Types of symmetry of virus particles

The genome and capsid of a virus are collectively called virus nucleo-capsid. Functions of Capsid : Capsid has three important functions: a. It protects the viral genome from getting digested by enzymes. It bears special sites on its exterior that enable the virion to connect to a host cell. It also produces proteins that facilitate the virion to insert the host cell membrane and, in some cases, to inject the infectious viral genome into the cytoplasm of the cell.

Envelope is the structure that surrounds the Nucleocapsid of a virus. Viruses without an envelope are called naked viruses. It is basically a hybrid structure made up of lipids derived from host cells and proteins derived from viruses.

The glycoprotein of the envelope projects out as spikes in some viral structures. These spikes are called peplomers and help in binding the virus to the host cell.

Functions of Envelope : a. Like capsid, the envelope also helps in viral genome protection, attachment and penetration to the host cell. Glycoproteins found on the envelope surface help to recognise and bind to receptor sites on the host cell membrane. In some cases, they possess spikes made of glycoprotein that help them to attach to specific cell surfaces. Viruses with envelopes are adapted for changes and may cause constant infections. Some species of virus contain various enzymes that help them to catalyze various processes.

Reverse transcriptase, lysozyme, viral protease, and integrase are some examples of enzymes found in certain species of viruses. Functions of Enzymes : a.

Some species of virus contain a special enzyme that helps them to infect the host. For example, a bacteriophage has lysozyme enzyme that helps to make a hole in a bacterial cell for entry of the viral genome. A few viruses contain their own enzyme called nucleic acid polymerase that transcribes the viral genetic material into mRNA during the replication process.

Practice Exam Questions. The arrangement of capsomeres in the capsid of a virus is called the symmetry of the virus. There are mainly three types of symmetry observed in viruses that resemble two primary shapes that are rod shaped and spherical shaped. The third one is complex symmetry seen in virus-like phages. Let us learn these structures with an example. Rod Shaped a. Helical Symmetry 2. Spherical Shaped a. Cubicle or Icosahedral Symmetry i. Pentagonal ii. Hexagonal 3. Complex Symmetry.

In this type of symmetry, the capsomere in the capsid and viral genome are woven together to form a helical or spiral tube-like structure. Helical symmetry: The nucleic acid and the capsomeres arewound together to form a spherical or spiral tube. The viruses with helical structure usually appear as rods and their capso-meres self-assemble on the RNA genome into rods extending to the length of the genome. These capsomeres cover and protect the RNA. The tubular nucleocapsid structure may be rigid as in tobacco mosaic virus, but may be pliable and may be coiled on itself in case of some other animal viruses.

Helical nucleo-capsids are usually demonstrated within the envelope of most negative-stranded RNA viruses. Complex symmetry: Some viruses may not exhibit either ico-sahedral or helical symmetry but instead may exhibit a complex symmetry. For example, poxvirus shows a complex symmetry. Most of the enveloped viruses are round or pleomorphic with exception of poxvirus and rhabdovirus.

Rhabdovirus is a bullet-shaped virus, whereas poxvirus is brick shaped. Developed by Therithal info, Chennai. Toggle navigation BrainKart. The virion consists of a nucleic acid core, the genome, surrounded by a protein coat, the capsid Fig. Related Topics Chlamydophila. Chlamydophila pneumonia. Chlamydophila psittaci.

General Properties of Viruses. Morphology of Viruses. Structure and Symmetry of Virus. Susceptibility to Physical and Chemical Agents of Virus. Replication of Viruses. Viral Genetics. Nomenclature and Taxonomy of Viruses. Stages of Viral Infections. Viral Pathogenesis at the Cellular Level. In addition, the RNA strand of a single-stranded genome may be either a sense strand plus strand , which can function as messenger RNA mRNA , or an antisense strand minus strand , which is complementary to the sense strand and cannot function as mRNA protein translation see Ch.

Sense viral RNA alone can replicate if injected into cells, since it can function as mRNA and initiate translation of virus-encoded proteins. Antisense RNA, on the other hand, has no translational function and cannot per se produce viral components. Schemes of 21 virus families infecting humans showing a number of distinctive criteria: presence of an envelope or double- capsid and internal nucleic acid genome.

DsRNA viruses, e. Each segment consists of a complementary sense and antisense strand that is hydrogen bonded into a linear ds molecule. The replication of these viruses is complex; only the sense RNA strands are released from the infecting virion to initiate replication. The retrovirus genome comprises two identical, plus-sense ssRNA molecules, each monomer 7—11 kb in size, that are noncovalently linked over a short terminal region.

