Complex viruses code for proteins that assist in the construction of their capsid. Proteins associated with nucleic acid are known as nucleoproteins, and the association of viral capsid proteins with viral nucleic acid is called a nucleocapsid.
The capsid and entire virus structure can be mechanically physically probed through atomic force microscopy. Viral diseases have an enormous impact on human health worldwide. Genomic technologies are providing infectious disease researchers an unprecedented capability to study at a genetic level the viruses that cause disease and their interactions with infected hosts.
An enormous variety of genomic structures can be seen among viral species; as a group, they contain more structural genomic diversity than plants, animals, archaea, or bacteria. There are millions of different types of viruses, although only about 5, of them have been described in detail.
Diagram of a virus : The location of the genome inside the virus. The vast majority of viruses have RNA genomes. Viral genomes are circular, as in the polyomaviruses, or linear, as in the adenoviruses.
The type of nucleic acid is irrelevant to the shape of the genome. Among RNA viruses and certain DNA viruses, the genome is often divided up into separate parts, in which case it is called segmented. For RNA viruses, each segment often codes for only one protein, and they are usually found together in one capsid. However, all segments are not required to be in the same virion for the virus to be infectious, as demonstrated by the brome mosaic virus and several other plant viruses.
A viral genome, irrespective of nucleic acid type, is almost always either single-stranded or double-stranded. Single-stranded genomes consist of an unpaired nucleic acid, analogous to one-half of a ladder split down the middle. Double-stranded genomes consist of two complementary paired nucleic acids, analogous to a ladder. The virus particles of some virus families, such as those belonging to the Hepadnaviridae, contain a genome that is partially double-stranded and partially single-stranded.
A host is an organism that harbors a parasite or a mutual or commensal symbiont, typically providing nourishment and shelter. Resistance to and recovery from viral infections depend on the interactions that occur between the virus and the host. The defenses mounted by the host may act directly on the virus or indirectly on virus replication by altering or killing the infected cell. Non-specific host defenses function early in an encounter with a virus to prevent or limit infection, while the specific host defenses function after infection in recovery to provide immunity for subsequent challenges.
The host defense mechanisms involved in a particular viral infection will vary depending on the virus, dose, and portal of entry. The host has many barriers against infection that are inherent in the organism. These represent the first line of defense, which functions to prevent or limit infection Examples of natural barriers include but are not limited to skin, the expression of surface receptors such as CD4, complement receptors, glycophorin, intercelullar adhesion molecule 1 ICAM-1 , mucus, a ciliated epithelium, low pH, and humoral and cellular components.
In general, mRNAs in eukaryotic cells are translated beginning at the first AUG initiation codon; as virtually all viruses encode more than one protein, RNA viruses must possess a mechanism for production of multiple proteins from the RNA genome. Different viruses have a wide variety of solutions to this problem. Both hepatitis C virus a flavivirus and poliovirus a picornavirus use this mechanism. For example, the alphaviruses e.
In a somewhat analogous strategy, coronaviruses produce a nested series of mRNAs encoding different viral proteins. However, in some viruses including cricket paralysis virus, an IRES in the interior of the viral RNA allows translation of an internally placed open reading frame Figure 4 d , bottom left. IRES elements derived from viral genomes have been extremely useful in the design of vectors for simultaneous expression of two genes from a single mRNA.
A somewhat analogous mechanism is called termination—reinitiation: here the AUG of a second open reading frame in the RNA overlaps the termination codon of a first reading frame within the 5-base sequence UAAUG Figure 4 d , right panel. Both of these mechanisms are also used by retroviruses Hatfield et al. Successful virus replication in these cases requires the optimal efficiency of suppression, yielding the proper ratio of extended to terminated translation products.
This ratio is often in the range of In some retroviruses, there are two successive frameshift sites, resulting in the production of three proteins from the same mRNA molecule.
The efficiency of utilization of the AUGs is largely governed by their surrounding sequence contexts. In some retroviruses, the viral RNA contains a CUG in an excellent context for initiation, with an AUG further downstream in the same reading frame; both of these codons are used for initiation, resulting in the production of two protein species, identical except for an N-terminal extension on one of them.
It is important to note that these diverse mechanisms for gene expression are not mutually exclusive: many viruses use more than one of these strategies. As far as we know, all living organisms are hosts to viruses.
