Exam 3 Powerpoint Notes.docx - Lecture 27 Viruses are the...

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Lecture 27 - Viruses are the most abundant biological entities - They alternate between two states in their replication cycle: o Replicative state o Virion/virus particle (a metabolically inert yet infectious structure) - Viruses are undead (not considered alive) - Fundamental viron (virus particle) is composed of a protein capsid and nucleic acid genome (nucleic acid tightly packed inside capsid) - Enveloped virus has a phospholipid bilayer membrane surrounding it - Virus capsids are usually symmetric - Two morphologies are common: o Cylindrical or tubular capsid o Spherical icosahedral capsid o (Less common = binal) - Enveloped viruses have host-derived lipid envelopes giving them irregular shapes - Capsids are composed of individual protein subunits called capsomeres - Viral genomes can be one or more chromosomes of either DNA or RNA, can be double or single stranded, circular or linear, segmented or non-segmented - Most viral genomes are comprised of several dozen genes, but range from as few as 3 to several hundred - Viral genomes are incomplete - Viral genomes are small because they do not encode most of the machinery needed for replication - Host cells provides the remaining molecular machinery needed for virus replication - Virions genomes encode; o Structural proteins of virus o Proteins necessary for replication of viral nucleic acid and virion assembly not encoded in host genome o Protein regulating gene expression o Proteins that release viruses from the host cell - Successful viral replication requires the combined genomes of virus and host cell - Viruses are classified into non-phylogenic groups based on their molecular biology, life cycles or hosts - These classification schemes do not provide insight into the relatedness ot evolution of all viruses - Phylogenic relatedness can and does get determined within specific viral lineages (ex. Flu virus, retrovirus) - Virus classification based on genome type - Tissue tropism: viruses only attach and infect certain hosts, certain tissues, or certain cells - Virus replication/multiplication is a five step algorithm
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o Attachment: virion binds to host cell using surface proteins that bind to specific structures. Highly specific. These interactions require tissue tropism. Most virus particles never encounter a host cell with the right chemical structure on it’s surface. o Penetration: viral nucleic acid gains access to site at which the nucleic acid will be expressed and replicated. Occurs by nucleic acid injection Plant viruses penetrate via wounds or insect feeding and move from cell to cell through plasmodesmata, but move long distances throughout the plant through the bundle sheath Uncoating in an animal host cell Latency: some viruses can enter a latent state, delaying replication until later o Macromolecule synthesis: viral nucleotide is replicated and the viral proteins including capsid proteins are made. Antiviral drugs interfere before and after this step. Expression of host genes is not required and competes with viral replication, transcription, and translation. Viral
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  • Spring '08
  • keen
  • cells, host cells

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