Page 1 :
Definition, Viruses occupy a special taxonomic position: they are not plants, animals, or bacteria (single-cell organisms without defined nuclei), and they are generally placed in their own kingdom. In fact, viruses should not even be considered organisms, in the strictest sense, because they are not free-living—i.e., they cannot reproduce and carry on metabolic processes without a host ., All true viruses contain —either (deoxyribonucleic acid) or (ribonucleic acid)—and . The nucleic acid encodes the genetic information unique for each virus. The infective, extracellular (outside the cell) form of a virus is called the . It contains at least one unique protein synthesized by specific genes in the of that virus. In virtually all viruses, at least one of these proteins forms a shell (called a ) around the nucleic acid. Certain viruses also have other proteins internal to the capsid; some of these proteins act as , often during the synthesis of viral nucleic acids. (meaning “viruslike”) are disease-causing organisms that contain only nucleic acid and have no structural proteins. Other virus like particles called are composed primarily of a protein tightly complexed with a small nucleic acid . Prions are very resistant to inactivation and appear to cause degenerative brain disease in mammals, including humans., Viruses are quintessential ; they depend on the host cell for almost all of their life-sustaining functions. Unlike true organisms, viruses cannot synthesize proteins, because they lack (cell organelles) for the translation of viral (mRNA; a complementary copy of the nucleic acid of the that associates with ribosomes and directs protein synthesis) into proteins. Viruses must use the ribosomes of their host cells to translate viral mRNA into viral proteins., Viruses are also energy parasites; unlike cells, they cannot generate or store energy in the form of (ATP). The virus derives energy, as well as all other metabolic functions, from the host cell. The invading virus uses the nucleotides and of the host cell to synthesize its nucleic acids and proteins, respectively. Some viruses use the and sugar chains of the host cell to form their membranes and glycoproteins (proteins linked to short consisting of several )., The true infectious part of any virus is its nucleic acid, either DNA or RNA but never both. In many viruses, but not all, the nucleic acid alone, stripped of its capsid, can infect (transfect) cells, although considerably less efficiently than can the intact ., The virion capsid has three functions: (1) to protect the viral nucleic acid from digestion by certain enzymes (), (2) to furnish sites on its surface that recognize and attach (adsorb) the virion to on the surface of the host cell, and, in some viruses, (3) to provide proteins that form part of a specialized component that enables the virion to penetrate through the cell surface membrane or, in special cases, to inject the infectious nucleic acid into the interior of the host cell., Virus: Structure and Symmetry, Virus are very small infectious agents with size ranging from 20-300nm in diameter., Viruses are non-cellular entities so they are also called as particles., Virus lacks their own independent metabolism and cannot replicate outside the host cell. So they are also called as obligate intracellular parasites., Virus that infects bacteria are called bacteriophage or simply phage. Animal virus infects animals and similarly plant virus infects plants., Host range and distribution, Logic originally dictated that viruses be identified on the basis of the host they infect. This is justified in many cases but not in others, and the host range and distribution of viruses are only one for their classification. It is still traditional to divide viruses into three categories: those that infect animals, plants, or bacteria., Virtually all are transmitted by insects or other organisms (vectors) that feed on plants. The hosts of vary from protozoans (single-celled animal organisms) to humans. Many viruses infect either invertebrate animals or vertebrates, and some infect both. Certain viruses that cause serious diseases of animals and humans are carried by . These vector-borne viruses multiply in both the invertebrate vector and the host., Certain viruses are limited in their host range to the various orders of . Some viruses appear to be adapted for growth only in (animals commonly referred to as , such as and ), possibly because they can reproduce only at low . Other viruses are limited in their host range to endothermic vertebrates (animals commonly referred to as warm-blooded, such as )., Size and shape, Dissect the anatomy of icosahedral, rod-shaped, and bacteriophage virus structures in electron micrographs, The amount and arrangement of the proteins and of viruses determine their