Virus: Structure and Symmetry -
A capsid is the protein shell of a virus. It consists of several oligomeric structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or may not correspond to individual proteins, are called capsomeres. .. Edit links. This page was last edited on 26 November , at (UTC). The number and arrangement of the capsomeres (morphologic subunits of the Also considered in viral classification is the site of capsid assembly and. The capsomere is a subunit of the capsid, an outer covering of protein that protects the genetic Edit links. This page was last edited on 5 November , at (UTC). Text is available under the Creative Commons Attribution- ShareAlike.
General Concepts Structure and Function Viruses are small obligate intracellular parasites, which by definition contain either a RNA or DNA genome surrounded by a protective, virus-coded protein coat. Viruses may be viewed as mobile genetic elements, most probably of cellular origin and characterized by a long co-evolution of virus and host.
For propagation viruses depend on specialized host cells supplying the complex metabolic and biosynthetic machinery of eukaryotic or prokaryotic cells. A complete virus particle is called a virion. The main function of the virion is to deliver its DNA or RNA genome into the host cell so that the genome can be expressed transcribed and translated by the host cell.
The viral genome, often with associated basic proteins, is packaged inside a symmetric protein capsid. The nucleic acid-associated protein, called nucleoprotein, together with the genome, forms the nucleocapsid. In enveloped viruses, the nucleocapsid is surrounded by a lipid bilayer derived from the modified host cell membrane and studded with an outer layer of virus envelope glycoproteins.
Classification of Viruses Morphology: Viruses are grouped on the basis of size and shape, chemical composition and structure of the genome, and mode of replication.
Helical morphology is seen in nucleocapsids of many filamentous and pleomorphic viruses.
Helical nucleocapsids consist of a helical array of capsid proteins protomers wrapped around a helical filament of nucleic acid. The number and arrangement of the capsomeres morphologic subunits of the icosahedron are useful in identification and classification. Many viruses also have an outer envelope. Chemical Composition and Mode of Replication: The genome of a virus may consist of DNA or RNA, which may be single stranded ss or double stranded dslinear or circular.
The entire genome may occupy either one nucleic acid molecule monopartite genome or several nucleic acid segments multipartite genome. The different types of genome necessitate different replication strategies. Nomenclature Aside from physical data, genome structure and mode of replication are criteria applied in the classification and nomenclature of viruses, including the chemical composition and configuration of the nucleic acid, whether the genome is monopartite or multipartite.
Also considered in viral classification is the site of capsid assembly and, in enveloped viruses, the site of envelopment. Structure and Function Viruses are inert outside the host cell. Viruses are unable to generate energy.Virus structure and classification - Cells - MCAT - Khan Academy
As obligate intracellular parasites, during replication, they fully depend on the complicated biochemical machinery of eukaryotic or prokaryotic cells. The main purpose of a virus is to deliver its genome into the host cell to allow its expression transcription and translation by the host cell. A fully assembled infectious virus is called a virion. The simplest virions consist of two basic components: Capsid proteins are coded for by the virus genome. Because of its limited size Table the genome codes for only a few structural proteins besides non-structural regulatory proteins involved in virus replication.
Capsids are formed as single or double protein shells and consist of only one or a few structural protein species. Therefore, multiple protein copies must self assemble to form the continuous three-dimensional capsid structure. Self assembly of virus capsids follows two basic patterns: Some virus families have an additional covering, called the envelope, which is usually derived in part from modified host cell membranes.
Viral envelopes consist of a lipid bilayer that closely surrounds a shell of virus-encoded membrane-associated proteins. The exterior of the bilayer is studded with virus-coded, glycosylated trans- membrane proteins. Therefore, enveloped viruses often exhibit a fringe of glycoprotein spikes or knobs, also called peplomers. In viruses that acquire their envelope by budding through the plasma or another intracellular cell membrane, the lipid composition of the viral envelope closely reflects that of the particular host membrane.
The outer capsid and the envelope proteins of viruses are glycosylated and important in determining the host range and antigenic composition of the virion. In addition to virus-specified envelope proteins, budding viruses carry also certain host cell proteins as integral constituents of the viral envelope.
