length of a helical capsid is related to the length of the nucleic acid contained within it, and the diameter is dependent on the size and arrangement of capsomeres. the well-studied tobacco mosaic virus and inovirus are examples of helical viruses. icosahedralmost animal viruses are icosahedral or near-spherical with chiral icosahedral symmetry. a regular icosahedron is the optimum way of forming a closed shell from identical subunits. the minimum number of capsomeres required for each triangular face is 3, which gives 60 for the icosahedron. many viruses, such as rotavirus, have more than 60 capsomers and appear spherical but they retain this symmetry. to achieve this, the capsomeres at the apices are surrounded by five other capsomeres and are called pentons. capsomeres on the triangular faces are surrounded by six others and are called hexons. hexons are in essence flat and pentons, which form the 12 vertices, are curved. the same protein may act as the subunit of both the pentamers and hexamers or they may be composed of different proteins. prolatethis is an icosahedron elongated along the fivefold axis and is a common arrangement of the heads of bacteriophages. this structure is composed of a cylinder with a cap at either end. envelopedsome species of virus envelop themselves in a modified form of one of the cell membranes, either the outer membrane surrounding an infected host cell or internal membranes such as a nuclear membrane or endoplasmic reticulum, thus gaining an outer lipid bilayer known as a viral envelope. this membrane is studded with proteins coded for by the viral genome and host genome; the lipid membrane itself and any carbohydrates present originate entirely from the host. influenza virus, hiv (which causes aids), and severe acute respiratory syndrome coronavirus 2 (which causes covid-19) use this strategy. most enveloped viruses are dependent on the envelope for their infectivity. complexthese viruses possess a capsid that is neither purely helical nor purely icosahedral, and that may possess extra structures such as protein tails or a complex outer wall. some bacteriophages, such as enterobacteria phage t4, have a complex structure consisting of an icosahedral head bound to a helical tail, which may have a hexagonal base plate with protruding protein tail fibres. this tail structure acts like a molecular syringe, attaching to the bacterial host and then injecting the viral genome into the cell. the poxviruses are large, complex viruses that have an unusual morphology. the viral genome 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. giant viruses mimivirus is one of the largest characterised viruses, with 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 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 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. genome genomic diversity among viruses propertyparameters nucleic acid dna rna both dna and rna (at different stages in the life cycle) shape linear circular segmented strandedness single-stranded (ss) double-stranded (ds) double-stranded with regions of single-strandedness sense positive sense (+) negative sense (−) ambisense (+/−) 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, 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. the vast majority of 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 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 up 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 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 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 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 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 genome size varies greatly between species. the smallest—the ssdna circoviruses, family circoviridae—code for only two proteins and have 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 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 the extreme of the ssrna virus case. genetic mutation and recombination antigenic shift, or reassortment, can result in novel and highly pathogenic strains of human flu 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 is a major change in the genome of the virus. this can be a result of recombination or reassortment. when this happens with influenza viruses, pandemics might result. rna viruses often exist as quasispecies or swarms of viruses of the same species but with slightly different genome nucleoside sequences. such quasispecies are 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 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 the polymerase during genome replication. this process appears to be an adaptation for coping with genome damage. replication cycle a typical virus replication cycle some bacteriophages inject their genomes into bacterial cells (not to scale) 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 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 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 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 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 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 –