Viruses Are Viruses Dead or Alive?: 7 Criteria for a Living Thing: 1. Living things must maintain homeostasis 2. Living things have different levels of organization 3. Living things reproduce 4. Living things grow 5. Living things use energy 6. Living things respond to stimuli 7. Living things adapt to their environment Lytic Phase: where the virus actively replicates in a host cell Lysogenic Phase: where the viral DNA incorporates itself into the cell’s DNA and multiplies whenever the cell multiplies Virus Structure & Classification: 4 things that define viruses: size, shape, nucleic acids (contain one type such as dsRNA, dsDNA, ssRNA, or ssDNA), and type of host Viruses have a capsid (protein coat made up of capsomers) and can make a couple different shapes (icosahedral, helical, and spherical) Nucleocapsid: a capsid containing the genome Viral Replication: Lytic & Lysogenic: Bacteriophages literally just inject their DNA into the cell Every other cell uses one of two ways: 1. Enveloped: bind to receptors on cell surface to let them in order directly fuse with the cell membrane 2. Non-enveloped: trick cell receptors into letting them in Lytic Cycle: begins to take over the cell’s machinery immediately in order to begin replicating Lysogenic Cycle: the virus combines with the host’s genetic material and the host beings replicating its genetic information for the virus (a certain condition such as UV light will cause the virus to be excised from the genome and enter the lytic cycle Retroviruses: Retroviruses are an enveloped ssRNA virus which carries three special proteins (reverse transcriptase - forms cDNA (same as any other polymerase) from the ssRNA template and then reads the cDNA strand in order to make it a dsDNA | integrase - cuts off the 3’ ends (shortening the genetic code) forming sticky ends and integrates it to the host’s DNA | protease - cleaves proteins within the virus in order to make sure they’re functional before infecting another cell) Uncoating occurs when the capsid of the virus is dissolved Subviral Particles: Viroids & Prions: 2 Types of Subviral Particles: 1. Viroids: smaller than viruses and only made of a single strand of circular RNA (can infect plants and humans alike) and is thought to be catalytic RNA (self cleave covalent bonds to create more viroids) a. Virions are not the same as viroids, virions are viruses 2. Prions: these are proteinaceous infectious particles and have no genetic material and is typically found in a beta sheet complex (acts to convert other alpha helices into beta sheets causing protein deposits)