth of crystals as they inherit genetic mutations while being subject to natural selection. Virus self-assembly within host cells has implications for
1134-446: A compartment for abiogenic processes. A genomic analysis supports this hypothesis as they found 355 genes that likely traced to LUCA upon 6.1 million sequenced prokaryotic genes. They reconstruct LUCA as a thermophilic anaerobe with a Wood-Ljungdahl pathway, implying an origin of life at white smokers. LUCA would also have exhibited other biochemical pathways such as gluconeogenesis , reverse incomplete Krebs cycle , glycolysis , and
1260-493: A different DNA (or RNA) molecule. This can occur when viruses infect cells simultaneously and studies of viral evolution have shown that recombination has been rampant in the species studied. Recombination is common to both RNA and DNA viruses. Coronaviruses have a single-strand positive-sense RNA genome. Replication of the genome is catalyzed by an RNA-dependent RNA polymerase . The mechanism of recombination used by coronaviruses likely involves template switching by
1386-399: A different scenario a single last universal ancestor, e.g. a "first cell" or a first individual precursor cell has never existed. Instead, the early biochemical evolution of life led to diversification through the development of a multiphenotypical population of pre-cells from which the precursor cells ( protocells ) of the three domains of life emerged. Thus, the formation of cells was
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#17330923391201512-559: A few species, or broad for viruses capable of infecting many. Viral infections in animals provoke an immune response that usually eliminates the infecting virus. Immune responses can also be produced by vaccines , which confer an artificially acquired immunity to the specific viral infection. Some viruses, including those that cause HIV/AIDS , HPV infection , and viral hepatitis , evade these immune responses and result in chronic infections. Several classes of antiviral drugs have been developed. The English word "virus" comes from
1638-549: A fluid, by Wendell Meredith Stanley , and the invention of the electron microscope in 1931 allowed their complex structures to be visualised. Scientific opinions differ on whether viruses are a form of life or organic structures that interact with living organisms. They have been described as "organisms at the edge of life", since they resemble organisms in that they possess genes , evolve by natural selection , and reproduce by creating multiple copies of themselves through self-assembly. Although they have genes, they do not have
1764-445: A genome size of only two kilobases; the largest—the pandoraviruses —have genome sizes of around two megabases which code for about 2500 proteins. Virus genes rarely have introns and often are arranged in the genome so that they overlap . In general, RNA viruses have smaller genome sizes than DNA viruses because of a higher error-rate when replicating, and have a maximum upper size limit. Beyond this, errors when replicating render
1890-506: A ladder. The virus particles of some virus families, such as those belonging to the Hepadnaviridae , contain a genome that is partially double-stranded and partially single-stranded. For most viruses with RNA genomes and some with single-stranded DNA (ssDNA) genomes, the single strands are said to be either positive-sense (called the 'plus-strand') or negative-sense (called the 'minus-strand'), depending on if they are complementary to
2016-634: A left and a right side that are mirror images of each other, appeared by 555 Ma (million years ago). Ediacara biota appeared during the Ediacaran period, while vertebrates , along with most other modern phyla originated about 525 Ma during the Cambrian explosion . During the Permian period, synapsids , including the ancestors of mammals , dominated the land. The Permian–Triassic extinction event killed most complex species of its time, 252 Ma . During
2142-436: A life form, because they carry genetic material, reproduce, and evolve through natural selection , although they lack some key characteristics, such as cell structure, that are generally considered necessary criteria for defining life. Because they possess some but not all such qualities, viruses have been described as "organisms at the edge of life" and as replicators . Viruses spread in many ways. One transmission pathway
2268-472: A limited range of hosts and many are species-specific. Some, such as smallpox virus for example, can infect only one species—in this case humans, and are said to have a narrow host range . Other viruses, such as rabies virus, can infect different species of mammals and are said to have a broad range. The viruses that infect plants are harmless to animals, and most viruses that infect other animals are harmless to humans. The host range of some bacteriophages
2394-443: A long way to go, since theoretical and empirical approaches are only beginning to make contact with each other. Even the simplest members of the three modern domains of life use DNA to record their "recipes" and a complex array of RNA and protein molecules to "read" these instructions and use them for growth, maintenance and self-replication. The discovery that some RNA molecules can catalyze both their own replication and
2520-480: A particular environment rapidly becomes dominant; and they can catalyze the formation of RNA molecules. Although this idea has not become the scientific consensus, it still has active supporters. Research in 2003 reported that montmorillonite could also accelerate the conversion of fatty acids into "bubbles" and that the "bubbles" could encapsulate RNA attached to the clay. These "bubbles" can then grow by absorbing additional lipids and then divide. The formation of
2646-408: A prime target for natural selection. Segmented genomes confer evolutionary advantages; different strains of a virus with a segmented genome can shuffle and combine genes and produce progeny viruses (or offspring) that have unique characteristics. This is called reassortment or 'viral sex'. Genetic recombination is a process by which a strand of DNA (or RNA) is broken and then joined to the end of
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#17330923391202772-577: A process which sometimes involved transfer of genes between them. Another hypothesis proposes that mitochondria were originally sulfur - or hydrogen -metabolising endosymbionts, and became oxygen-consumers later. On the other hand, mitochondria might have been part of eukaryotes' original equipment. There is a debate about when eukaryotes first appeared: the presence of steranes in Australian shales may indicate eukaryotes at 2.7 Ga; however, an analysis in 2008 concluded that these chemicals infiltrated
