Parts viruses

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. Chemical reactions in your cells constantly change molecules into forms of energy we can use.

The energy you use to run and jump came from breaking big food molecules into smaller pieces that can be used or stored in the cell. Viruses are too small and simple to collect or use their own energy — they just steal it from the cells they infect. Viruses only need energy when they make copies of themselves, and they don't need any energy at all when they are outside of a cell. Finally, living things maintain homeostasis , meaning keeping conditions inside the body stable.

Your body sweats to cool you down and shivers to warm you up if its temperature changes from Millions of adjustments throughout the day keep your temperature and the chemicals in your body balanced.

Viruses have no way to control their internal environment and they do not maintain their own homeostasis. So, since viruses cannot reproduce on their own and have no metabolism or homeostasis, they are usually not thought of as truly alive.

They do have a huge effect on living things during infections, though! What do you think? Should viruses be included with other living things? After you decide why you think they should or should not be considered alive, listen to biochemist Nick Lane and Dr.

Biology discuss if they think viruses are alive. By volunteering, or simply sending us feedback on the site. Scientists, teachers, writers, illustrators, and translators are all important to the program. If you are interested in helping with the website we have a Volunteers page to get the process started. Digging Deeper. Digging Deeper: Depression and the Past. Digging Deeper: Germs and Disease. Digging Deeper: Milk and Immunity.

Capsid: a protective shell around the genome of a virus. Homeostasis: the ability to keep a system at a constant condition. View Citation You may need to edit author's name to meet the style formats, which are in most cases "Last name, First name. In general, the shapes of viruses are classified into four groups: filamentous, isometric or icosahedral , enveloped, and head and tail.

Filamentous viruses are long and cylindrical. Many plant viruses are filamentous, including TMV tobacco mosaic virus. Isometric viruses have shapes that are roughly spherical, such as poliovirus or herpesviruses. Enveloped viruses have membranes surrounding capsids. Animal viruses, such as HIV, are frequently enveloped. Head and tail viruses infect bacteria. They have a head that is similar to icosahedral viruses and a tail shape like filamentous viruses.

Many viruses use some sort of glycoprotein to attach to their host cells via molecules on the cell called viral receptors. For these viruses, attachment is a requirement for later penetration of the cell membrane, allowing them to complete their replication inside the cell. The receptors that viruses use are molecules that are normally found on cell surfaces and have their own physiological functions. Viruses have simply evolved to make use of these molecules for their own replication.

Overall, the shape of the virion and the presence or absence of an envelope tell us little about what disease the virus may cause or what species it might infect, but they are still useful means to begin viral classification. Among the most complex virions known, the T4 bacteriophage, which infects the Escherichia coli bacterium, has a tail structure that the virus uses to attach to host cells and a head structure that houses its DNA.

Adenovirus, a non-enveloped animal virus that causes respiratory illnesses in humans, uses glycoprotein spikes protruding from its capsomeres to attach to host cells.

Non-enveloped viruses also include those that cause polio poliovirus , plantar warts papillomavirus , and hepatitis A hepatitis A virus.

Examples of virus shapes : Viruses can be either complex in shape or relatively simple. This figure shows three relatively-complex virions: the bacteriophage T4, with its DNA-containing head group and tail fibers that attach to host cells; adenovirus, which uses spikes from its capsid to bind to host cells; and HIV, which uses glycoproteins embedded in its envelope to bind to host cells.

Enveloped virions like HIV consist of nucleic acid and capsid proteins surrounded by a phospholipid bilayer envelope and its associated proteins. Glycoproteins embedded in the viral envelope are used to attach to host cells. Other envelope proteins include the matrix proteins that stabilize the envelope and often play a role in the assembly of progeny virions. Chicken pox, influenza, and mumps are examples of diseases caused by viruses with envelopes.

Because of the fragility of the envelope, non-enveloped viruses are more resistant to changes in temperature, pH, and some disinfectants than are enveloped viruses. The virus core contains the genome or total genetic content of the virus. Viral genomes tend to be small, containing only those genes that encode proteins that the virus cannot obtain from the host cell.

This genetic material may be single- or double-stranded. Since viral glycoproteins are one of the key ways viruses can infect cells, many scientists are working on medicines that can impact how the glycoproteins work in order to prevent viral illnesses in people, pets, and plants.

In addition to being varied in their shapes and sizes, viruses also demonstrate diversity when it comes to their nucleic acid genomes.

The primary function of a viral genome is to store the instructions for building more virus particles. Regardless of which type of genome a virus has, there are two main routes for packing it: viruses can either assemble their capsid shell around their nuclear genome, or viruses can make a capsid shell, and insert their nuclear genome into it.

Viruses also need to make sure that they are packaging their genomes, and not the genomes of their host cells. Because there are millions of different viruses, there are millions of different viral genomes. So far, scientists have mapped the genomes of 75, viruses, but that is merely a fraction of what is out there.

As next generation sequencing and analysis continues to grow in its sophistication, scientists will continue building knowledge when it comes to viral genomes! Gelderblom, H. Structure and classification of viruses. Baron Ed. University of Texas Medical Branch. Holmes, E. What does virus evolution tell us about virus origins? Journal of Virology, 85 , Knipe, D.