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Unformatted text preview: CHAPTER 24 VIRUSES AND PROKARYOTES • Pathogens o Agents that cause disease VIRUSES A virus consists of nucleic acid surrounded by a protein coat • Virus – virion o Tiny, infectious particle consisting of a nucleic acid core surrounded by a protein coat • Capsid o The protein coat surrounding the nucleic acid core • Host cells o Living cells that are infected by viruses • Obligate intracellular parasites o Survive only by using the resources of a host cell A virus may contain either DNA or RNA • Viruses have DNA or RNA, not both The capsid is a protective protein coat • Capsomeres o Protein subunits that make up the capsid o Determine the shape of the virus • Bacteriophages – phages o Viruses that infect bacteria • Enveloped viruses o Viruses surrounded by an outer membranous envelope Viruses may have evolved from cells • Escaped gene hypothesis o States that viruses derive from bits of nucleic acid that “escaped” from cellular organisms The International Committee on Taxonomy of Viruses classifies viruses • Host range o Types of organisms viruses infect Bacteriophages are viruses that attack bacteria Viruses reproduce only inside host cells Lytic reproductive cycles destroy host cells • Lytic cycle o Virus lyses (destroys) the host cell • Virulent o Viruses that have only a lytic cycle o Cause disease, often death • Five steps in lytic viral reproduction o Attachment • Virus attaches to receptors on the host cell o Penetration • Virus penetrates the host plasma membrane and moves into the cytoplasm o Replication and synthesis • Virus degrades the host cell nucleic acid and uses the molecular machinery of the host cell to replicate its own nucleic acid o Assembly • Newly synthesized viral components are assembled into new viruses o Release • Assembled viruses are released from the cell • Restriction enzymes o Enzymes that cut up the foreign DNA of the phage Temperate viruses integrate their DNA into the host DNA • Temperate viruses o Do not always destroy their hosts • Lysogenic cycle o Viral genome usually becomes integrated into the host bacterial DNA • Provirus – Prophage o The name of the viral and bacterial genomes combined • Lysogenic cells o Bacterial cells carrying proviruses • Lysogenic conversion o Bacterial cells containing certain temperate viruses may exhibit new properties Many viruses infect vertebrates Many vertebrate viruses have lytic reproductive cycles • Retroviruses o RNA viruses that have a DNA polymerase • Reverse transcriptase o The DNA polymerase in retroviruses o Used to transcribe the RNA genome into a DNA intermediate •...
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- Fall '08