2898-444: A pseudo T=25 symmetry. The capsid has two layers. The outer layer is a protein structure of 240 capsid proteins trimers, and the inner one is a proteinaceous lipid membrane which envelopes the virus genome . Apical spikes extending about 20 nanometers (nm) protrude from the icosahedrons vertices. The genome is a single molecule of linear double-stranded DNA of 15 kilo bases in length, and has 30 open reading frames. It forms
3024-412: A single viral particle that is released from the cell and is capable of infecting other cells of the same type. Viruses are found wherever there is life and have probably existed since living cells first evolved . The origin of viruses is unclear because they do not form fossils, so molecular techniques are used to infer how they arose. In addition, viral genetic material occasionally integrates into
3150-456: A small part of the total diversity of viruses has been studied. As of 2022, 6 realms, 10 kingdoms, 17 phyla, 2 subphyla, 40 classes, 72 orders, 8 suborders, 264 families, 182 subfamilies , 2,818 genera, 84 subgenera , and 11,273 species of viruses have been defined by the ICTV. The general taxonomic structure of taxon ranges and the suffixes used in taxonomic names are shown hereafter. As of 2022,
3276-409: A successive process. See § Metabolism first: Pre-cells, successive cellularisation , below. Life on Earth is based on carbon and water . Carbon provides stable frameworks for complex chemicals and can be easily extracted from the environment, especially from carbon dioxide . There is no other chemical element whose properties are similar enough to carbon's to be called an analogue; silicon ,
3402-401: A tightly packed coil and encodes several structural proteins. It encodes about 30 proteins that are transcribed in operons. At least 9 structural proteins are present in the viron. The genome is about 66 mega daltons in weight and constitutes 14–15% of the virion by weight. Lipids constitute a further 15% by weight. Carbohydrates are not present. Viral replication is cytoplasmic. Entry into
3528-533: A very small percentage of species have been identified: one estimate claims that Earth may have 1 trillion species, because "identifying every microbial species on Earth presents a huge challenge." Only 1.75–1.8 million species have been named and 1.8 million documented in a central database. The currently living species represent less than one percent of all species that have ever lived on Earth. The oldest meteorite fragments found on Earth are about 4.54 billion years old; this, coupled primarily with
3654-605: A wide diversity of sizes and shapes, called ' morphologies '. In general, viruses are much smaller than bacteria and more than a thousand bacteriophage viruses would fit inside an Escherichia coli bacterium's cell. Many viruses that have been studied are spherical and have a diameter between 20 and 300 nanometres . Some filoviruses , which are filaments, have a total length of up to 1400 nm; their diameters are only about 80 nm. Most viruses cannot be seen with an optical microscope , so scanning and transmission electron microscopes are used to visualise them. To increase
3780-553: A wide variety of unusual shapes, ranging from spindle-shaped structures to viruses that resemble hooked rods, teardrops or even bottles. Other archaeal viruses resemble the tailed bacteriophages, and can have multiple tail structures. An enormous variety of genomic structures can be seen among viral species ; as a group, they contain more structural genomic diversity than plants, animals, archaea, or bacteria. There are millions of different types of viruses, although fewer than 7,000 types have been described in detail. As of January 2021,
3906-416: Is a feature of many bacterial and some animal viruses. Some viruses undergo a lysogenic cycle where the viral genome is incorporated by genetic recombination 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 ". Whenever the host divides, the viral genome is also replicated. The viral genome is mostly silent within
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4032-405: Is a major change in the genome of the virus. This can be a result of recombination or reassortment . The Influenza A virus is highly prone to reassortment; occasionally this has resulted in novel strains which have caused pandemics . RNA viruses often exist as quasispecies or swarms of viruses of the same species but with slightly different genome nucleoside sequences. Such quasispecies are
4158-966: Is an excellent solvent and has two other useful properties: the fact that ice floats enables aquatic organisms to survive beneath it in winter; and its molecules have electrically negative and positive ends, which enables it to form a wider range of compounds than other solvents can. Other good solvents, such as ammonia , are liquid only at such low temperatures that chemical reactions may be too slow to sustain life, and lack water's other advantages. Organisms based on alternative biochemistry may, however, be possible on other planets. Research on how life might have emerged from non-living chemicals focuses on three possible starting points: self-replication , an organism's ability to produce offspring that are very similar to itself; metabolism, its ability to feed and repair itself; and external cell membranes , which allow food to enter and waste products to leave, but exclude unwanted substances. Research on abiogenesis still has
4284-400: Is associated with proteins within a central disc structure known as a nucleoid . The nucleoid is surrounded by a membrane and two lateral bodies of unknown function. The virus has an outer envelope with a thick layer of protein studded over its surface. The whole virion is slightly pleomorphic , ranging from ovoid to brick-shaped. Mimivirus is one of the largest characterised viruses, with
4410-439: Is caused by cessation of its normal activities because of suppression by virus-specific proteins, not all of which are components of the virus particle. The distinction between cytopathic and harmless is gradual. Some viruses, such as Epstein–Barr virus , can cause cells to proliferate without causing malignancy, while others, such as papillomaviruses , are established causes of cancer. Some viruses cause no apparent changes to
4536-611: Is controversy over whether the bornavirus , previously thought to cause neurological diseases in horses, could be responsible for psychiatric illnesses in humans. Evolutionary history of life The history of life on Earth traces the processes by which living and extinct organisms evolved, from the earliest emergence of life to the present day. Earth formed about 4.5 billion years ago (abbreviated as Ga , for gigaannum ) and evidence suggests that life emerged prior to 3.7 Ga. The similarities among all known present-day species indicate that they have diverged through
