4 classifications of virus

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: those through which the fivefold symmetry axes pass and the distribution of capsomeres between them. 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. This symmetry and number of capsomeres is typical of all members of the adenovirus family.

Except in helical nucleocapsids, little is known about the packaging or organization of the viral genome within the core. Small virions are simple nucleocapsids containing 1 to 2 protein species. The larger viruses contain in a core the nucleic acid genome complexed with basic protein s and protected by a single- or double layered capsid consisting of more than one species of protein or by an envelope Fig.

Two-dimensional diagram of HIV-1 correlating immuno- electron microscopic findings with the recent nomenclature for the structural components in a 2-letter code and with the molecular weights of the virus structural glyco- proteins. SU stands for more Because of the error rate of the enzymes involved in RNA replication, these viruses usually show much higher mutation rates than do the DNA viruses. Mutation rates of 10 -4 lead to the continuous generation of virus variants which show great adaptability to new hosts.

The viral RNA may be single-stranded ss or double-stranded ds , and the genome may occupy a single RNA segment or be distributed on two or more separate segments segmented genomes. 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. Summary Viruses can store their genetic information in six different types of nucleic acid which are named based on how that nucleic acid eventually becomes transcribed to the viral mRNA.

The "polymerase" part of the name refers what type of nucleic acid is being synthesized. Questions Study the material in this section and then write out the answers to these questions. State what criteria are used in viral classification. Contributors and Attributions Dr.

The Invaluable Contribution of Nurses Across a variety of roles and specializations, nursing professionals fight viruses in numerous ways. Slowing the Spread Advanced practice nurses are uniquely qualified to conduct research and assist government leaders and public health officials in creating an informed response to viral outbreaks.

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Who is this brochure for? However, the proportion of influenza B viruses from each lineage that circulate can vary by geographic location and by season. Figure 2 — This image shows how influenza viruses are named. The name starts with the virus type, followed by the place the virus was isolated, followed by the virus strain number often a sample identifier , the year isolated, and finally, the virus subtype.

CDC follows an internationally accepted naming convention for influenza viruses. This convention was accepted by WHO in and published in February in the Bulletin of the World Health Organization, 58 4 see A revision of the system of nomenclature for influenza viruses: a WHO Memorandum pdf icon[ KB, 7 pages]external icon pdf icon external icon. The approach uses the following components:. Getting a flu vaccine can protect against these viruses as well as additional flu viruses that are antigenically similar to the viruses used to make the vaccine.

Seasonal flu vaccines do not protect against influenza C or D viruses or against zoonotic animal-origin flu viruses that can cause human infections, such as variant or avian flu viruses.

In addition, flu vaccines will NOT protect against infection and illness caused by other viruses that also can cause influenza-like symptoms. There are many other viruses besides influenza that can result in influenza-like illness ILI that spread during flu season.

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