This preview shows pages 1–3. Sign up to view the full content.
This preview has intentionally blurred sections. Sign up to view the full version.View Full Document
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...
View Full Document
- Fall '08