4662-512: Is correct. It seems unlikely that all currently known viruses have a common ancestor, and viruses have probably arisen numerous times in the past by one or more mechanisms. The first evidence of the existence of viruses came from experiments with filters that had pores small enough to retain bacteria. In 1892, Dmitri Ivanovsky used one of these filters to show that sap from a diseased tobacco plant remained infectious to healthy tobacco plants despite having been filtered. Martinus Beijerinck called
4788-498: Is first recorded in 1728, long before the discovery of viruses by Dmitri Ivanovsky in 1892. The English plural is viruses (sometimes also vira ), whereas the Latin word is a mass noun , which has no classically attested plural ( vīra is used in Neo-Latin ). The adjective viral dates to 1948. The term virion (plural virions ), which dates from 1959, is also used to refer to
4914-607: Is identical in sequence to the viral mRNA and is thus a coding strand, while negative-sense viral ssDNA is complementary to the viral mRNA and is thus a template strand. Several types of ssDNA and ssRNA viruses have genomes that are ambisense in that transcription can occur off both strands in a double-stranded replicative intermediate. Examples include geminiviruses , which are ssDNA plant viruses and arenaviruses , which are ssRNA viruses of animals. Genome size varies greatly between species. The smallest—the ssDNA circoviruses, family Circoviridae —code for only two proteins and have
5040-431: Is limited to a single strain of bacteria and they can be used to trace the source of outbreaks of infections by a method called phage typing . The complete set of viruses in an organism or habitat is called the virome ; for example, all human viruses constitute the human virome . A novel virus is one that has not previously been recorded. It can be a virus that is isolated from its natural reservoir or isolated as
5166-446: Is sequestered into calcium carbonate ( calcite ), phosphate concentrations are able to increase to levels necessary for facilitating biomolecule creation. Though carbonate-rich lakes have alkaline chemistry in modern times, models suggest that carbonate lakes had a pH low enough for prebiotic synthesis when placed in the acidifying context of Earth's early carbon dioxide rich atmosphere . Rainwater rich in carbonic acid weathered
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5292-477: Is shown in the adjacent figure, where important evolutionary improvements are indicated by numbers. Wet-dry cycles at geothermal springs are shown to solve the problem of hydrolysis and promote the polymerization and vesicle encapsulation of biopolymers. The temperatures of geothermal springs are suitable for biomolecules. Silica minerals and metal sulfides in these environments have photocatalytic properties to catalyze biomolecules. Solar UV exposure also promotes
5418-402: Is solved in carbonate -rich environments. When in the presence of carbonate, calcium readily reacts to form calcium carbonate instead of apatite minerals. With the free calcium ions removed from solution , phosphate ions are no longer precipitated from solution. This is specifically seen in lakes with no inflow, since no new calcium is introduced into the water body. After all of the calcium
5544-453: Is that the Earth and Moon started to coalesce at the same time but the Earth, having a much stronger gravity than the early Moon, attracted almost all the iron particles in the area. Until 2001, the oldest rocks found on Earth were about 3.8 billion years old, leading scientists to estimate that the Earth's surface had been molten until then. Accordingly, they named this part of Earth's history
5670-475: Is through disease-bearing organisms known as vectors : for example, viruses are often transmitted from plant to plant by insects that feed on plant sap , such as aphids ; and viruses in animals can be carried by blood-sucking insects. Many viruses spread in the air by coughing and sneezing, including influenza viruses , SARS-CoV-2 , chickenpox , smallpox , and measles . Norovirus and rotavirus , common causes of viral gastroenteritis , are transmitted by
5796-398: Is water, which is much more plentiful than the geologically produced reducing agents required by the earlier non-oxygenic photosynthesis. From this point onwards life itself produced significantly more of the resources it needed than did geochemical processes. Oxygen became a significant component of Earth's atmosphere about 2.4 Ga. Although eukaryotes may have been present much earlier,
5922-400: The capsid , which surrounds and protects the genetic material; and in some cases (iii) an outside envelope of lipids . The shapes of these virus particles range from simple helical and icosahedral forms to more complex structures. Most virus species have virions too small to be seen with an optical microscope and are one-hundredth the size of most bacteria. The origins of viruses in
6048-474: The CD4 molecule—a chemokine receptor —which is most commonly found on the surface of CD4+ T-Cells . This mechanism has evolved to favour those viruses that infect only cells in which they are capable of replication. Attachment to the receptor can induce the viral envelope protein to undergo changes that result in the fusion of viral and cellular membranes, or changes of non-enveloped virus surface proteins that allow
6174-510: The Hadean . However, analysis of zircons formed 4.4 Ga indicates that Earth's crust solidified about 100 million years after the planet's formation and that the planet quickly acquired oceans and an atmosphere , which may have been capable of supporting life. Evidence from the Moon indicates that from 4 to 3.8 Ga it suffered a Late Heavy Bombardment by debris that was left over from the formation of
6300-540: The International Committee on Taxonomy of Viruses (ICTV) was formed. The system proposed by Lwoff, Horne and Tournier was initially not accepted by the ICTV because the small genome size of viruses and their high rate of mutation made it difficult to determine their ancestry beyond order. As such, the Baltimore classification system has come to be used to supplement the more traditional hierarchy. Starting in 2018,
6426-675: The Latin vīrus , which refers to poison and other noxious liquids. Vīrus comes from the same Indo-European root as Sanskrit viṣa , Avestan vīša , and Ancient Greek ἰός ( iós ), which all mean "poison". The first attested use of "virus" in English appeared in 1398 in John Trevisa 's translation of Bartholomeus Anglicus 's De Proprietatibus Rerum . Virulent , from Latin virulentus ('poisonous'), dates to c. 1400 . A meaning of 'agent that causes infectious disease'
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#17330923391206552-484: The NCBI Virus genome database has more than 193,000 complete genome sequences, but there are doubtlessly many more to be discovered. A virus has either a DNA or an RNA genome and is called a DNA virus or an RNA virus , respectively. Most viruses have RNA genomes. Plant viruses tend to have single-stranded RNA genomes and bacteriophages tend to have double-stranded DNA genomes. Viral genomes are circular, as in
