Microbiology Lecture 5 Structure of Viruses Professor T.J. Foster
Relative Sizes of Viruses, Bacterial and Mammalian Cells
Infectious agents Viruses DNA or RNA genomes Nucleic acid protected by protein capsid Depends on host cell for energy, building blocks (amino acids, nucleotides) and ribosomes. Only propagates within host cell Obligate intracellular parasites Bacterial, plant, and animal viruses
Definitions and Terminology Virion: complete virus particle Capsid: protein shell encasing genome Capsomer: protein subunit of capsid Nucleocapsid: capsid containing nucleic acid Attachment protein: surface component for attachment to receptor on host cell eg spike. Visible (by electron microscopy) attachment appendage (see adenovirus, next slide)
Diversity of Virus Capsid Shapes Spherical (Icosohedron) 20-faced polyhedron eg. Adenovirus 252 identical capsomers. Each capsomer has 6 neighbours (hexon). Except in 12 vertices, 5 neighbours (pentons)
Diversity of Virus Capsid Shapes Rod (rigid filament) Tobacco Mosaic Virus Central holow core 49 capsomers every 3 turns Helix 1 capsomer protects 3 nucleotides
Tobacco Mosaic Virus X Ray crystal structure
Filamentous Viruses Nucleocapsid - flexible helix eg the paramyxovirus, the Mumps virus
Bullet Shaped eg the rabies virus
Complex Cowpox virus (Vaccinia) External surface series of parallel rows Complex particle >100 proteins Contains 2 membranes Core nucleoprotein with at least 10 enzymes
Envelopes Some animal viruses have a membranous outer envelope Derived from host cell membrane Contains viral glycoproteins (proteins with polysaccharide chains attached, synthesized in endoplasmic reticulum) Host receptor recognition, attachment
Herpes Virus Structure
The Five Basic Structural Forms Naked Icosahedron Adenovirus, poliovirus, Naked helical (not found in animals) Tobacco mosaic virus Enveloped Icosohedron Herpes virus, rubella virus, Enveloped helical Rabies virus, influenza virus, mumps virus, measles virus Complex Pox viruses
Bacteriophage Viruses that Infect Bacteria Icosohedral capsid contains nucleic acid Tail fibres. Attachment to bacterial cell surface
Bacteriophage Bacteriophage nucleocapsid does NOT enter bacterial cell. DNA is INJECTED through cell wall. Tail lysozyme degrades peptidoglycan. Sheath contracts. Syringe-like mechanism injects DNA into cytoplasm.
Classification. The Baltimore Scheme Based on Nucleic Acid Composition of Genome, Replication and Information Flow Classification imposes order out of chaos
DNA Double stranded ds DNA
DNA REPLICATION ds DNA DNA polymerase ds DNA
The Central Dogma Information Flow DNA > mrna > Protein
+ _ TRANSCRIPTION +
TRANSCRIPTION
Transcription ds DNA + ss mrna +
Group Genome Baltimore Classification of Viruses Genetic Information. From genome to message Example 1 ds DNA dsdna mrna Herpes simplex virus 2 ss DNA ssdna dsdna mrna Parvovirus 3 ds RNA dsrna mrna Reovirus 4 + ss RNA Serves as mrna 5 - ss RNA mrna template dsrna +ssrna (mrna) Enterovirus dsrna -ssrna mrna Influenza 6 ssrna ssrna dsdna mrna Retrovirus 7 Nicked ds DNA nicked ds intact ds mrna DNA DNA RNA replicate thru RNA Hepatitis B virus intermediate
Adenovirus Baltimore 1. Double stranded DNA genome Non-enveloped icosohedron Common human pathogen Self-limiting upper respiratory tract infections
Paramyxoviruses Mumps and Measles Viruses Baltimore 5. Negative single stranded RNA genomes Enveloped, filamentous Vaccination. Measles Mumps Rubella (MMR) Measles rash, raised spots. Middle ear infection. Pneumonia, Encephalitis (1:1000), Fatal (1:3000) Mumps Swollen parotid gland
Baltimore 5 Negative single strand RNA 8 segmented genome Enveloped filamentous Seasonal flu vaccine Influenza Virus
Herpes Viruses Baltimore 1. Double stranded DNA Enveloped icosohedron Herpes infections of man. Herpes Simplex Viruses (HSV) Cold sore on lip Genital
Herpes Viruses HSV-1 Enters nerve cell Migrates to trigeminal ganglia near ear. Virus remains dormant Re-activation Migrates to tip of nerve cell. Infects epithelial cells in lip.
Rubella Virus Baltimore 4. Positive single-stranded RNA genome Enveloped icosohedron German measles, mild rash Congenital rubella syndrome Foetal damage MMR Vaccine
Retrovirus Baltimore 6 Positive single stranded RNA plus reverse transcriptase Diploid Enveloped non-icosohedral elongated Human Immunodeficiency Virus (HIV) Acquired Immunodeficiency Diseases (AIDS) No vaccine Anti-retrovirus drugs
Pox Viruses Baltimore 1. Double stranded DNA Complex structure Smallpox 20% mortality. Scarring Vaccinia (cowpox) virus Jenner s pioneering vaccination Eradicated by WHO vaccination programme