Learning Objectives MICB202-2019W / Virology 1 Topic 1: Virus = poison, pathological agents - obligate intracellular parasites - Completely dependent on host cells, can't replicate without host, reproduce at the expense of the host - Its genetic material is simply covered with a protein coat and glycolipid layer - No cell membrane No metabolic capabilities, can't transcribe or translate independently (lacks these proteins) - Genome is made of either ssRNA, dsRNA, ssDNA, or dsDNA to encode their structural proteins - Virus particles are formed via self-assembly within host - Genome may be linear, circular, or even in several segments Naked viruses are just the genome surrounded by protein coat (capsid) a. Capsid composed of capsomer subunits b. Higher survival rate than enveloped viruses c. Surrounded by an envelope with modified proteins that help it infect hosts 2. Icosahedral capsids - symmetrical capsid with maximized internal volume 3. Helical capsids - ones in which the capsid proteins have affinity for the viral genome 4. Complex capsids - complex because it consists of multiple layers of proteins and structures Enveloped viruses have a lipid bilayer around capsid - Some have glycoproteins to help with attachment to host cells - Some enveloped viruses have a core replacing the capsid coat - Bilayer is sensitive to desiccation, heat, detergents and pH changes Extracellular state - (virion) Metabolically inert virus particle Intracellular state - Replicating nucleic acids, metabolically active and undergo genetic mutation/variation Replication Cycle 1. Virus adsorption and attachment a. Random collisions - binds host cell by chance without any energy expended b. Virus has evolved to bind to natural host structures with an 'anti-receptor' c. If the host cell has a matching virus receptor, its susceptible to invasion 2. Genome and other necessary proteins enter cell a. A given species of virus always uses the same strategy to entry into host cell b. If the virus can replicate within the host cell, its permissive (unrelated to susceptibility) c. Virus entry into cytoplasm is energy dependent 3. Synthesis of viral mRNA, proteins and, genome replication a. Virus is uncoated and host enzymes replicate the viral genome - self-assembly b. Mechanism success is dependent on location in cell (available host enzymes) and the types of nucleic acids in the virus c. DNA viruses: DNA - RNA - Protein d. RNA viruses: RNA -> Protein e. Retroviruses: RNA - DNA -> RNA -> Protein 4. Assemble of virus progeny a. Viral components have an affinity for each other b. Triggered when the concentration of viral genomes and proteins exceeds a threshold c. Capsid formation is energetically favorable -> stable, low free energy, inherent affinity 5. Release from host cell a. Once certain concentration of progeny is reached, the host cell is lysed (naked viruses) or enveloped virus progeny bud out in host cell membrane vesicles, taking its lipid envelope and inserting viral proteins that aid in attachment to another host in the future The virus must synthesize an mRNA for each gene in order to be