6678-680: The Solar System , and the Earth should have experienced an even heavier bombardment due to its stronger gravity. While there is no direct evidence of conditions on Earth 4 to 3.8 Ga, there is no reason to think that the Earth was not also affected by this late heavy bombardment. This event may well have stripped away any previous atmosphere and oceans; in this case gases and water from comet impacts may have contributed to their replacement, although outgassing from volcanoes on Earth would have supplied at least half. However, if subsurface microbial life had evolved by this point, it would have survived
6804-539: The common cold , influenza , chickenpox , and cold sores . Many serious diseases such as rabies , Ebola virus disease , AIDS (HIV) , avian influenza , and SARS are caused by viruses. The relative ability of viruses to cause disease is described in terms of virulence . Other diseases are under investigation to discover if they have a virus as the causative agent, such as the possible connection between human herpesvirus 6 (HHV6) and neurological diseases such as multiple sclerosis and chronic fatigue syndrome . There
6930-588: The domain Archea and finally to the domain Eucarya . For the development of cells ( cellularisation ), the pre-cells had to be protected from their surroundings by envelopes (i.e. membranes, walls). For instance, the development of rigid cell walls by the invention of peptidoglycan in bacteria (domain Bacteria) may have been a prerequisite for their successful survival, radiation and colonisation of virtually all habitats of
7056-477: The endosymbiont mitochondria provided a more abundant source of biological energy . Around 1.6 Ga, some eukaryotes gained the ability to photosynthesize via endosymbiosis with cyanobacteria, and gave rise to various algae that eventually overtook cyanobacteria as the dominant primary producers . At around 1.7 Ga, multicellular organisms began to appear, with differentiated cells performing specialised functions. While early organisms reproduced asexually ,
7182-407: The evolutionary history of life are still unclear. Some viruses may have evolved from plasmids , which are pieces of DNA that can move between cells. Other viruses may have evolved from bacteria. In evolution, viruses are an important means of horizontal gene transfer , which increases genetic diversity in a way analogous to sexual reproduction . Viruses are considered by some biologists to be
7308-422: The faecal–oral route , passed by hand-to-mouth contact or in food or water. The infectious dose of norovirus required to produce infection in humans is fewer than 100 particles. HIV is one of several viruses transmitted through sexual contact and by exposure to infected blood. The variety of host cells that a virus can infect is called its host range : this is narrow for viruses specialized to infect only
7434-430: The germline of the host organisms, by which they can be passed on vertically to the offspring of the host for many generations. This provides an invaluable source of information for paleovirologists to trace back ancient viruses that existed as far back as millions of years ago. There are three main hypotheses that aim to explain the origins of viruses: In the past, there were problems with all of these hypotheses:
7560-508: The pentose phosphate pathway , including biochemical reactions such as reductive amination and transamination . The Panspermia hypothesis does not explain how life arose originally, but simply examines the possibility of its coming from somewhere other than Earth. The idea that life on Earth was "seeded" from elsewhere in the Universe dates back at least to the Greek philosopher Anaximander in
7686-454: The polyomaviruses , or linear, as in the adenoviruses . The type of nucleic acid is irrelevant to the shape of the genome. Among RNA viruses and certain DNA viruses, the genome is often divided into separate parts, in which case it is called segmented. For RNA viruses, each segment often codes for only one protein and they are usually found together in one capsid. All segments are not required to be in
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#17330923391207812-447: The three domains . This discovery has led modern virologists to reconsider and re-evaluate these three classical hypotheses. The evidence for an ancestral world of RNA cells and computer analysis of viral and host DNA sequences give a better understanding of the evolutionary relationships between different viruses and may help identify the ancestors of modern viruses. To date, such analyses have not proved which of these hypotheses
7938-527: The ICTV began to acknowledge deeper evolutionary relationships between viruses that have been discovered over time and adopted a 15-rank classification system ranging from realm to species. Additionally, some species within the same genus are grouped into a genogroup . The ICTV developed the current classification system and wrote guidelines that put a greater weight on certain virus properties to maintain family uniformity. A unified taxonomy (a universal system for classifying viruses) has been established. Only
8064-415: The ability to tolerate and then to use oxygen, possibly via endosymbiosis , where one organism lives inside another and both of them benefit from their association. Cyanobacteria have the most complete biochemical "toolkits" of all the mat-forming organisms. Hence they are the most self-sufficient, well-adapted to strike out on their own both as floating mats and as the first of the phytoplankton , provide
8190-762: The assembly of vesicles. Exergonic reactions at hydrothermal vents are suggested to have been a source of free energy that promoted chemical reactions, synthesis of organic molecules, and are inducive to chemical gradients. In small rock pore systems, membranous structures between alkaline seawater and the acidic ocean would be conducive to natural proton gradients. Nucleobase synthesis could occur by following universally conserved biochemical pathways by using metal ions as catalysts. RNA molecules of 22 bases can be polymerized in alkaline hydrothermal vent pores. Thin pores are shown to only accumulate long polynucleotides whereas thick pores accumulate both short and long polynucleotides. Small mineral cavities or mineral gels could have been
8316-496: The atmosphere, leading to the Great Oxygenation Event around 2.4 Ga. The earliest evidence of eukaryotes (complex cells with organelles ) dates from 1.85 Ga, likely due to symbiogenesis between anaerobic archaea and aerobic proteobacteria in co-adaptation against the new oxidative stress . While eukaryotes may have been present earlier, their diversification accelerated when aerobic cellular respiration by
8442-428: The basis of most marine food chains. Eukaryotes may have been present long before the oxygenation of the atmosphere, but most modern eukaryotes require oxygen, which is used by their mitochondria to fuel the production of ATP , the internal energy supply of all known cells. In the 1970s, a vigorous debate concluded that eukaryotes emerged as a result of a sequence of endosymbiosis between prokaryotes . For example:
