Viruses pp. 355-360

Why study viruses? Microscopic parasites that cause numerous diseases A.I.D.S., rabies, polio, mumps, warts, measles, common cold, cold sores (herpes simplex), various plants diseases, some cancers Copyright Pearson Prentice Hall

What Is a Virus? Non-cellular particle made up of genetic material (DNA or RNA), protein, and in some cases, lipids. Nucleic acid (DNA or RNA) core surrounded by a protein coat (capsid) Vary in size (10-400nm) & shape Specific viruses affect specific organisms Viruses can reproduce only by infecting living cells Virus “injects” its nucleic acid into the cell & alters the normal functioning of the cell

T4 Bacteriophage Tobacco Mosaic Virus Influenza Virus RNA DNA Head RNA Capsid Tail sheath Tail fiber Membrane envelope Surface proteins Capsid proteins Viruses come in a wide variety of sizes and shapes. A typical virus is composed of a core of either DNA or RNA, which is surrounded by a protein coat, or capsid. Photo Credits: l. ©M.Wurtz/Biozentrum, University of Basel/Science Photo Library/Photo Researchers, Inc. m. ©Dr. O. Bradfute/Peter Arnold, Inc. r. ©National Institute for Biological Standards and Control, England/Photo Researchers, Inc. Copyright Pearson Prentice Hall

Fibrils (recognize target cells) Enzyme site (“burns” hole in cell membrane/wall

Origins of Viruses p.360 Three possibilities: Fragments of ancient, extinct cells Primitive “life-nonlife” forms left over from the beginning of life 3-4 billion years ago Specialized parasitic cells which have lost most of their cell structures via natural selection

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Viral Infection Once the virus is inside the host cell, two different processes may occur. Some viruses replicate immediately, killing the host cell (lytic) Others replicate, but do not kill the host cell immediately (lysogenic) Copyright Pearson Prentice Hall

Bacteriophage injects DNA into bacterium Bacteriophage DNA forms a circle Lytic Infection Lysogenic Infection Bacteriophage may infect cells in two ways: lytic infection and lysogenic infection. Copyright Pearson Prentice Hall

Lytic Infection Viruses that cause cells to burst (lyse) are lytic viruses (ex. T4 bacteriophage) Lytic cycle: 1. Infection By chance, a virus attaches to host cell & injects its genetic material 2. Growth Host cell cannot distinguish between its own DNA & the virus’ Host cell begins to make messenger RNA which will start shutting down & taking over

3. Replication 4. Cell lysis Virus uses materials from host cell to make copies of its own genetic material & protein coat genetic material & protein coats are assembled into hundreds of new viruses 4. Cell lysis Infected cell bursts & releases new viruses See Fig.17-4 p. 358 Copyright Pearson Prentice Hall

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Lysogenic Infection  virus integrates its genetic material into the DNA of the host cell, and the viral genetic information replicates along with the host cell's DNA. Examples: HIV, herpes (cold sores), chicken pox, Hepatitis C

Lysogenic cycle: 1.Infection (same as lytic cycle) 2. Incorporation Virus’ DNA is incorporated into host’s DNA (prophage/provirus = viral DNA once incorporated) 3. Dormancy Host cell continues as if it wasn’t infected May replicate for many generations Advantage: presence of prophage may prevent other viral infections & add useful DNA

4. Lytic cycle Certain conditions cause prophage DNA to become active, removes itself from host’s DNA, & enters the lytic cycle Host cell lysis NOTE: Retroviruses (ex. HIV) Contain RNA After infection, virus produces a DNA copy of its RNA genes Then, the viral DNA is inserted into host’s DNA Has been linked to some cancers **Normally, a cell starts with DNA & produces RNA (a retrovirus does the opposite!)