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Viruses
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Viruses
• Non-cellular particles of nucleic acid, protein, and in some cases lipids that can reproduce only by infecting living cells
• Differ widely in terms of size and structure
• They enter living cells and, once inside, use the machinery of the infected cell to produce more viruses
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Structure
• Typical virus is composed of a core of either DNA or RNA surrounded by a protein coat
• Outer protein coat is called its capsid
• Includes proteins that enable a virus to enter a host cell
• Bind to the surface of a cell and “trick” the cell into allowing it inside
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Viral Infection
• Most viruses are highly specific to the cells they infect
• Plant viruses do not infect animal cells; most animal viruses infect only certain species of animals; and bacterial viruses infect only certain types of bacteria
• Bacteriophages are viruses that infect bacteria
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2 Modes of Infection• Lytic Infection
– Virus enters a cell, makes copies of itself, and causes the cell to burst
– Host cell is lysed and destroyed • Lysogenic Infection
– Host cell makes copies of the virus indefinitely – Virus embeds its DNA into the DNA of the
host cell and is replicated along with the host cell’s DNA
– Does not lyse the host cell right away
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Viruses and Disease• Cause human diseases such as polio,
measles, AIDS, mumps, influenza, yellow fever, rabies, and the common cold
• Oncogenic viruses cause cancer in animals
• Those that contain RNA as their genetic information are called retroviruses
• Prions contain only the protein portion and can cause mad cow disease and CJD
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Prevention• Best way to protect against most viral
diseases lies in prevention via vaccines
• Vaccine made up of weakened or killed virus or viral proteins
• Injected into the body, a vaccine stimulates the immune system, sometimes producing permanent immunity to the disease
• Only effective before an infection begins
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Are Viruses Alive?• Living things are made up of units called
cells. • Living things reproduce. • Living things are based on a universal
genetic code. • Living things grow and develop. • Living things obtain and use materials and
energy. • Living things respond to their environment. • Living things maintain a stable internal
environment. • Taken as a group, living things change over
time.