8568-484: The basis of similarities. In 1962, André Lwoff , Robert Horne , and Paul Tournier were the first to develop a means of virus classification, based on the Linnaean hierarchical system. This system based classification on phylum , class , order , family , genus , and species . Viruses were grouped according to their shared properties (not those of their hosts) and the type of nucleic acid forming their genomes. In 1966,
8694-464: The bombardment. The earliest identified organisms were minute and relatively featureless, and their fossils looked like small rods that are very difficult to tell apart from structures that arise through abiotic physical processes. The oldest undisputed evidence of life on Earth, interpreted as fossilized bacteria, dates to 3 Ga. Other finds in rocks dated to about 3.5 Ga have been interpreted as bacteria, with geochemical evidence also seeming to show
8820-427: The bottom layer is oxygen-free and often dominated by hydrogen sulfide emitted by the organisms living there. Oxygen is toxic to organisms that are not adapted to it, but greatly increases the metabolic efficiency of oxygen-adapted organisms; oxygenic photosynthesis by bacteria in mats increased biological productivity by a factor of between 100 and 1,000. The source of hydrogen atoms used by oxygenic photosynthesis
8946-574: The capabilities of individual organisms. Ribozymes remain as the main components of ribosomes , the "protein factories" in modern cells. Evidence suggests the first RNA molecules formed on Earth prior to 4.17 Ga. Although short self-replicating RNA molecules have been artificially produced in laboratories, doubts have been raised about whether natural non-biological synthesis of RNA is possible. The earliest "ribozymes" may have been formed of simpler nucleic acids such as PNA , TNA or GNA , which would have been replaced later by RNA. In 2003, it
9072-654: The case of PRD1, is achieved with the aid of virus-encoded lysis machinery consisting of four proteins: P15 ( endolysin ), P35 (holin), P36 and P37 (homologues of the Rz/Rz1 proteins of phage lambda). Tectiviridae contains the following genera and species: Other unassigned phages: Viruses A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism . Viruses infect all life forms , from animals and plants to microorganisms , including bacteria and archaea . Viruses are found in almost every ecosystem on Earth and are
9198-534: The construction of proteins led to the hypothesis of earlier life-forms based entirely on RNA. These ribozymes could have formed an RNA world in which there were individuals but no species, as mutations and horizontal gene transfers would have meant that offspring were likely to have different genomes from their parents, and evolution occurred at the level of genes rather than organisms. RNA would later have been replaced by DNA, which can build longer, more stable genomes, strengthening heritability and expanding
9324-624: The continuous exposure to sunlight as well as their cell walls with ion pumps to maintain their intracellular metabolism after they entered the oceans. Catalytic mineral particles and transition metal sulfides at these environments are capable of catalyzing organic compounds. Scientists simulated laboratory conditions that were identical to white smokers and successfully oligomerized RNA, measured to be 4 units long. Long chain fatty acids can be synthesized via Fischer-Tropsch synthesis . Another experiment that replicated conditions also similar white smokers, with long chain fatty acids present resulted in
9450-410: The contrast between viruses and the background, electron-dense "stains" are used. These are solutions of salts of heavy metals, such as tungsten , that scatter the electrons from regions covered with the stain. When virions are coated with stain (positive staining), fine detail is obscured. Negative staining overcomes this problem by staining the background only. A complete virus particle, known as
9576-479: The cytoplasm of modern cells. Fatty acids in acidic or slightly alkaline geothermal springs assemble into vesicles after wet-dry cycles as there is a lower concentration of ionic solutes at geothermal springs since they are freshwater environments, in contrast to seawater which has a higher concentration of ionic solutes. For organic compounds to be present at geothermal springs, they would have likely been transported by carbonaceous meteors. The molecules that fell from
9702-416: The dating of ancient lead deposits, has put the estimated age of Earth at around that time. The Moon has the same composition as Earth's crust but does not contain an iron -rich core like the Earth's. Many scientists think that about 40 million years after the formation of Earth, it collided with a body the size of Mars , throwing crust material into the orbit that formed the Moon. Another hypothesis
9828-461: The development of a multiphenotypical population of pre-cells , i.e. evolving entities of primordial life with different characteristics and widespread horizontal gene transfer . From this pre-cell population the founder groups A, B, C and then, from them, the precursor cells (here named proto-cells) of the three domains of life arose successively, leading first to the domain Bacteria , then to
9954-451: The dominant form of life in the early Archean eon, and many of the major steps in early evolution are thought to have taken place in this environment. The evolution of photosynthesis by cyanobacteria , around 3.5 Ga, eventually led to a buildup of its waste product, oxygen , in the oceans. After free oxygen saturated all available reductant substances on the Earth's surface , it built up in
10080-746: The earliest terrestrial ecosystems at least 2.7 Ga, the evolution of plants from freshwater green algae dates back to about 1 billion years ago. Microorganisms are thought to have paved the way for the inception of land plants in the Ordovician period. Land plants were so successful that they are thought to have contributed to the Late Devonian extinction event as early tree Archaeopteris drew down CO 2 levels, leading to global cooling and lowered sea levels, while their roots increased rock weathering and nutrient run-offs which may have triggered algal bloom anoxic events . Bilateria , animals having
10206-451: The earliest cells may have been aided by similar processes. A similar hypothesis presents self-replicating iron-rich clays as the progenitors of nucleotides , lipids and amino acids . A series of experiments starting in 1997 showed that early stages in the formation of proteins from inorganic materials including carbon monoxide and hydrogen sulfide could be achieved by using iron sulfide and nickel sulfide as catalysts . Most of
