Core virus made up

And like other coronaviruses that infect people, the new coronavirus causes respiratory disease, among other symptoms. The single-stranded RNA acts as a molecular message that enables production of proteins needed for other elements of the virus. Bound to this string of RNA are nucleoproteins — dark blue discs —proteins that help give the virus its structure and enable it to replicate.

Encapsulating the RNA genome is the viral envelope teal , which protects the virus when it is outside of a host cell. 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. 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.

What does virus evolution tell us about virus origins? Journal of Virology, 85 , Knipe, D. Fields virology. Woolhouse, M. Human viruses: discovery and emergence. Update your browser to view this website correctly. Update my browser now. The Building Blocks of Viruses It is hard for scientists to know exactly when viruses first emerged, but we do know that viruses originated at least as early as the first cells, around 4 billion years ago.

The Viral Capsid A simple sketch of a virus: nucleic acid genome, surrounded by a protein coat capsid , additionally surrounded by a membrane envelope. References Gelderblom, H. Science Saturday.

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