Viruses are obligate parasites

Morphology: Viruses are grouped on the basis of size and shape, chemical composition and structure of the genome, and mode of replication. Helical morphology is seen in nucleocapsids of many filamentous and pleomorphic viruses. Helical nucleocapsids consist of a helical array of capsid proteins protomers wrapped around a helical filament of nucleic acid.

The number and arrangement of the capsomeres morphologic subunits of the icosahedron are useful in identification and classification. Many viruses also have an outer envelope. The entire genome may occupy either one nucleic acid molecule monopartite genome or several nucleic acid segments multipartite genome. The different types of genome necessitate different replication strategies. 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.

Also considered in viral classification is the site of capsid assembly and, in enveloped viruses, the site of envelopment. Viruses are inert outside the host cell. Small viruses, e. Viruses are unable to generate energy. As obligate intracellular parasites, during replication, they fully depend on the complicated biochemical machinery of eukaryotic or prokaryotic cells.

The main purpose of a virus is to deliver its genome into the host cell to allow its expression transcription and translation by the host cell. A fully assembled infectious virus is called a virion.

The simplest virions consist of two basic components: nucleic acid single- or double-stranded RNA or DNA and a protein coat, the capsid, which functions as a shell to protect the viral genome from nucleases and which during infection attaches the virion to specific receptors exposed on the prospective host cell. Capsid proteins are coded for by the virus genome.

Because of its limited size Table the genome codes for only a few structural proteins besides non-structural regulatory proteins involved in virus replication. Capsids are formed as single or double protein shells and consist of only one or a few structural protein species.

Therefore, multiple protein copies must self assemble to form the continuous three-dimensional capsid structure. Self assembly of virus capsids follows two basic patterns: helical symmetry, in which the protein subunits and the nucleic acid are arranged in a helix, and icosahedral symmetry, in which the protein subunits assemble into a symmetric shell that covers the nucleic acid-containing core.

Some virus families have an additional covering, called the envelope, which is usually derived in part from modified host cell membranes. Viral envelopes consist of a lipid bilayer that closely surrounds a shell of virus-encoded membrane-associated proteins. The exterior of the bilayer is studded with virus-coded, glycosylated trans- membrane proteins.

Therefore, enveloped viruses often exhibit a fringe of glycoprotein spikes or knobs, also called peplomers. In viruses that acquire their envelope by budding through the plasma or another intracellular cell membrane, the lipid composition of the viral envelope closely reflects that of the particular host membrane.

The outer capsid and the envelope proteins of viruses are glycosylated and important in determining the host range and antigenic composition of the virion. In addition to virus-specified envelope proteins, budding viruses carry also certain host cell proteins as integral constituents of the viral envelope. Virus envelopes can be considered an additional protective coat.

Larger viruses often have a complex architecture consisting of both helical and isometric symmetries confined to different structural components. Viruses are classified on the basis of morphology, chemical composition, and mode of replication. The viruses that infect humans are currently grouped into 21 families, reflecting only a small part of the spectrum of the multitude of different viruses whose host ranges extend from vertebrates to protozoa and from plants and fungi to bacteria.

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. Such nucleocapsids form rigid, highly elongated rods or flexible filaments; in either case, details of the capsid structure are often discernible by electron microscopy.

In addition to classification as flexible or rigid and as naked or enveloped, helical nucleocapsids are characterized by length, width, pitch of the helix, and number of protomers per helical turn. The most extensively studied helical virus is tobacco mosaic virus Fig. Many important structural features of this plant virus have been detected by x-ray diffraction studies. Figure shows Sendai virus, an enveloped virus with helical nucleocapsid symmetry, a member of the paramyxovirus family see Ch.

The helical structure of the rigid tobacco mosaic virus rod. About 5 percent of the length of the virion is depicted. Individual 17,Da protein subunits protomers assemble in a helix with an axial repeat of 6. Each more Fragments of flexible helical nucleocapsids NC of Sendai virus, a paramyxovirus, are seen either within the protective envelope E or free, after rupture of the envelope.

The intact nucleocapsid is about 1, nm long and 17 nm in diameter; its pitch more An icosahedron is a polyhedron having 20 equilateral triangular faces and 12 vertices Fig.

These viruses may cause local inflammation, such as various enteric fevers various diarrheas, e. Poliovirus, for example, initially replicates in the gut with few symptoms but is borne by the blood to the central nervous system, where it infects specific cells to cause devastating effects. The viruses that replicate in the intestinal cells are often shed into the feces and passed on to others by the fecal—oral route of transmission.