10332-481: The element directly below carbon on the periodic table , does not form very many complex stable molecules, and because most of its compounds are water-insoluble and because silicon dioxide is a hard and abrasive solid in contrast to carbon dioxide at temperatures associated with living things, it would be more difficult for organisms to extract. The elements boron and phosphorus have more complex chemistries but suffer from other limitations relative to carbon. Water
10458-498: The external membranes of cells may have been an essential first step. Experiments that simulated the conditions of the early Earth have reported the formation of lipids, and these can spontaneously form liposomes , double-walled "bubbles", and then reproduce themselves. Although they are not intrinsically information-carriers as nucleic acids are, they would be subject to natural selection for longevity and reproduction. Nucleic acids such as RNA might then have formed more easily within
10584-431: The extreme of the ssRNA virus case. Viruses undergo genetic change by several mechanisms. These include a process called antigenic drift where individual bases in the DNA or RNA mutate to other bases. Most of these point mutations are "silent"—they do not change the protein that the gene encodes—but others can confer evolutionary advantages such as resistance to antiviral drugs . Antigenic shift occurs when there
10710-446: The filtered, infectious substance a "virus" and this discovery is considered to be the beginning of virology. The subsequent discovery and partial characterization of bacteriophages by Frederick Twort and Félix d'Herelle further catalyzed the field, and by the early 20th century many viruses had been discovered. In 1926, Thomas Milton Rivers defined viruses as obligate parasites. Viruses were demonstrated to be particles, rather than
10836-484: The form of fossilized microorganisms in hydrothermal vent precipitates from the Nuvvuagittuq Belt , that may have lived as early as 4.28 billion years ago, not long after the oceans formed 4.4 billion years ago, and after the Earth formed 4.54 billion years ago. These earliest fossils, however, may have originated from non-biological processes. Microbial mats of coexisting bacteria and archaea were
10962-399: The form of single-stranded nucleoprotein complexes, through pores called plasmodesmata . Bacteria, like plants, have strong cell walls that a virus must breach to infect the cell. Given that bacterial cell walls are much thinner than plant cell walls due to their much smaller size, some viruses have evolved mechanisms that inject their genome into the bacterial cell across the cell wall, while
11088-467: The geosphere and hydrosphere. This scenario may explain the quasi-random distribution of evolutionarily important features among the three domains and, at the same time, the existence of the most basic biochemical features (genetic code, set of protein amino acids etc.) in all three domains (unity of life), as well as the close relationship between the Archaea and the Eucarya. A scheme of the pre-cell scenario
11214-473: The host cell is achieved by adsorption into the host cell. After adsorption to the host cell surface the virion extrudes a tail-tube structure through a vertex for genome delivery into the host. Replication follows the DNA strand displacement model. DNA-templated transcription is the method of transcription. Capsid proteins polymerize around a lipoprotein vesicle translocated in the cytoplasm by virion assembly factors. Mature virons are released by lysis, which, in
11340-406: The host. At some point, the provirus or prophage may give rise to the active virus, which may lyse the host cells. Enveloped viruses (e.g., HIV) typically are released from the host cell by budding . During this process, the virus acquires its envelope, which is a modified piece of the host's plasma or other, internal membrane. The genetic material within virus particles, and the method by which
11466-517: The infected cell. Cells in which the virus is latent and inactive show few signs of infection and often function normally. This causes persistent infections and the virus is often dormant for many months or years. This is often the case with herpes viruses . Viruses are by far the most abundant biological entities on Earth and they outnumber all the others put together. They infect all types of cellular life including animals, plants, bacteria and fungi . Different types of viruses can infect only
11592-669: The lake - allowing the first prebiotic syntheses on Earth to occur. Microbial mats are multi-layered, multi-species colonies of bacteria and other organisms that are generally only a few millimeters thick, but still contain a wide range of chemical environments, each of which favors a different set of microorganisms. To some extent each mat forms its own food chain , as the by-products of each group of microorganisms generally serve as "food" for adjacent groups. Stromatolites are stubby pillars built as microorganisms in mats slowly migrate upwards to avoid being smothered by sediment deposited on them by water. There has been vigorous debate about
11718-420: The liposomes than outside. RNA is complex and there are doubts about whether it can be produced non-biologically in the wild. Some clays , notably montmorillonite , have properties that make them plausible accelerators for the emergence of an RNA world: they grow by self-replication of their crystalline pattern; they are subject to an analogue of natural selection, as the clay "species" that grows fastest in
11844-416: The material is replicated, varies considerably between different types of viruses. The range of structural and biochemical effects that viruses have on the host cell is extensive. These are called ' cytopathic effects '. Most virus infections eventually result in the death of the host cell. The causes of death include cell lysis, alterations to the cell's surface membrane and apoptosis . Often cell death
11970-406: The meteors were then accumulated in geothermal springs. Geothermal springs can accumulate aqueous phosphate in the form of phopshoric acid . Based on lab-run models, these concentrations of phoshate are insufficient to facilitate biosynthesis . As for the evolutionary implications, freshwater heterotrophic cells that depended upon synthesized organic compounds later evolved photosynthesis because of
12096-551: The most credible sources are Mars and Venus ; by alien visitors , possibly as a result of accidental contamination by microorganisms that they brought with them; and from outside the Solar System but by natural means. Experiments in low Earth orbit, such as EXOSTACK , have demonstrated that some microorganism spores can survive the shock of being catapulted into space and some can survive exposure to outer space radiation for at least 5.7 years. Meteorite ALH84001 , which
