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.
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. Reassortment can occur when two influenza viruses infect a host at the same time and swap genetic information. This graphic shows the two types of influenza viruses A and B that cause most human illness and that are responsible for flu seasons each year.
Both influenza A and B viruses can be further classified into clades and sub-clades which are sometimes called groups and sub-groups. Note that this graphic is an example, and currently circulating influenza clades and subclades may differ from those presented here.
Figure 1 — This is a picture of a phylogenetic tree. Each sequence from a specific influenza virus has its own branch on the tree. The degree of genetic difference between viruses is represented by the length of the horizontal lines branches in the phylogenetic tree. The further apart viruses are on the horizontal axis of a phylogenetic tree, the more genetically different the viruses are to one another. An influenza clade or group is a further subdivision of influenza viruses beyond subtypes or lineages based on the similarity of their HA gene sequences.
See the Genome Sequencing and Genetic Characterization page for more information. Clades and subclades are shown on phylogenetic trees as groups of viruses that usually have similar genetic changes i.
Dividing viruses into clades and subclades allows flu experts to track the proportion of viruses from different clades in circulation. Note that clades and sub-clades that are genetically different from others are not necessarily antigenically different. These proteins act as antigens. Antigens are molecular structures on the surface of viruses that are recognized by the immune system and can trigger an immune response such as antibody production.
This is more common in people with cardiopulmonary disease, people with weakened immune systems, infants, and older adults. In the United States, people usually get infected with common human coronaviruses in the fall and winter, but you can get infected at any time of the year. Young children are most likely to get infected, but people can have multiple infections in their lifetime.
There is no vaccine to protect you against human coronaviruses and there are no specific treatments for illnesses caused by human coronaviruses. Most people with common human coronavirus illness will recover on their own. However, to relieve your symptoms you can:. Sometimes, respiratory secretions are tested to figure out which specific germ is causing your symptoms.
Skip directly to site content Skip directly to page options Skip directly to A-Z link. Antiviral drugs can treat viruses by inhibiting viral development and slowing down disease progression. These drugs help fight the flue, chickenpox and forms of hepatitis. Vaccines create a herd immunity that helps prevent an outbreak. There are five different types of viruses: Conjugate vaccines, inactivated vaccines, live, attenuated vaccines, subunit vaccines and toxoid vaccines.
There are several ways people can slow the spread of a virus in lieu of drugs or vaccination. These include thorough and frequent hand washing, eating a fruit and vegetable-rich diet, using an alcohol-based sanitizer and getting enough sleep each night.
Around the world, nurses contribute to the prevention, management and containment of viral outbreaks by caring for infected patients and educating the public on prevention strategies. Advanced practice nurses also fill a leadership role that involves working with government leaders and advocating for health care equality.
Across a variety of roles and specializations, nursing professionals fight viruses in numerous ways. Some of their methods are direct, such as preventing surgical infections.
Others are legislative in nature, such as advocating for care equality by questioning imbalanced care delivery systems.
Nurses also share their expertise with the public on a host of vital topics, such as care delivery models, infection prevention and the distribution of important resources. Public health nurses were involved in managing the severe acute respiratory syndrome SARS outbreak in They did so by tracing contacts, educating the public regarding disease signs and symptoms and serving in research teams in related case control studies.
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