Chapter 12 structure of viruses revised

Microbiology: A Clinical Approach © Garland Science CHAPTER 12 THE STRUCTURE AND INFECTION CYCLE OF VIRUSES © Eye of Science / Science Photo Library OVERVIEW Viruses Viruses: • are noncellular or Acellular infectious agents Virology: • study of viruses Virologists: • scientists who study viruses General features of Viruses  Viruses are infectious agents with both living and nonliving characteristics. 1. Living characteristics of viruses  a. They reproduce at a fantastic rate, but only in living host cells. b. They can mutate. …General features of Viruses 2. Nonliving characteristics of viruses  They are acellular, that is, they contain no cytoplasm or cellular organelles.  They carry out no metabolism on their own and must replicate using the host cell's metabolic machinery.  Virus particles contains either DNA or RNA (not both)  Nucleic Acid is surrounded or coated by a protein shell (capsid)  Some viruses possess a membrane-like envelope surrounding the particle …General features of Viruses …General Properties of viruses  Consists of ≥1 molecule of DNA or RNA enclosed in coat of protein  May have additional layers  Cannot reproduce independent of living cells nor carry out cell division as procaryotes and eucaryotes do  An intact viral particle is called a virion. The Size and Morphology of Selected Viruses Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 8 Generalized Structure of Viruses Viral components  Nucleic acids  Capsid  Envelope 9 …Generalized Structure of Viruses [...]...The Structure of Viruses  Virion size range is ~10-400 nm  All virions contain a nucleocapsid which is composed of nucleic acid (DNA or RNA) and a protein coat (capsid)  Some viruses consist only of a nucleocapsid, others have additional components  Envelopes  virions having envelopes = enveloped viruses  virions lacking envelopes = naked viruses VIRAL ENVELOPES • Many viruses that infect... Companies, Inc Permission required for reproduction or display 19 HELICAL VIRUSES ICOSAHEDRAL VIRUSES  Their shape is derived from 20 triangular faces that make up the capsid  The capsid ‘has’ 12 points of symmetry Icosahedral capsids THE INFECTION CYCLE • The infection cycle was first worked out in bacteriophages (bacterial viruses) • Animal virus infections can be either lytic or lysogenic LYTIC... macromolecular structures which serve as protein coat of virus  Protect viral genetic material and aid in its transfer between host cells  Made of protein subunits called protomers Copyright © The McGraw-Hill Companies, Inc Permission required for reproduction or display 15 GENOMIC PACKAGING • Genome packaging has an important role in the infection • Viral genomes are packaged in one of three ways:... with the plasma membrane of the host cell • All envelopes have a phospholipid bilayer VIRAL ENVELOPES Envelopes vary in:  Size  Morphology  Complexity  Composition ENVELOPE GLYCOPROTEINS • They are firmly embedded in the envelope bilayer • This is facilitated by domains of host membrane proteins called spanners • They can form spikes or other structures on the outside of the virion • These can... viruses, there are six steps in lytic infection: • Attachment • Penetration • Uncoating • Biosynthesis • Maturation • Release Attachment Receptor sites:  Specific surface structures on host to which viruses attach  Specific for each virus  Can be proteins, lipopolysaccharides, techoic acids, etc Copyright © The McGraw-Hill Companies, Inc Permission required for reproduction or display 27 TYPES OF. .. reproduction or display 27 TYPES OF RECEPTOR BINDING • Non-enveloped viruses • Binding takes place between viral capsid and receptor Enveloped viruses • Binding takes place between viral envelope proteins and receptor • Entry into the Host  Most bacterial viruses (bacteriophages) inject their nucleic acid into host  Eucaryotic viruses usually enter the cytoplasm with the genome still enclosed Copyright... 29 Bacteriophages 30 Viruslike Agents  Prions 31 Copyright © The McGraw-Hill Companies, Inc Permission required for reproduction or display 32 Assignment  Homework: Important group of RNA viruses Viral Diseases (Next chapter) A B C D E Influenza Diseases Caused by the Herpes Family Viral Hepatitis Human Immunodeficiency Virus Miscellaneous Viral Diseases Bibliography  Microbiology, A clinical Approach... required for reproduction or display 15 GENOMIC PACKAGING • Genome packaging has an important role in the infection • Viral genomes are packaged in one of three ways: • Directly in the capsid-inner side of the protein coat • Enclosed in special proteins-nucleic acid binding protein • Enclosed in proteins from the host cell VIRUS CLASSIFICATION Helical Capsids-Tobacco Mosaic Virus  TMV are shaped like . © Garland Science CHAPTER 12 THE STRUCTURE AND INFECTION CYCLE OF VIRUSES © Eye of Science / Science Photo Library OVERVIEW Viruses Viruses: • are noncellular. acids  Capsid  Envelope 9 …Generalized Structure of Viruses The Structure of Viruses  Virion size range is ~10-400 nm  All virions contain a nucleocapsid which is composed of nucleic