12222-416: The most numerous type of biological entity. Since Dmitri Ivanovsky 's 1892 article describing a non-bacterial pathogen infecting tobacco plants and the discovery of the tobacco mosaic virus by Martinus Beijerinck in 1898, more than 11,000 of the millions of virus species have been described in detail. The study of viruses is known as virology , a subspeciality of microbiology . When infected,
12348-601: The origin of life since it is a critical component of nucleotides , phospholipids , and adenosine triphosphate . Phosphate is often depleted in natural environments due to its uptake by microbes and its affinity for calcium ions. In a process called ' apatite precipitation', free phosphate ions react with the calcium ions abundant in water to precipitate out of solution as apatite minerals. When attempting to simulate prebiotic phosphorylation , scientists have only found success when using phosphorus levels far above modern day natural concentrations. This problem of low phosphate
12474-522: The origin of life. Similar to the process predicted by geothermal hot spring hypotheses , changing lake levels and wave action deposited phosphorus-rich brine onto dry shore and marginal pools. This drying of the solution promotes polymerization reactions and removes enough water to promote phosphorylation, a process integral to biological energy storage and transfer. When washed away by further precipitation and wave action, researchers concluded these newly formed biomolecules may have washed back into
12600-461: The original virus. Their life cycle differs greatly between species, but there are six basic stages in their life cycle: Attachment is a specific binding between viral capsid proteins and specific receptors on the host cellular surface. This specificity determines the host range and type of host cell of a virus. For example, HIV infects a limited range of human leucocytes . This is because its surface protein, gp120 , specifically interacts with
12726-445: The oxygenation of the atmosphere was a prerequisite for the evolution of the most complex eukaryotic cells, from which all multicellular organisms are built. The boundary between oxygen-rich and oxygen-free layers in microbial mats would have moved upwards when photosynthesis shut down overnight, and then downwards as it resumed on the next day. This would have created selection pressure for organisms in this intermediate zone to acquire
12852-416: The polymerase during genome replication. This process appears to be an adaptation for coping with genome damage. Viral populations do not grow through cell division, because they are acellular. Instead, they use the machinery and metabolism of a host cell to produce multiple copies of themselves, and they assemble in the cell. When infected, the host cell is forced to rapidly produce thousands of copies of
12978-537: The presence of life 3.8 Ga. However, these analyses were closely scrutinized, and non-biological processes were found which could produce all of the "signatures of life" that had been reported. While this does not prove that the structures found had a non-biological origin, they cannot be taken as clear evidence for the presence of life. Geochemical signatures from rocks deposited 3.4 Ga have been interpreted as evidence for life. Evidence for fossilized microorganisms considered to be 3.77 billion to 4.28 billion years old
13104-463: The primary method of reproduction for the vast majority of macroscopic organisms, including almost all eukaryotes (which includes animals and plants ), is sexual reproduction , the fusion of male and female reproductive cells ( gametes ) to create a zygote . The origin and evolution of sexual reproduction remain a puzzle for biologists, though it is thought to have evolved from a single-celled eukaryotic ancestor. While microorganisms formed
13230-486: The process of evolution from a common ancestor . The earliest clear evidence of life comes from biogenic carbon signatures and stromatolite fossils discovered in 3.7 billion-year-old metasedimentary rocks from western Greenland . In 2015, possible "remains of biotic life " were found in 4.1 billion-year-old rocks in Western Australia . There is further evidence of possibly the oldest forms of life in
13356-960: The ranks of subrealm, subkingdom, and subclass are unused, whereas all other ranks are in use. The Nobel Prize-winning biologist David Baltimore devised the Baltimore classification system. The ICTV classification system is used in conjunction with the Baltimore classification system in modern virus classification. The Baltimore classification of viruses is based on the mechanism of mRNA production. Viruses must generate mRNAs from their genomes to produce proteins and replicate themselves, but different mechanisms are used to achieve this in each virus family. Viral genomes may be single-stranded (ss) or double-stranded (ds), RNA or DNA, and may or may not use reverse transcriptase (RT). In addition, ssRNA viruses may be either sense (+) or antisense (−). This classification places viruses into seven groups: Examples of common human diseases caused by viruses include
13482-575: The recovery from this catastrophe, archosaurs became the most abundant land vertebrates; one archosaur group, the dinosaurs , dominated the Jurassic and Cretaceous periods. After the Cretaceous–Paleogene extinction event 66 Ma killed off the non-avian dinosaurs, mammals increased rapidly in size and diversity . Such mass extinctions may have accelerated evolution by providing opportunities for new groups of organisms to diversify. Only
13608-422: The regressive hypothesis did not explain 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 contravened the definition of viruses in that they require host cells. Viruses are now recognised as ancient and as having origins that pre-date the divergence of life into
13734-479: The result of spread to an animal or human host where the virus had not been identified before. It can be an emergent virus , one that represents a new virus, but it can also be an extant virus that has not been previously identified . The SARS-CoV-2 coronavirus that caused the COVID-19 pandemic is an example of a novel virus. Classification seeks to describe the diversity of viruses by naming and grouping them on
13860-417: The rock on the surface of the Earth at rates far greater than today. With high phosphate influx, no phosphate precipitation, and no microbial usage of phosphate at this time, models show phosphate reached concentrations approximately 100 times greater than they are today. Modeled pH and phosphate levels of early Earth carbonate-rich lakes nearly match the conditions used in current laboratory experiments on
13986-438: The rocks less than 2.2 Ga and prove nothing about the origins of eukaryotes. Fossils of the algae Grypania have been reported in 1.85 billion-year-old rocks (originally dated to 2.1 Ga but later revised ), indicating that eukaryotes with organelles had already evolved. A diverse collection of fossil algae were found in rocks dated between 1.5 and 1.4 Ga. The earliest known fossils of fungi date from 1.43 Ga. Plastids ,
