Viruses. Early events. Overview. Virus Characteristics. Growth of Viruses. Viral Entry. Microbiology Chapters 16-18

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1 Overview Viruses Microbiology Chapters What is a virus? Genetic element within a protein coat that requires a host cell for replication, and has an extracellular phase (virion( virion). Virus characteristics Virus growth Virus Life Cycle Bacteriophages Virus Characteristics Nucleic Acid genome single-stranded(ss stranded(ss) ) or double- stranded(ds) RNA or DNA kb generally have protein coat (capsid)) in the form of filaments, icosahedrons or both may be surrounded by envelope nm No intrinsic metabolism - parasitic on host (energy, building blocks) Growth of Viruses Usually injected into chicken eggs when being used for vaccine production. Grown in cultured host cells - virus growth and lysis of host cells leads to plaque formation Viral Entry Steps in generic viral life cycle Attachment to cell surface Receptors - site of host/tissue specificity flagella, pili,, maltose transporter In bacteriophages,, nucleic acids are injected into cell restriction enzymes are defense against viruses What about host DNA? endocytosis or membrane fusion with eukaryotic viruses Early events Production viral proteins in preparation for later steps method of mrna synthesis varies with virus mrna (coding) strand designated as + + strand ssdna viruses generally converted to ds (replicative form - Mu phagemids) + strand RNA viruses can be translated (producing replicase to make - strand and then more + strands) - strand RNA viruses must contain replicase in virion to synthesize + strand retroviruses (ssrna( ssrna) ) contain reverse transcriptase (RNA --> > DNA) 1

2 Generic Viral Life Cycle Bacteriophage lambda life cycle Replication of viral genome Synthesis of structural proteins Assembly/packaging of virions Release from cell lysis budding Temperate phage - has both lytic and lysogenic pathways Lysogeny - integration of viral DNA into host chromosome Attachment to maltose transporter, injection of dsdna, circularization Bacteriophage lambda life cycle ciii protein stabilizes cii (subject to degradation by HflA protease, whose abundance is controlled by nutrient/dna status of cell). cii acts as an activator of ci (lambda repressor) and int (integrase)) expression ci repressor inhibits expression of all other phage genes integrase catalyzes insertion of lambda DNA to form prophage Bacteriophage lambda life cycle Lytic phase DNA damage (SOS response) activates protease activity of reca,, which breaks down lambda repressor cro expressed, blocks expression of ci,, allows expression of other lambda genes necessary for virion production Extra credit projects Viruses-2 Microbiology March 26, 2004 Chapters 18,38 Projects may be done as teams (points get split among team members) Projects will be displayed in Microbiology Lab, therefore must include an easily viewable key with your names, date and bulleted chart detailing features of your project Winogradsky column Key should include sources of materials and types of microbes expected to be growing at various locations 2

3 Extra credit projects Specific Bacterial Physical Model (40 pts max) Scale is 100,000x (i.e. 10 cm = 1 um) Demonstrate ribosomes how many, size Chromosomes, plasmids length Cell surface layers, structures (capsule, flagella, pili) Inclusion bodies Viral Physical Model (30 pts max) Scale is 1,000,000x (i.e. 1 cm = 10 nm) Demonstrate Nucleic acid length Layers, envelope Surface proteins Animal Viruses Classification based on nucleic acid, presence of envelope, size Picornaviruses picornaviruses - + strand RNA poliovirus, hepatitis A, Common cold, plant viruses RNA translated to polyprotein, which self cleaves Replicase synthesizes neg. strand (up to 10 3 copies), then more positive strands(10 6 ) a viral protein inhibits host cell protein synthesis some + strands translated virus packaged into protein coat, lyses cell - strand RNA viruses - strand RNA viruses - enveloped Rhabdoviruses (rabies), orthomyxoviruses (influenza), paramyxoviruses (measles, mumps) membrane envelope containing viral proteins that bind receptors Rabies - acetylcholine receptor paramyxovirus - respiratory epithelium rabies transmitted by animal bites (saliva) causes drooling, difficulty swallowing, neurological problems, respiratory failure Measles/German Measles Both cause rashes, controlled by MMR Rubella - German severe damage to fetuses Rubeola - more severe fever + Koplik s spots can cause encephalitis Influenza Influenza virus has segmented genome - causing antigenic shift hemagglutinin,, neuraminidase on surface Main reservoir is pigs often infected with multiple strains farming practices in asia allow easy transmission to humans Genomes can recombine, creating new variants that the human population has not been expsoed to before - lack immunity Creates pandemics (world-wide wide epidemics) about every 20 years 3

4 Ds DNA viruses herpesviruses - mononucleosis, chickenpox, shingles, herpes (simplex), Epstein-Barr enveloped - from nuclear membrane latent form in sensory ganglia pox viruses - smallpox, cowpox, rabbit myxomatosis virus, vaccinia Baculovirus - used for protein production in insect cell cultures Chickenpox/Shingles Varicella-Zoster virus a herpesvirus 3 million US cases/yr lesions appear 2 weeks after exposure Virus can become latent in neurons reactivation causes shingles Shingles lesions typically form along neuron in which virus was reactivated Herpes Painful sores most infectious when sores are present virus becomes latent in neurons Poxviruses Smallpox eradicated Monkeypox - danger of jump to humans Molluscum contagiosum - induces tumor-like growths 4

5 Warts Caused by papillomaviruses generally removed by cryotherapy 5