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UW-Milwaukee BIOSCI 152 - General Features of Viruses

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BIO SCI 152 1st Edition Lecture 6 Current LectureGeneral Features of Viruses I. Size Much smaller than cells 20-300 nm diameter (E. coli cell is 1000 nm diameter, Human cell is 20,000 nm diameter) in general viruses cannot be seen with a light microscope, but can be viewed by electron microscope II. Structure Much simpler in structure than cells Typical virus is composed of nucleic acid surrounded by protein coat. Nucleic acid genome may be RNA or DNA RNA or DNA may be double stranded (ds) or single stranded (ss) Some viruses have other features: Envelope Tail and tail fibers Terminology: Capsid- protein coat of virus Nucleocapsid- capsid + nucleic acid Envelope- membrane surrounding a nucleocapsid (lipid and protein) Viral capsids come in a few basic shapes: Helical Icosahedral Complex (combination of helical and icosahedral, or other complex structure)III. Replication of Viruses Viruses grow only in living cells Viruses do not have metabolism of their own and are metabolically inert when outside of host cell Viruses in general lack ribosomes, have few enzymes . General steps in viral life cycle: 1. Attachment 2. Entry into cell 3. Biosynthesis of viral components4. Assembly or maturation 5. Release from cell IV. Importance of Viruses to Humans Bacteriophage: can transfer genes between bacteria (such as antibiotic resistance genes or virulence genes) Plant viruses: crop diseases Animal viruses: human and animal diseases Viruses and Human Disease Examples of major human diseases caused by viruses 1. Common cold 2. influenza 3. Rabies 4. Measles These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.5. Mumps 6. Herpes (cold sores) 7. Hepatitis (liver disease) 8. Warts 9. Chickenpox 10. smallpox 11. AIDS V. Influenza Influenza virus is an enveloped ss RNA virus. Genome in multiple pieces (segmented genome) Epidemics of influenza occur every few years, usually seasonaly (November to April). Pandemics (worldwide epidemic) occur less frequently, several times in a century. In some pandemic years influenza kills millions. In 1918, a pandemic occurred that killed at least 20 million worldwide. This pandemic, which occurred during World War I, killed more people than battlefield wounds. This pandemic hit young (20-30 years old) apparently healthy individuals extremely hard. A person could be healthy in the morning, and dead 48 hours later. Human Influenza viruses are rather specific to humans, but related influenza viruses are common in many animals (birds, horses, pigs, seals, etc.). Why do epidemics and pandemics of influenza arise? Antigenic Drift : Yearly Epidemics usually arise because of mutations that cause minor in viral genome. This ‘antigenic drift’ allows the virus to evade the immune system of previously infected individuals. Antigenic Shift : Pandemics are usually the result of antigenic shift, acquisition of drastically different genes and not just small mutations. VI. Retroviruses: HIV and AIDS HIV infects T cells, critical components of the immune system. Eventually the Immune system collapses (T cell number declines drastically) and many infections (fungal, bacterial, viral) occur. These other infections are often the final cause of death.HIV is a retrovirus 1) Retroviruses have an RNA genome, but convert this into DNA during replication. They use the enzyme reverse transcriptase to do this. goes against previous dogma of DNA-->RNA-->Protein. 2) The DNA form of the genome integrates into a host chromosome, 3) Transcription produces messenger RNAs that migrate to the cytoplasm where they are translated to make long ‘polyproteins’. Cleavage of the polyproteins produces the final proteins needed for viral assembly.4) Genome replication is a simple transcription of the integrated virus DNA as described above to produce the full length RNA genome 5) virus assembles in cytoplasm and is released by budding from membrane Antiviral drugs have been developed to treat AIDS by targeting unique features of the virus. 1. Drugs that inhibit reverse transcriptase 2. Protease inhibitors (prevent cleavage of


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