Slide 1Why is this important?Characteristics of VirusesCharacteristics of VirusesCharacteristics of VirusesCharacteristics of VirusesGenetic Material of VirusesHosts of VirusesHosts of VirusesSizes of VirusesCapsid MorphologyViral ShapesThe Viral EnvelopeThe Viral EnvelopeViral ReplicationLytic Replication Cycle of Bacteriophages - AttachmentLytic Replication Cycle of Bacteriophages - EntryLytic Replication Cycle of Bacteriophages - SynthesisLytic Replication Cycle of Bacteriophages - AssemblyLytic Replication Cycle of Bacteriophages - ReleaseLytic Replication Cycle of BacteriophagesLytic Replication Cycle of BacteriophagesLysogeny (Lysogenic Replication Cycle)Lysogeny - AttachmentLysogeny - Entry and Prophage FormationLysogeny - Replication of Chromosomes and Cell DivisionLysogeny - InductionLysogeny - SynthesisLysogeny - AssemblyLysogeny - ReleaseLysogenic Replication Cycle of BacteriophagesReplication of Animal VirusesAttachment of Animal VirusesEntry and Uncoating of Animal VirusesEntry and Uncoating of Animal VirusesEntry and Uncoating of Animal VirusesSynthesis of Animal VirusesAssembly and Release of Animal VirusesAssembly and Release of Animal VirusesLatency of Animal VirusesViral ReplicationM I C R O B I O L O G YWITH DISEASES BY BODY SYSTEM SECOND EDITIONChapter 13Characterizing and Classifying Viruses, Viroids, and Prions THIRDWhy is this important?•Although viruses are not living entities, they do have shape and structure.•In order to understand viral diseases, we need to have an understanding of viral structure.Characteristics of Viruses•Many infections of humans, animals, and plants, and even of bacteria, are caused by acellular (noncellular) agents, including viruses•Recall from Chapter 3 that viruses–Lack a cellular structure–Lack most of the characteristics of life–Cannot carry out any metabolic pathway–Can neither grow nor respond to the environment–Cannot reproduce independently–Recruit the cell’s metabolic pathways to increase their numbersCharacteristics of Viruses•Cause most of the diseases that plague the industrialized world (colds, herpes, AIDS, and flu)–Characteristics of viruses and the means by which they attack their hosts make cures for viral diseases elusiveCharacteristics of Viruses•How are all viruses similar?–Have one or several pieces of either DNA or RNA–No cytoplasmic membrane, cytosol, functional organelles–Have extracellular and intracellular state–Extracellular State (virion)–Protein coat (capsid) surrounding nucleic acid core–Nucleic acid and capsid also called nucleocapsid–Some have phospholipid membrane called an envelope surrounding the nucleocapsid–Outermost layer provides protection and recognition sites that bind to complementary chemicals on the surfaces of cells–Intracellular State–Capsid removed–Virus exists as nucleic acidCharacteristics of Viruses•How do viruses differ from one another?–The type of genetic material they contain–The kinds of cells they attack–Their size–The nature of their capsid coat–Their shapes–The presence or absence of an envelopeGenetic Material of Viruses•Show more variety in nature of their genomes than do cells–May be DNA or RNA, but never both–Primary way scientists categorize and classify viruses•Can be dsDNA, ssDNA, dsRNA, ssRNA–ssDNA and dsRNA are almost nonexistent in cells•May be linear and composed of several segments or single and circular•Much smaller than genomes of cellsHosts of Viruses•Most viruses infect only particular host’s cells–Due to affinity of viral surface proteins or glycoproteins for complementary proteins or glycoproteins on host cell surface–May be so specific they only infect particular kind of cell in a particular host–Generalists – infect many kinds of cells in many different hosts•Cells infected include archaea, bacteria, fungi, plants, protozoa, and vertebrates–Bacteriophage/phage – a virus that infects bacteria–Scientists have determined that bacteriophages outnumber all bacteria, archaea, and eukaryotes put togetherHosts of VirusesSizes of VirusesCapsid Morphology•Capsids – protein coats that provide protection for viral nucleic acid and means of attachment to host’s cells•Capsid composed of proteinaceous subunits called capsomeres–Some capsomeres are composed of only a single type of protein, whereas others are composed of several different kinds of proteinsViral ShapesThe Viral Envelope•Some viruses, particularly animal viruses, have a membrane similar in composition to a cytoplasmic membrane surrounding their capsids–Matrix proteins fill the region between capsid and envelope–Enveloped virion versus nonenveloped (naked) virion•Enveloped virus acquires its envelope from its host cell during viral replication or release–Envelope is a portion of the membrane system of a host cell•Viral envelope is composed of phospholipid bilayer and proteins–Some proteins are virally encoded glycoproteins, which appear as spikes protruding outward from the envelope’s surface•Envelope’s proteins and glycoproteins often play role in host recognitionThe Viral EnvelopeCopyright © Gary E. KaiserViral Replication•Viruses lack the genes for all the enzymes necessary for replication and do not possess functional ribosomes for protein synthesis•Viruses are dependent on their hosts’ enzymes and organelles to produce new virions•Viral replication cycle usually results in the death and lysis of the host cell•Lytic replication cycle (example – T4 phage)–Attachment–Entry–Synthesis–Assembly–ReleaseLytic Replication Cycle of Bacteriophages - Attachment•Random collision•Tail fibers are responsible for attachment•Chemical attraction (not random) and precise fit to receptor sites•Due to attachment proteins on tail fibers and complementary receptor proteins on the surface of the host’s cell wall (for other viruses, the flagella or pili can also be attachment sites)•Very specificCopyright © Gary E. KaiserLytic Replication Cycle of Bacteriophages - Entry•Virus must overcome barrier posed by the cell wall and cytoplasmic membrane•T4 lysozyme weakens peptidoglycan of the cell wall•Tail sheath contracts and forces internal hollow tube through the cell wall and cytoplasmic membrane•Viral genome moves through tail sheath into the bacterial cellCopyright © Gary E. KaiserLytic Replication Cycle of Bacteriophages - Synthesis•Viral enzymes