14112-451: The same virion for the virus to be infectious, as demonstrated by brome mosaic virus and several other plant viruses. A viral genome, irrespective of nucleic acid type, is almost always either single-stranded (ss) or double-stranded (ds). Single-stranded genomes consist of an unpaired nucleic acid, analogous to one-half of a ladder split down the middle. Double-stranded genomes consist of two complementary paired nucleic acids, analogous to
14238-437: The sixth century BCE . In the twentieth century it was proposed by the physical chemist Svante Arrhenius , by the astronomers Fred Hoyle and Chandra Wickramasinghe , and by molecular biologist Francis Crick and chemist Leslie Orgel . There are three main versions of the "seeded from elsewhere" hypothesis: from elsewhere in our Solar System via fragments knocked into space by a large meteor impact, in which case
14364-423: The steps required temperatures of about 100 °C (212 °F) and moderate pressures, although one stage required 250 °C (482 °F) and a pressure equivalent to that found under 7 kilometres (4.3 mi) of rock. Hence it was suggested that self-sustaining synthesis of proteins could have occurred near hydrothermal vents. In this scenario, the biochemical evolution of life led to diversification through
14490-399: The structure-mediated self-assembly of the virus particles, some modification of the proteins often occurs. In viruses such as HIV, this modification (sometimes called maturation) occurs after the virus has been released from the host cell. Release – Viruses can be released from the host cell by lysis , a process that kills the cell by bursting its membrane and cell wall if present: this
14616-457: The study of the origin of life , as it lends further credence to the hypothesis that life could have started as self-assembling organic molecules . The virocell model first proposed by Patrick Forterre considers the infected cell to be the "living form" of viruses and that virus particles (virions) are analogous to spores . Although the living versus non-living debate continues, the virocell model has gained some acceptance. Viruses display
14742-415: The superclass of organelles of which chloroplasts are the best-known exemplar, are thought to have originated from endosymbiotic cyanobacteria. The symbiosis evolved around 1.5 Ga and enabled eukaryotes to carry out oxygenic photosynthesis . Three evolutionary lineages of photosynthetic plastids have since emerged: chloroplasts in green algae and plants, rhodoplasts in red algae and cyanelles in
14868-407: The synthesis of biomolecules like RNA nucleotides. An analysis of hydrothermal veins at a 3.5 Gya (giga years ago or 1 billion years) geothermal spring setting were found to have elements required for the origin of life, which are potassium, boron, hydrogen, sulfur, phosphorus, zinc, nitrogen, and oxygen. Mulkidjanian and colleagues find that such environments have identical ionic concentrations to
14994-400: The validity of alleged stromatolite fossils from before 3 Ga, with critics arguing that they could have been formed by non-biological processes. In 2006, another find of stromatolites was reported from the same part of Australia, in rocks dated to 3.5 Ga. In modern underwater mats the top layer often consists of photosynthesizing cyanobacteria which create an oxygen-rich environment, while
15120-446: The viral messenger RNA (mRNA). Positive-sense viral RNA is in the same sense as viral mRNA and thus at least a part of it can be immediately translated by the host cell. Negative-sense viral RNA is complementary to mRNA and thus must be converted to positive-sense RNA by an RNA-dependent RNA polymerase before translation. DNA nomenclature for viruses with genomic ssDNA is similar to RNA nomenclature, in that positive-strand viral ssDNA
15246-824: The viral capsid remains outside. Uncoating is a process in which the viral capsid is removed: This may be by degradation by viral enzymes or host enzymes or by simple dissociation; the end-result is the releasing of the viral genomic nucleic acid. Replication of viruses involves primarily multiplication of the genome. Replication involves the synthesis of viral messenger RNA (mRNA) from "early" genes (with exceptions for positive-sense RNA viruses), viral protein synthesis , possible assembly of viral proteins, then viral genome replication mediated by early or regulatory protein expression. This may be followed, for complex viruses with larger genomes, by one or more further rounds of mRNA synthesis: "late" gene expression is, in general, of structural or virion proteins. Assembly – Following
15372-503: The virus to enter. Penetration or viral entry follows attachment: Virions enter the host cell through receptor-mediated endocytosis or membrane fusion . The infection of plant and fungal cells is different from that of animal cells. Plants have a rigid cell wall made of cellulose , and fungi one of chitin, so most viruses can get inside these cells only after trauma to the cell wall. Nearly all plant viruses (such as tobacco mosaic virus) can also move directly from cell to cell, in
15498-460: The virus useless or uncompetitive. To compensate, RNA viruses often have segmented genomes—the genome is split into smaller molecules—thus reducing the chance that an error in a single-component genome will incapacitate the entire genome. In contrast, DNA viruses generally have larger genomes because of the high fidelity of their replication enzymes. Single-strand DNA viruses are an exception to this rule, as mutation rates for these genomes can approach
15624-547: Was found in the Nuvvuagittuq Greenstone Belt in Quebec, Canada, although the evidence is disputed as inconclusive. Some biologists reason that all living organisms on Earth must share a single last universal ancestor , because it would be virtually impossible that two or more separate lineages could have independently developed the many complex biochemical mechanisms common to all living organisms. According to
15750-491: Was once part of the Martian crust, shows evidence of carbonate-globules with texture and size indicative of terrestrial bacterial activity. Scientists are divided over the likelihood of life arising independently on Mars, or on other planets in our galaxy . One theory traces the origins of life to the abundant carbonate-rich lakes which would have dotted the early Earth . Phosphate would have been an essential cornerstone to
15876-411: Was proposed that porous metal sulfide precipitates would assist RNA synthesis at about 100 °C (212 °F) and ocean-bottom pressures near hydrothermal vents . Under this hypothesis, lipid membranes would be the last major cell components to appear and, until then, the protocells would be confined to the pores. It has been suggested that double-walled "bubbles" of lipids like those that form
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