BIO SCI 152 1st EditionLecture 7General Features of VirusesI. SizeMuch 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 microscopeII. StructureMuch simpler in structure than cellsTypical virus is composed of nucleic acid surrounded by protein coat.Nucleic acid genome may be RNA or DNARNA or DNA may be double stranded (ds) or single stranded (ss)Some viruses have other features:EnvelopeTail and tail fibersTerminology:Capsid- protein coat of virus Nucleocapsid- capsid + nucleic acidEnvelope- membrane surrounding a nucleocapsid (lipid and protein)Viral capsids come in a few basic shapes: HelicalIcosahedralComplex (combination of helical and icosahedral, or other complex structure)III. Replication of VirusesViruses grow only in living cellsViruses do not have metabolism of their own and are metabolically inert when outsideof host cellViruses in general lack ribosomes, have few enzymes .General steps in viral life cycle:1. Attachment 2. Entry into cell 3. Biosynthesis of viral components 4. Assembly or maturation 5. Release from cell 1. Attachment:Proteins on surface of virus interact with proteins (receptors) on surface of cellAttachment tends to be specific (virus only attaches to specific host cells)2. EntryIn bacteria, capsid remains outside cell and nucleic acid is injected into cytoplasm. Ineukaryotes nucleocapsid enters the cell3. BiosynthesisVirus steals machinery of cell to carry out replication of new viral particlesTranscription of viral RNATranslation of Viral proteinsReplication of viral DNA or RNA4. AssemblyIn general viruses assemble spontaneously once the component parts have beensynthesized5. Release from cellMost bacteriophage produce enzymes to lyse the host cell allowing release of phageIn eukaryotes, some viruses are released by lysis of cell, while others bud out of cell andthus acquire an envelopeIV. Importance of Viruses to HumansBacteriophage: can transfer genes between bacteria (such as antibiotic resistance genes or virulence genes)Plant viruses: crop diseasesAnimal viruses: human and animal diseasesViruses and Human DiseaseExamples of major human diseases caused by viruses1. Common cold 2. influenza3. Rabies 4. Measles 5. Mumps 6. Herpes (cold sores) 7. Hepatitis (liver disease) 8. Warts 9. Chickenpox 10. smallpox 11. AIDS V. InfluenzaInfluenza 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 thatkilled 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 arecommon 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 differentgenes and not just small mutations.VI. Retroviruses: HIV and AIDSHIV 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 retrovirus1) 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 aretranslated to make long ‘polyproteins’. Cleavage of the polyproteins produces the finalproteins needed for viral assembly. 4) Genome replication is a simple transcription of the integrated virus DNA as describedabove 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