Virus envelopes can be considered an additional protective coat. Larger viruses often have a complex architecture consisting of both helical and isometric symmetries confined to different structural components. Classification of Viruses Viruses are classified on the basis of morphology, chemical composition, and mode of replication. The viruses that infect humans are currently grouped into 21 families, reflecting only a small part of the spectrum of the multitude of different viruses whose host ranges extend from vertebrates to protozoa and from plants and fungi to bacteria.
Morphology Helical Symmetry In the replication of viruses with helical symmetry, identical protein subunits protomers self-assemble into a helical array surrounding the nucleic acid, which follows a similar spiral path.
Such nucleocapsids form rigid, highly elongated rods or flexible filaments; in either case, details of the capsid structure are often discernible by electron microscopy.
In addition to classification as flexible or rigid and as naked or enveloped, helical nucleocapsids are characterized by length, width, pitch of the helix, and number of protomers per helical turn. The most extensively studied helical virus is tobacco mosaic virus Fig. Many important structural features of this plant virus have been detected by x-ray diffraction studies.
Figure shows Sendai virus, an enveloped virus with helical nucleocapsid symmetry, a member of the paramyxovirus family see Ch.
Capsid - Wikipedia
Figure The helical structure of the rigid tobacco mosaic virus rod. About 5 percent of the length of the virion is depicted. Individual 17,Da protein subunits protomers assemble in a helix with an axial repeat of 6.
Figure Fragments of flexible helical nucleocapsids NC of Sendai virus, a paramyxovirus, are seen either within the protective envelope E or free, after rupture of the envelope. The intact nucleocapsid is about 1, nm long and 17 nm in diameter; its pitch more Icosahedral Symmetry An icosahedron is a polyhedron having 20 equilateral triangular faces and 12 vertices Fig.
Lines through opposite vertices define axes of fivefold rotational symmetry: Lines through the centers of opposite triangular faces form axes of threefold rotational symmetry; twofold rotational symmetry axes are formed by lines through midpoints of opposite edges.
Virus: Structure and Symmetry
An icosaheron polyhedral or spherical with fivefold, threefold, and twofold axes of rotational symmetry Fig. Figure Icosahedral models seen, left to right, on fivefold, threefold, and twofold axes of rotational symmetry. These axes are perpendicular to the plane of the page and pass through the centers of each figure. Both polyhedral upper and spherical lower forms more Viruses were first found to have symmetry by x-ray diffraction studies and subsequently by electron microscopy with negative-staining techniques.
In most icosahedral viruses, the protomers, i. The arrangement of capsomeres into an icosahedral shell compare Fig. This requires the identification of the nearest pair of vertex capsomeres called penton: Figure Adenovirus after negative stain electron microscopy. A The capsid reveals the typical isometric shell made up from 20 equilateral triangular faces. The net axes are formed by lines of the closest-packed neighboring capsomeres.
In adenoviruses, the h and k axes also coincide with the edges of the triangular faces. The capsomere number C can be determined to be from the h and k indices and the equation: Viral Structure Viruses are composed of two fundamental units, a protein coat and a nucleic acid core. The protein coat is termed a capsid and is composed of subunits called capsomeres.
The nucleic acid may be either RNA or DNA and can be either single-stranded, double-stranded, or partially double-stranded. Some viruses contain lipid coverings which are termed envelopes. In most cases, these are aquired from cellular membranes nuclear, golgi, plasma when the mature viruses bud from the cell. In at least once case, poxvirues, the virus codes for enzymes that make some of the lipids. Viruses which do not have membranes are termed "naked".
The protein coat capsid and the nucleic acid together are the nucleocapsid. In the case of an enveloped virus, the virion is the entire virus and the nucleocapsid is the capsid and nucleic acid, minus the envelope.
In the case of a naked virus, the nucleocapsid and virion are the same thing.
The function of the virion is, fundamentally, to ensure that the viral nucleic acid gets delivered from one cell to another cell. This involves many functions, including protection of the genome from nucleolytic enzymes, delivery of the genome, and interactions of the virus with cells.
Virus nucleocapsids come in two basic shapes, helical and icosahedral.