size and shape. The nucleic acid and proteins of each class of viruses assemble themselves into a structure called a , or nucleocapsid. Some viruses have more than one layer of surrounding the nucleic acid; still others have a membrane (called an envelope), derived from the membrane of the host , that surrounds the nucleocapsid core. Penetrating the membrane are additional proteins that determine the specificity of the virus to host cells. The protein and nucleic acid have properties unique for each class of virus; when assembled, they determine the size and shape of the virus for that specific class. The genomes of Mimiviruses and Pandoraviruses, which are some of the largest known viruses, range from 1 to 2.5 Mb (1 Mb = 1,000,000 base pairs of DNA)., Most viruses vary in diameter from 20 nanometres (nm; 0.0000008 inch) to 250–400 nm; the largest, however, measure about 500 nm in diameter and are about 700–1,000 nm in length. Only the largest and most complex viruses can be seen under the light microscope at the highest resolution. Any determination of the size of a virus also must take into account its shape, since different classes of viruses have distinctive shapes., Shapes of viruses are predominantly of two kinds: rods, or filaments, so called because of the linear array of the nucleic acid and the protein subunits; and spheres, which are actually 20-sided (icosahedral) polygons. Most viruses are small and are either filaments or polygons, as are many bacterial viruses. The larger and more-complex bacteriophages, however, contain as their genetic information double-stranded DNA and combine both filamentous and polygonal shapes. The classic T4 is composed of a polygonal head, which contains the DNA genome and a special-function rod-shaped tail of long fibres. Structures such as these are unique to the bacteriophages., Animal viruses exhibit extreme variation in size and shape. The smallest viruses belong to the families and and measure about 20 nm and about 30 nm in diameter, respectively. Viruses of these two families are icosahedrons and contain nucleic acids with limited genetic information. Viruses of the family are about 250 to 400 nm in their longest dimension, and they are neither polygons nor filaments. Poxviruses are structurally more complex than simple , despite their close resemblance. Animal viruses that have rod-shaped (helical) nucleocapsids are those enclosed in an envelope; these viruses are found in the families , , , and . Not all enveloped viruses contain helical nucleocapsids, however; those of the families , , and have polygonal nucleocapsids. Most enveloped viruses appear to be spherical, although the rhabdoviruses are elongated cylinders., The used for classifying viruses into families and genera are primarily based on three structural considerations: (1) the type and size of their nucleic acid, (2) the shape and size of the capsids, and (3) the presence of a envelope, derived from the host cell, surrounding the viral nucleocapsid., (a double helix). In viruses, however, genetic information can com, Bacteriophages, The most important discovery was that of the Bacteriophage era. In 1915 Twort was working with vaccinia virus and found that the viruses grew in cultures of bacteria. He called then bacteriophage., Diseases caused by viruses, Japanese Encephalitis, Japanese encephalitis (JE) is a dangerous form of brain inflammation caused by the Japanese encephalitis virus, which is common in Asia and the West Pacific territories. The JE virus is a flavivirus, which brings it into the same family as the dengue virus. The life cycle of the virus is completed in two hosts, the mosquito and another vertebrate, whether man, pig or wading bird. The bites of culex mosquitoes transmit the virus, mainly by C. tritaeniorhynchus., Symptoms, .Headache , fever, nausea ,body pain., POLIO, Polio, or poliomyelitis, is a disabling and life-threatening disease caused by the poliovirus., The virus spreads from person to person and can infect a person’s spinal cord, causing paralysis (can’t move parts of the body)., Symptoms, Post-polio syndrome (PPS) is a condition that can affect polio survivors decades after they recover from their initial poliovirus infection., Most people who get infected with poliovirus (about 72 out of 100) will not have any visible symptoms., About 1 out of 4 people with poliovirus infection will have flu-like symptoms that may include:, Sore throat, Fever, Tiredness, Nausea, Headache, Stomach pain, These symptoms usually last 2 to 5 days, then go away on their own., A smaller proportion of people with poliovirus infection will develop other, more serious symptoms that affect the brain and spinal cord:, Paresthesia (feeling of pins and needles in the legs), Meningitis (infection of the covering of the spinal cord and/or brain) occurs in