<p> Unit 3 Edit 6.09 </p><p>Viruses</p><p>What is a virus?</p><p> Obligate</p><p> Intracellular</p><p> Parasite</p><p>They use the host cell machinery to make and release large quantities of viruses.</p><p>Taxonomy of Viruses Viral species are a group of viruses sharing the same type of genetic information or ecological niche.</p><p>Grouped according to :</p><p> Type of nucleic acid – RNA or DNA but never both; double or single stranded; linear or circular or in pieces.</p><p> Replication strategy o Lytic cycle – multiplication phase that result in host death and the release of more viruses</p><p> o Lysogenic phase – multiplication phase where the virus remains latent in the host cell DNA, host cell is not lysed</p><p> Morphology </p><p>1 Unit 3 Edit 6.09 </p><p> o Protein coat – subunits called capsomeres, arrangement of these subunits is indicative of viral type.</p><p> o Envelope – some have, some don’t – envelope is added when the virus pushes itself out through the host cell’s plasma membrane.</p><p> o Spikes – some have, some don’t – reliable way to ID, stimulate host’s immune system. Mutate easily</p><p>Examples:</p><p>Tobacco mosaic Virus – helical</p><p>Adenovirus – Polyhedral</p><p>2 Unit 3 Edit 6.09 </p><p>Bacteriophage – Complex</p><p>Culturing Viruses is difficult</p><p> Often can only be cultured with host cells</p><p>Culture Methods</p><p> In living animals (host cells)</p><p> o Advantages </p><p> o Problems </p><p>3 Unit 3 Edit 6.09 </p><p> Embroynated eggs</p><p> o Advantages</p><p> o Problems</p><p> Cell cultures</p><p> o Advantages</p><p> o Problems </p><p>Bacterial Replication – most of this information known by studying bacteriophages</p><p>Most viruses only contain a few genes (structural components), the host provides enzymes for protein synthesis, replication, etc.</p><p>Two types: Lytic cycle and Lysogenic cycle</p><p>Lytic Cycle stages</p><p> Attachment</p><p> Penetration</p><p> Biosynthesis</p><p>4 Unit 3 Edit 6.09 </p><p> Maturation</p><p> Release</p><p>Lysogenic Cycle Stages</p><p> Penetration</p><p> Phage DNA combines with host cell DNA</p><p> Spontaneous event can lead to excision of phage DNA and initiation of lytic cycle</p><p> o Results of lysogenic cycle</p><p>1. Immune to infection by another of same virus</p><p>2. Phage conversion = new properties</p><p>3. Specialized transduction – pieces of bacterial DNA are packaged in virus coat and moved</p><p>Terminology</p><p> Burst time</p><p>5 Unit 3 Edit 6.09 </p><p> Burst size</p><p> Eclipse time</p><p>Animal Virus Replication is different</p><p> Penetration by endocytosis</p><p> Exit by budding</p><p> Receptor sites are inherited</p><p> Uncoating</p><p>Viruses and Cancer</p><p> Can go unrecognized because the viruses invade but do not cause cancer immediately</p><p> Not contagious as most viral diseases</p><p> Virus can be activated to abnormal function by mutagenic agents</p><p>Vertical gene transfer – plants and animals transfer genes to their offspring</p><p>6 Unit 3 Edit 6.09 </p><p>Horizontal gene transfer – Bacteria can transfer genetic material to each other as well as their offspring.</p><p> Conjugation </p><p> Transduction</p><p> Transformation</p><p>Conjugation </p><p> Plasmid = circular DNA</p><p> Requires cell to cell contact</p><p> Opposite mating type</p><p> Plasmid carries gene for sex pili</p><p>Transduction</p><p> Bacterial DNA broken down by viral enzyme</p><p> Pieces of bacterial DNA packaged in new viruses</p><p>7 Unit 3 Edit 6.09 </p><p> Transferred to new host cell</p><p> Recombination</p><p> Specialized transduction – only particular genes are transferred via transduction</p><p>Transformation </p><p> Genes are transferred as naked DNA from one bacteria to another and recombination occurs</p><p> Frederick Griffith, 1928</p><p>8</p>