Some viruses are efficiently transmitted by direct inoculation into the bloodstream. In nature these viruses often require insect vectors to effect this transmission. Well-known examples include the yellow fever virus, dengue fever virus, and the encephalitis viruses, all transmitted by blood-feeding arthropods such as mosquitoes and ticks. Many viruses, for example, hepatitis B virus and HIV, although not transmitted by direct inoculation into the bloodstream in nature, can be transmitted by blood inoculation through medical procedures transfusions, injections or trauma.

The venereal route of transmission is also utilized by some viruses. After the local infection of susceptible cells with an initial round of viral multiplication, the initial viremia primary viremia serves to transport the virus to specific target cells or tissues in the body where the virus may replicate further, giving rise to additional virus in the blood secondary viremia.

Often, the immunological responses of the individual are provoked only by massive secondary viremia because the primary viremia may be inadequate in duration or intensity to do so. Certain unusual modes of virus transmission have been observed, for example, in rabies, where the virus enters the tissues by trauma, often an animal bite, whereupon it enters the peripheral nerve cells and the virus migrates along the nerves to the central nervous system where it then replicates and causes damage.

The virus can find its way, perhaps by the bloodstream or by the nerves, to the salivary glands where it can be excreted through the saliva and thereby transmitted to another susceptible host. Table 2 summarizes the modes of transmission and average incubation periods of some common human pathogenic viruses.

Most virus infections are asymptomatic or, at most, cause such common and inconsequential symptoms that the infection passes unnoticed. Analysis of the antiviral antibodies in normal human serum shows that we have many antibodies, which indicates a history of prior encounters with viruses of which we have been unaware.

Likewise, many individuals who cannot recall having fever blisters carry herpes simplex virus type 1 in a latent form in their bodies. Infection with poliovirus in infancy often provokes only a mild, self-limited febrile illness in contrast to the devastating infections of the central nervous system seen upon primary infection in older children and adults.

The first cellular response to infection by many viruses seems to be the induction of interferon-specific proteins that are secreted by the infected cells and function to render neighboring cells more resistant to virus replication.

The interferon response aims at producing local resistance to virus infection so as to limit the spread of the virus. This response is immediate and occurs within hours to days of the initial infection.

Some side effects of the production of interferon include fever as well as the general malaise associated with many virus infections. The viremic phase of virus infection allows the cells of the immune system to detect and respond to the presence of virus. If the virus is sufficiently immunogenic recognized as foreign to the body , the immune system produces a primary antibody response in about a week.

This primary immune response results in the production of long-lasting memory-B-lymphocytes, which can be activated later by subsequent exposure to the same virus to provide a more rapid and more intense secondary immune response. This immunological memory is the primary reason that we usually are more or less immune for life once we have survived a particular virus infection. The specific antibodies produced by the primary immune response can combine with the virus in the blood and result in circulating immune complexes that facilitate the destruction and clearance of the virus from the body.

Such circulating immune complexes, however, also result in activation of some other processes such as the production of fever. Some viruses, such as the herpes simplex virus, cause local immunological reactions of such intensity that much of the inflammation and pain at the site of the infection is the result of the action of the immune cells rather than the destruction of the infected cells by the virus alone. Another unusual immune reaction is observed in the case of infection by Epstein—Barr virus.

The normal immune surveillance mechanism that involves the T-lymphocyte system is activated to respond to these transformed B-cells and kill them. That is, it was named for the cells that reacted to the virus infection rather than for the virus-infected cells themselves.

Some viruses that enter into a latent or symbiotic state within the host cell can provoke the cell to behave in abnormal ways.

Many such viruses carry extra genes that regulate cell division and can result in the malignant transformation of the cell to produce a cancer. These cancer-causing viruses oncogenic viruses are a special group of viruses that are of great current interest for both their special biology and their practical importance.

A virus can be in one of two states. Either they are Intracellular, a state in which they are active inside of a host cell, or they are Extracellular, a state in which they are inactive, but have the ability to transfer that DNA or RNA from cell to cell. They can be as small as a few micrometers, and as huge as several meters. Parasites are classified as eukaryotes an organism made up of a cell or cells , and therefore share certain characteristics with human cells, such as the possession of a nucleus.

Well, viruses are only active while intracellular. In enveloped viruses, the nucleocapsid is surrounded by a lipid bilayer derived from the modified host cell membrane and studded with an outer layer of virus envelope glycoproteins. Excerpt Viruses are small obligate intracellular parasites, which by definition contain either a RNA or DNA genome surrounded by a protective, virus-coded protein coat.