Retroviruses contain 2 envelope proteins encoded by the env-gene, 4—6 nonglycosylated core proteins and 3 non-structural functional proteins reverse transcriptase, integrase, protease: RT, IN, PR specified by the gag-gene Fig. This DNA, mediated by the viral integrase, becomes covalently bonded into the DNA of the host cell to make possible the subsequent transcription of the sense strands that eventually give rise to retrovirus progeny.

After assembly and budding, retroviruses show structural and functional maturation. In immature virions the structural proteins of the core are present as a large precursor protein shell. After proteolytic processing by the viral protease the proteins of the mature virion are rearranged and form the dense isometric or cone-shaped core typical of the mature virion, and the particle becomes infectious.

Most DNA viruses Fig. The papovaviruses, comprising the polyoma- and papillomaviruses, however, have circular DNA genomes, about 5. Three or 2 structural proteins make up the papovavirus capsid: in addition, nonstructural proteins are encoded that are functional in virus transcription, DNA replication and cell transformation.

Single-stranded linear DNA, 4—6 kb in size, is found with the members of the Parvovirus family that comprises the parvo-, the erythro- and the dependoviruses.

The virion contains 2—4 structural protein species which are differently derived from the same gene product see Ch. The adeno-associated virus AAV, a dependovirus is incapable of producing progeny virions except in the presence of helper viruses adenovirus or herpesvirus. It is therefore said to be replication defective.

Circular single-stranded DNA of only 1. The isometric capsid measures 17 nm and is composed of 2 protein species only. On the basis of shared properties viruses are grouped at different hierarchical levels of order, family, subfamily, genus and species. More than 30, different virus isolates are known today and grouped in more than 3, species, in genera and 71 families. Viral morphology provides the basis for grouping viruses into families.

A virus family may consist of members that replicate only in vertebrates, only in invertebrates, only in plants, or only in bacteria. Certain families contain viruses that replicate in more than one of these hosts. This section concerns only the 21 families and genera of medical importance. Besides physical properties, several factors pertaining to the mode of replication play a role in classification: the configuration of the nucleic acid ss or ds, linear or circular , whether the genome consists of one molecule of nucleic acid or is segmented, and whether the strand of ss RNA is sense or antisense.

Also considered in classification is the site of viral capsid assembly and, in enveloped viruses, the site of nucleocapsid envelopment. Table lists the major chemical and morphologic properties of the families of viruses that cause disease in humans. The use of Latinized names ending in -viridae for virus families and ending in -virus for viral genera has gained wide acceptance. The names of subfamilies end in -virinae. Vernacular names continue to be used to describe the viruses within a genus.

In this text, Latinized endings for families and subfamilies usually are not used. Table shows the current classification of medically significant viruses. In the early days of virology, viruses were named according to common pathogenic properties, e. From the early s until the mids, when many new viruses were being discovered, it was popular to compose virus names by using sigla abbreviations derived from a few or initial letters.

Thus the name Picornaviridae is derived from pico small and RNA; the name Reoviridae is derived from respiratory, enteric, and orphan viruses because the agents were found in both respiratory and enteric specimens and were not related to other classified viruses; Papovaviridae is from papilloma, polyoma, and vacuolating agent simian virus 40 [SV40] ; retrovirus is from reverse transcriptase; Hepadnaviridae is from the replication of the virus in hepatocytes and their DNA genomes, as seen in hepatitis B virus.

Hepatitis A virus is classified now in the family Picornaviridae, genus Hepatovirus. Although the current rules for nomenclature do not prohibit the introduction of new sigla, they require that the siglum be meaningful to workers in the field and be recognized by international study groups. Several viruses of medical importance still remain unclassified. Some are difficult or impossible to propagate in standard laboratory host systems and thus cannot be obtained in sufficient quantity to permit more precise characterization.

Hepatitis E virus, the Norwalk virus and similar agents see Ch. The fatal transmissible dementias in humans and other animals scrapie in sheep and goat; bovine spongiform encephalopathy in cattle, transmissible mink encephalopathy; Kuru, Creutzfeldt-Jakob disease, and Gerstmann-Straussler-Scheinker syndrome in humans see Ch.

The agents causing transmissible subacute spongiform encephalopathies have been linked to viroids or virinos i. Some of the transmissible amyloidoses show a familial pattern and can be explained by defined mutations which render a primary soluble glycoprotein insoluble, which in turn leads to the pathognomonic accumulation of amyloid fibers and plaques. The pathogenesis of the sporadic amyloidoses, however, is still a matter of highly ambitious research.

Turn recording back on. National Center for Biotechnology Information , U. Show details Baron S, editor. Search term. 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.

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. 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.

Structure and Function Viruses are inert outside the host cell. Classification of Viruses Viruses are classified on the basis of morphology, chemical composition, and mode of replication. 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.

Figure The helical structure of the rigid tobacco mosaic virus rod. 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.