about 1 out of 25 people with poliovirus infection, Paralysis (can’t move parts of the body) or weakness in the arms, legs, or both, occurs in about 1 out of 200 people with poliovirus infection, Paralysis is the most severe symptom associated with polio, because it can lead to permanent disability and death. Between 2 and 10 out of 100 people who have paralysis from poliovirus infection die, because the virus affects the muscles that help them breathe., Even children who seem to fully recover can develop new muscle pain, weakness, or paralysis as adults, 15 to 40 years later. This is called ., Note that “poliomyelitis” (or “polio” for short) is defined as the paralytic disease. So only people with the paralytic infection are considered to have the disease., Mumps Virus, Mumps was a common childhood disease before a vaccination became available for the virus. When people think of mumps, they often think of swollen cheeks due to the swelling of the salivary glands located next to the ears. However, because of the types of tissues that the mumps virus can infect, complications such as encephalitis, meningitis, swelling of the ovaries, and swelling of the testes can occur. Before the vaccination was introduced, mumps was the most common cause of acquired deafness in children. This lesson will cover the structure of the mumps virus and how the virus leads to disease., HIV /AIDS, HIV (Human Immunodeficiency Virus) is composed of two strands of RNA, 15 types of viral proteins, and a few proteins from the last host cell it infected, all surrounded by a lipid bilayer membrane. Together, these molecules allow the virus to infect cells of the immune system and force them to build new copies of the virus. Each molecule in the virus plays a role in this process, from the first steps of viral attachment to the final process of budding., Since 1986, research on the structural biology of HIV have revealed the atomic details of these proteins. These structures are all publicly available in the Protein Data Bank (PDB) archive. Using these data, researchers have designed new treatments for HIV infection, including effective drug regimens that halt the growth of the virus. The structures also provide new hope for development of a vaccine., SMALLPOX, Smallpox is an extremely contagious disease, which is caused by the deadly virus called Variola. This contagious disease came into existence since 10,000 BC. In 1980, this syndrome was declared as completely eradicated after the global immunization campaign led by the World Health Organization (WHO) with the help of the smallpox vaccine. The first effective vaccine to be discovered was the smallpox one as it was developed in 1796 by Edward Jenner. The last symptoms of this were found in the year 1977., Smallpox – An Extremely Contagious Disease, Smallpox is a contagious disease, which transmits from one person to another by infective droplets of an infected person. There is no treatment found for this contagious disease but could be prevented through the ., Variola- The Smallpox Virus, The variola virus has two forms namely the Variola minor and Variola major. Variola virus, is a deadly virus, being a member of the orthopoxvirus family. The structure of this virus resembles that of brick ones and the core of the virus is made up of a genetic material DNA which resembles a dumbbell in shape. The DNA in the core comprises necessary proteins, which are required to replicate the host’s cell. The incubation period for this disease is about 17 days, which later results in severe fever with the appearance of rashes on the face, legs, hands, arms, etc., Causes of Smallpox, Smallpox is an airborne disease that spreads at a faster rate and is mainly caused by an infection of a deadly type of virus variola., It transmits through the droplets released from coughing, sneezing, and face to face contact with an infected person., This infection is also transmitted by sharing drinks, exchange of body fluids like blood transfusion and etc., Caused by even touching any contaminated area., By using unclean syringes or the used ones., Symptoms of Smallpox, Usually, after the infection of the variola virus, the symptoms occur after 17 days. Below Listed general symptoms are seen after the incubation period, High fever followed with chills., Vomiting or nausea., A severe headache, followed by other body pains., Development of rashes, filled with pus or fluid on the face, legs, hands, arms, etc., Treatment for Smallpox, Since it is a deadly disease, there is no such cure available for this syndrome. It could be still prevented by vaccinating with the smallpox vaccine. This vaccine helps in preventing the disease from illness and causing fatal conditions to humans. The antibodies present in this vaccine protects the body from invading and to destroy the virus.