The main function of the virion is to deliver its DNA or RNA genome into the host cell so that the genome can be expressed transcribed and translated by the host cell. The viral genome, often with associated basic proteins, is packaged inside a symmetric protein capsid. The nucleic acid-associated protein, called nucleoprotein, together with the genome, forms the nucleocapsid.
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.
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.
Lines through the centers of opposite triangular faces form axes of threefold rotational symmetry; twofold rotational symmetry axes are formed by lines through midpoints of opposite edges. An icosaheron polyhedral or spherical with fivefold, threefold, and twofold axes of rotational symmetry Fig.
Icosahedral models seen, left to right, on fivefold, threefold, and twofold axes of rotational symmetry. 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 laboratories send representative viruses to six World Health Organization WHO Collaborating Centers external icon for Reference and Research on Influenza, which are located in the following places:.
They review the results of surveillance , laboratory , and clinical studies , and the availability of vaccine viruses and make recommendations on the composition of the influenza vaccine. The WHO recommends specific vaccine viruses for inclusion in influenza vaccines, but then each country makes their own decision about which viruses should be included in influenza vaccines licensed in their country.
Information about the circulation of influenza viruses and available vaccine viruses is summarized and presented to an advisory committee of the FDA in February each year for the U. The influenza viruses in the seasonal flu vaccine are selected each year based on surveillance data indicating which viruses are circulating and forecasts about which viruses are the most likely to circulate during the coming season. The degree of similarity between available vaccine viruses and circulating viruses also is important.
Vaccine viruses must be similar to the influenza viruses predicted to circulate most commonly during the upcoming season. Another important factor is whether there is a good vaccine virus available; that is, a virus that could be used in vaccine production and which would likely protect against the viruses likely to circulate during the upcoming season.
Historically, vaccine viruses were required by FDA to be isolated and grown in chicken eggs, but now the FDA allows vaccine viruses to be grown in cells, too. Regardless of how they are grown, vaccine viruses must be tested and available in time to allow for manufacturers to produce the large amount of vaccine virus needed to make vaccine. There are a number of factors that can make getting a good vaccine virus for vaccine production challenging, including both scientific issues and issues of timing.
FDA regulatory requirements now allow influenza vaccine viruses to be grown in chicken eggs or cells. Today, the majority of influenza vaccines are still grown in chicken eggs. Some influenza viruses, like influenza A H3N2 viruses, grow poorly in eggs, making it challenging to obtain good candidate vaccine viruses at times.
In some years certain influenza viruses may not circulate until later in the influenza season, making it difficult to prepare a candidate vaccine virus in time for vaccine production. Airborne transmission: Pathogens can also spread when residue from evaporated droplets or dust particles containing microorganisms are suspended in air for long periods of time.
Diseases spread by airborne transmission include measles and hantavirus pulmonary syndrome. Bacteria and viruses are almost unimaginably small. To give a sense of these measures, consider that the period at the end of this sentence is about microns, or , nanometers, in diameter.
If we magnify the period to one thousand times its actual size see far left , a nearby Pseudomonas aeruginosa , the bacterium that causes hospital-acquired pneumonia and bloodstream infections, becomes visible. If, in turn, we magnify Pseudomonas 75 more times, or to 75, times its actual size, an adjacent influenza virus particle also becomes visible.
Infection does not necessarily lead to disease. Infection occurs when viruses, bacteria, or other microbes enter your body and begin to multiply. Disease, which typically happens in a small proportion of infected people, occurs when the cells in your body are damaged as a result of infection, and signs and symptoms of an illness appear.
In response to infection, your immune system springs into action. White blood cells, antibodies, and other mechanisms go to work to rid your body of the foreign invader. Indeed, many of the symptoms that make a person suffer during an infection—fever, malaise, headache, rash—result from the activities of the immune system trying to eliminate the infection from the body.
Pathogenic microbes challenge the immune system in many ways. Viruses make us sick by killing cells or disrupting cell function. Many bacteria make us sick the same way, but they also have other strategies at their disposal. Sometimes bacteria multiply so rapidly they crowd out host tissues and disrupt normal function.
Sometimes they kill cells and tissues outright. Turn recording back on. National Center for Biotechnology Information , U. Search term. I How Infection Works. Figure From the moment we are born, microbes begin to colonize our bodies. Figure Lactobacillus bacteria, which produce lactic acid to help with digestion. Types of Microbes There are five major categories of infectious agents: Viruses, bacteria, fungi, protozoa, and helminths.
Viruses Viruses are tiny, ranging in size from about 20 to nanometers in diameter see page 9. Figure An electron micrograph of an influenza virus particle, showing details of its structure. Bacteria Bacteria are 10 to times larger than viruses and are more self-sufficient. Figure E. Other Infectious Agents The other three major types of infectious agents include fungi spore-forming organisms that range from bread mold to ringworm to deadly histoplasmosis , protozoa such as the agents behind malaria and dysentery , and helminths parasitic worms like those that cause trichinosis, hookworm, and schistosomiasis.
Figure Grand Prismatic Spring, a geothermal hot spring in Yellowstone and home to microbes that have adapted to this extreme environment. Encountering Microbes Microbes have inhabited the earth for billions of years and may be the earliest life forms on the planet.
New Meeting Places Any changes that create new intersections between microbes and people pave the way for disease-causing agents to enter our species. Entering the Human Host Microorganisms capable of causing disease—pathogens—usually enter our bodies through the mouth, eyes, nose, or urogenital openings, or through wounds or bites that breach the skin barrier. Figure Evidence for why it is important to cover your mouth when you sneeze.
Trichinella spiralis , the helminth that causes trichinosis, enters the body encased in cysts residing in undercooked meat. Pepsin and hydrochloric acid in our bodies help free the larvae in the cysts to enter the small intestine, where they molt, mature, and ultimately produce more larvae that pass through the intestine and into the bloodstream.
At that point they are free to reach various organs. Those that reach skeletal muscle cells can survive and form new cysts, thus completing their life cycle. Histoplasma capsulatum , a fungus that transmits histoplasmosis, grows in soil contaminated with bird or bat droppings. Spores of the fungus emerge from disturbed soil and, once inhaled into the lungs, germinate and transform into budding yeast cells. In its acute phase, the disease causes coughing and flu-like symptoms.
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