An Introduction to the Viruses Chapter 6 Copyright © The McGraw-Hill Companies, Inc) Permission required for reproduction or display.

Learning Objectives: Differentiate a virus from a bacterium. Define viral species. Give an example of a family, genus, and a common name for a virus. Describe the structure of an enveloped and a nonenveloped virus. Describe viral infectious cycle including lytic and lysogenic cycles. Describe how bacteriophages and animal viruses are cultured Describe the relationship between viruses and cancer. Provide an example of a latent viral infection Discuss how a protein can be infectious

Why are viruses important? Health—HIV is a leading cause of death in Africa Economics—Banana Bunchy Top Virus (BBTV) damages banana crops in Asia and the Pacific annually costing millions Important participant in the evolution of life forms % of human genome came from viruses! Gene Therapy—Viruses are efficient at introducing foreign DNA into a cell

The Scale of Viruses Too small for the light microscope Range from about 20 to 200 nm ( µm) 1. (1) (2) (3) (4) (6) (7) (8) (9) (10) BACTERIA CELLS Rickettsia 0.3 µm Viruses Mimivirus 450nm Herpes simplex 150 nm Rabies 125 nm HIV 110 nm Influenza 100 nm T2 bacteriophage 65 nm Poliomyelitis 30 nm yellow fever 22 nm Protein Molecule Hemoglobin 15 nm molecule (5) E. coli 2 µm long Streptococcus 1 µm Adenovirus 75 nm Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Viral Classification Type of nucleic acid (DNA, RNA) Strandedness (single vs double) Capsid morphology Presence or absence of envelope Host range

Virus taxonomy International Committee on the Taxonomy of Viruses (ICTV): 3 orders, 73 families, 287 genera Order names end in virales Family names end in viridae Genus names end in virus As an example, consider the virus from the family Herpesviridae belonging to the genus Simplexvirus, human herpesvirus 2

7 Viral Components

Generalized Structure of Viruses 8

Helical Capsid 9 Rod-shaped capsomeres assembled into a series of hollow disks Naked: Tobacco mosaic virus (rigid) Enveloped : Influenza virus (more flexible)

Icosahedral Viruses Ring- or dome- shaped capsomeres Spherical or cubical capsid with nucleic acid packed inside 10

Complex Virus Structure Found in bacteriophages 11

Viral Envelope Found in some, but not all viruses Derived from the membranes of the host cell Formed by budding Viral proteins (spikes) Functions: protection, host penetration, generation of immune response (c) Envelope Nucleocapsid Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Viral Enzymes Found In nucleocapsid Polymerases Reverse transcriptase Found in envelope Hemagglutinin Neuraminidase

Viral Genome DNA or RNA, but not both Single or double stranded RNA: sense or anti-sense, segmented or non- segmented RNA virus with encoded reverse transcriptase (retrovirus)

Virus Infectious Cycle Adsorption – specific attachment Penetration – entry of viral genome Uncoating – release of viral genome Synthesis – new viral products made Assembly – new viruses are made in the cell Release – often causes the host cell to lyse

16 Virus Infectious Cycle

Virus Adsorption Receptors are glycoproteins necessary for cell’s normal function Enveloped viruses use envelope spikes Naked viruses use capsid proteins Host range

Penetration/Uncoating (a)Endocytosis (b)Fusion with cell membrane

Replication of DNA Viruses

Release

Lytic Bacteriophage Virus binds DNA injected Capsid stays outside Expression Assembly Cell destroyed Head Bacterial cell wall Tube Viral nucleic acid Cytoplasm Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Lytic and Lysogenic Cycles Adsorption E. coli host Lysogenic State Release Penetration Bacteriophage + Sheath Tail fibersTail Viral DNA becomes latent as prophage. DNA splits Viral DNA Bacterial DNA molecule Spliced viral genome Replication Lytic Cycle Bacterial DNA Viral DNA Lysis Maturation CapsidDNA Assembly Bacteriophage The lysogenic state in bacteria. Infection followed by integration into the genome (prophage) Lysogenic conversion

Growing Bactriophages in the Lab Bacteria cultures Plaque assays (used to visualize the presence of virus)

Growing Animal Viruses in the Lab Living animals Embrionated eggs (pocks, embrio death) Cell cultures (cytopathic effects)

Oncogenic Viruses Oncogene: cancer causing alteration of cellular DNA Transformation HPV-16: cervical cancer Epstein-Barr virus: Burkitt’s lymphoma HBV: liver cancer HTLV-1 and 2: T-cell leukemia

Latent Viral Infections Latency: the ability of a pathogenic virus to lie dormant (latent) within a cell. Herpes simplex virus Chicken pox virus

Viroids Plant pathogens Naked RNA Single-stranded Circles Very small (100 to 300 nucleotides)

Prions Animal pathogens “Slow viruses” Spongiform encephalopathy Kuru Mad Cow Disease Creutzfeld-Jakob Disease Defectively folded host protein