We have attempted here to briefly indicate the amazing diversity of information-transmission mechanisms found among viruses. Indeed, it is striking that viruses exhibit far more diversity in this regard than do cellular organisms, whose genomes are exclusively dsDNA.
However, all viral genomes larger than those of coronaviruses are also composed of dsDNA. Perhaps this is because dsDNA is more resistant than other nucleic acids to the vicissitudes of mutation and chemical and physical damage. In any case, the study of viruses can only inspire wonder at the unimaginable variety found among living things. The primary literature on viral nucleic acids is massive. We have cited a few reviews here, but two textbooks Flint et al. We apologize to the many researchers whose results we have summarized without attribution here.
Work of I. She began studying retroviruses in Dr. After a postdoctoral fellowship at the Massachusetts Institute of Technology, he has performed basic research on retroviruses at or near the National Cancer Institute. He has studied both the biology of these viruses and the molecular mechanisms by which they replicate, and focused both on murine retroviruses and HIV He has published over papers in the field and is a Fellow of the American Academy of Microbiology.
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Abstract Viral genomes exhibit extraordinary diversity with respect to nucleic acid type, size, complexity, and the information transfer pathways they follow.
Glossary Cis -acting signal A sequence or structure in a nucleic acid molecule that confers some functional property on the molecule, but this property is confined to the molecule containing the signal and cannot be transferred to other molecules. Encapsidation Incorporation of nucleic acid into an assembling virus particle. Icosahedron A solid with 20 faces. Trans -acting factor A factor, typically a protein, produced within a cell and capable of conferring a functional property on other molecules or complexes.
For example, a viral genome might contain a cis -acting signal enabling it to be packaged into assembling virus particles, while a protein supplied by an expression vector might be incorporated into the virus particles and affect their host range. Virion Virus particle. Introduction Viruses are the most abundant organisms on earth Breitbart and Rohwer, , Suttle, Table 1 Diversity of viral genomes a. Open in a separate window. For each group, the range of genome size is given. It is important to remember that this table indicates the size-range of genomes for all members of a given group, and not merely for the example shown in the last column.
Figure 1. The whole virion is slightly pleiomorphic, ranging from ovoid to brick shape. Mimivirus is the largest characterized virus, with a capsid diameter of nm. Protein filaments measuring nm project from the surface. The capsid appears hexagonal under an electron microscope, therefore the capsid is probably icosahedral. In , researchers discovered a larger virus on the ocean floor off the coast of Las Cruces, Chile.
Provisionally named Megavirus chilensis , it can be seen with a basic optical microscope. Some viruses that infect archaea have complex structures unrelated to any other form of virus.
These include a wide variety of unusual shapes, ranging from spindle-shaped structures, to viruses that resemble hooked rods, teardrops, or even bottles. Other archaeal viruses resemble the tailed bacteriophages, and can have multiple tail structures. Privacy Policy. Skip to main content. Search for:. Structure of Viruses. Viral Morphology Viruses of all shapes and sizes consist of a nucleic acid core, an outer protein coating or capsid, and sometimes an outer envelope.
Learning Objectives Describe the relationship between the viral genome, capsid, and envelope. Key Takeaways Key Points Viruses are classified into four groups based on shape: filamentous, isometric or icosahedral , enveloped, and head and tail. Many viruses attach to their host cells to facilitate penetration of the cell membrane, allowing their replication inside the cell.
Non-enveloped viruses can be more resistant to changes in temperature, pH, and some disinfectants than are enveloped viruses. The virus core contains the small single- or double-stranded genome that encodes the proteins that the virus cannot get from the host cell.
Key Terms capsid : the outer protein shell of a virus envelope : an enclosing structure or cover, such as a membrane filamentous : Having the form of threads or filaments isometric : of, or being a geometric system of three equal axes lying at right angles to each other especially in crystallography.
General Morphology Viruses have a variety of shapes and structures. Learning Objectives Distinguish between the 5 main morphological virus types. Key Takeaways Key Points Viruses are very small and to reliably visualize them, stains and electron microscopy are needed.
Viruses encode capsid proteins which encase the nucleic acid. Sometimes, viral proteins combine with host proteins to make the envelope. The shape of a viral coat has implications on how a virus infects a host.
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. High School. Learning Resources. Each collection features resources to Know about, Show, Explore, and Relate to an engaging theme for learners and educators. Data for the People.
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