Ch.6 Notes PDF

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Ch.6 Notes for Exam...

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- Viruses are a unique group of biological entities known to infect every type of cell, including bacteria, algae, fungi, protozoa, plants, and animals, and are extremely abundant on our planet. -best to describe viruses as infectious particles, and as either active or inactive -Viruses are not just agents of disease. They have many positive uses. -By infecting other cells, and sometimes influencing their genetic makeup, they have shaped the way cells, tissues, bacteria, plants, and animals have evolved to their present forms. -scientists think that the human genome contains up to 80,000 viral genes, sequences that come from viruses that have incorporated their genetic material permanently into human DNA. -The sum total of the viruses associated with your body is called the virome -Having said all this, the vast majority of viruses in the human body are not pathogenic. -They are a type of obligate intracellular parasite that cannot multiply unless they invade a specific host cell and instruct its genetic and metabolic machinery to make and release quantities of new viruses. -Many viruses are much smaller than the average bacterium. These viruses cannot be seen with a light microscope; an electron microscope is necessary to detect them or examine their fine structure -It is important to realize that viruses bear no real resemblance to cells and that they lack any of the protein-synthesizing machinery found in even the simplest cells. -At their simplest, viruses need only a piece of genetic material and a protein coat. -Viruses contain only those parts needed to invade and control a host cell: an external coating and a core containing one or more nucleic acid strands of either DNA or RNA and, sometimes, one or two enzymes. -All viruses have a protein capsid, or shell, that surrounds the nucleic acid in the central core -Together, the capsid and the nucleic acid are referred to as the nucleocapsid - Members of many families of animal viruses possess an additional covering external to the capsid called an envelope, which is usually a modified piece of the host’s cell membrane. Viruses that consist of only a nucleocapsid are considered naked viruses -A fully formed virus that is able to establish an infection in a host cell is often called a virion -In general, each capsid is constructed from identical subunits called capsomeres, that are constructed from protein molecules - Depending on how the capsomeres are shaped and arranged, this assembly results in two different types for animal viruses: helical and icosahedral. - Although the capsids of all icosahedral viruses have this sort of symmetry, they can have major variations in the number of capsomeres; for example, a poliovirus has 32, and an adenovirus has 252 capsomeres. - When enveloped viruses are released from the host cell, they take with them a bit of its membrane in the form of an envelope. - Whichever avenue of escape, the viral envelope differs significantly from the host’s membranes. Viruses place their own proteins in the membrane, which they then proceed to use as their envelope

- These protruding molecules, called spikes when they are on enveloped viruses, are essential for the attachment of viruses to the next host cell. - The sum total of the genetic information carried by any organism is known as its genome. So far, one biological constant is that the genetic information of living cells is carried by nucleicPage 146 acids (DNA, RNA). Viruses, although not technically alive and definitely not cells, are no exception to this rule, but there is a significant difference. Unlike cells, which contain both DNA and RNA, viruses contain either DNA or RNA but not both. - In fact, viruses exhibit wide variety in how their RNA or DNA is configured. DNA viruses can have single-stranded (ss) or double-stranded (ds) DNA; the dsDNA can be arranged linearly or in ds circles. RNA viruses can be double-stranded but are more often single-stranded. - At the minimum, they must carry genes for synthesizing the viral capsid and genetic material, for regulating the actions of the host, and for packaging the mature virus How viruses are classified and named: -The main criteria presently used to group viruses are structure, chemical composition, and similarities in genetic makeup. Modes of Viral Multiplication: - Viruses are minute parasites that seize control of the synthetic and genetic machinery of cells. -Life cycle of animal viruses are:  adsorption,  penetration,  uncoating,  synthesis,  assembly, and  release from the host cell. - entire multiplication cycle varies from 8 hours in polioviruses to 36 hours in some herpesviruses - The membrane receptors that viruses attach to are usually glycoproteins the cell requires for its normal function. - Because a virus can invade its host cell only through making an exact fit with a specific host molecule, the range of hosts it can infect in a natural setting is limited. This limitation, known as the host range - It also explains why viruses usually have tissue specificities called tropisms (troh′pizmz) for certain cells in the body. - Animal viruses exhibit some impressive mechanisms for entering a host cell. The flexible cell membrane of the host is penetrated either by the whole virus or just by its nucleic acid (figure 6.13). In penetration by endocytosis - When enzymes in the vacuole dissolve the envelope and capsid, the virus is said to be uncoated, a process that releases the viral nucleic acid into the cytoplasm. - Enveloped viruses are liberated by budding, or exocytosis,1 from the membranes of the cytoplasm, nucleus, endoplasmic reticulum, or vesicles - But regardless of how the virus leaves, most active viral infections are ultimately lethal to the cell because of accumulated damage. The number of viruses released by infected cells is variable, controlled by factors such as the size of the virus and the health of the

host cell. About 3,000 to 4,000 virions are released from a single cell infected with poxviruses, whereas a poliovirus-infected cell can release over 100,000 virions. If even a small number of these virions happen to meet another susceptible cell and infect it, the potential for rapid viral proliferation is immense. - Virus-induced damage to the cell that alters its microscopic appearance is termed cytopathic effects - It is common to find inclusion bodies, or compacted masses of viruses or damaged cell organelles, in the nucleus and cytoplasm - When viral DNA is incorporated into the DNA of the host, it is called a provirus - Some animal viruses enter their host cell and permanently alter its genetic material, leading to cancer....