19_LectureW2011

19_LectureW2011 - Chapter 19 Overview A Borrowed Life...

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1 Chapter 19 VIRUSES Overview : A Borrowed Life Viruses called bacteriophages can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli Viruses lead “a kind of borrowed life” between life-forms and chemicals he origins of molecular biology lie in early The origins of molecular biology lie in early studies of viruses that infect bacteria A virus consists of a nucleic acid surrounded by a protein coat What Are Viruses? Viruses are very small (10 – 400nm). not cells. a genome surrounded by a protective protein coat. infectious. dependent on a host cell to provide the machinery they need to reproduce. Avian influenza AH5N1 virus CDC intracellular parasites. formed from the assembly of newly synthesized components made in a host cell. How Do Viruses Differ From Living Organisms? Viruses are not living organisms because they are incapable of rrying out all life processes carrying out all life processes Viruses are not made of cells can not reproduce on their own do not grow or undergo division do not transform energy lack machinery for protein synthesis
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2 Structure of Viruses Viruses are not cells Viruses are very small infectious particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope Viral Genomes Viral genomes may consist of either ouble r single tranded DNA or Double- or single-stranded DNA, or Double- or single-stranded RNA Depending on its type of nucleic acid, a virus is called a DNA virus or an RNA virus Capsids and Envelopes A capsid is the protein shell that encloses the viral genome Capsids are built from protein subunits called capsomeres A capsid can have various structures
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3 How Are Viruses Classified? Genetic material DNA viruses contain DNA as their enetic material. Helical Marburg virus ourtesy of CDC ge e c a e a RNA viruses contain RNA as their genetic material. Capsid structure Helical (rod-shaped) Polyhedral Courtesy of CDC Polyhedral SV40 virus Courtesy of CDC Complex Presence or absence of a membranous envelope surrounding the capsid Complex bacteriophage Courtesy of H.Ackermann Some viruses have membranous envelopes that help them infect hosts These viral envelopes surround the capsids of influenza viruses and many other viruses found in animals iral envelopes, which are derived from the Viral envelopes, which are derived from the host cell’s membrane, contain a combination of viral and host cell molecules Bacteriophages , also called phages , are viruses that infect bacteria They have the most complex capsids found among viruses Phages have an elongated capsid head that ncloses their DNA encloses their DNA A protein tail piece attaches the phage to the host and injects the phage DNA inside Viruses reproduce only in host cells Viruses are obligate intracellular parasites,
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19_LectureW2011 - Chapter 19 Overview A Borrowed Life...

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