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The Genetics of Viruses I. II. III. Background Structure Life Cycles Viral infections, past & present Viral infection have been one of the major infectious challenges of the human species, for as far back as we can tell. Polio Ebola Virus Herpes Virus Severe acute respiratory syndrome (SARS) (b) The SARS-causing agent is a coronavirus (a) Young ballet students in Hong Kong like this one (colorized TEM), so named for the wear face masks to protect themselves “corona” of glycoprotein spikes protruding from from the virus causing SARS. the envelope. Figure 18.11 A, B Let’s size them up… Compare the size of: eukaryotic cell, bacterial cell and a virus I. Background Everyone is at risk for infection! Bacteriophages infect bacteria only All eukaryotes (animals, plants, fungi, protist) all vulnerable Size 20nm-250nm Ghost phage 0.5 m Viral Diseases in Plants More than 2,000 types of viral diseases of plants are known. Spots on leaves and fruits, stunted growth, and damaged flowers or roots Tobacco Mosaic Virus I. Background Virus Discovery: Tobacco mosaic disease (1930’s) Stunts growth produces the speckled coloration I. Background Viral Evolution proposal: Fragments of cellular nucleic acid Reproduce within host cells only (non-living) Obligate intracellular parasites Host range II. Structure of viruses II. Structure of Viruses 1. Nucleic acid Enclosed in a protein coat Genomes may be ds/ss DNA ds/ss RNA II. Structure of Viruses 2. Capsids Protein Various shapes & structures Capsids are produced by host Capsomere of capsid RNA Capsomere DNA Glycoprotein 70–90 nm (diameter) 18 250 mm 20 nm 50 nm (a) Tobacco mosaic virus (b) Adenoviruses Viral structure II. Structure of Viruses 3. Envelopes Found in many animal viruses Glycoprotein and lipids Derived by host “Spikes” fuse with membrane or receptor mediated entry Membranous envelope Capsid RNA Glycoprotein 80–200 nm (diameter) 50 nm (c) Influenza viruses Receptor mediated model Fusion Model Basic Infection: Just Genome and Capsid QuickTime™ and a Cinepak decompressor are needed to see this picture. III. Life Cycles Bacteria and Eukaryotic Models III. Life Cycles: 1. Bacteriophages complete two reproductive mechanisms: Head Tail sheath lytic cycle lysogenic cycle DNA Tail fiber 80 225 nm Figure 18.4d 50 nm (d) Bacteriophage T4 III. Life Cycles A. Lytic Cycle: Digests the host’s cell wall, releasing the progeny viruses Kills host Virulent phage 1 Attachment. The T4 phage uses its tail fibers to bind to specific receptor sites on the outer surface of an E. coli cell. 5 Release. The phage directs production of an enzyme that damages the bacterial cell wall, allowing fluid to enter. The cell swells and finally bursts, releasing 100 to 200 phage particles. 2 Entry of phage DNA and degradation of host DNA. The sheath of the tail contracts, injecting the phage DNA into the cell and leaving an empty capsid outside. The cell’s DNA is hydrolyzed. Phage assembly 4 Assembly. Three separate sets of proteins self-assemble to form phage heads, tails, and tail fibers. The phage genome is packaged inside the capsid as the head forms. Head Tails Tail fibers 3 Synthesis of viral genomes and proteins. The phage DNA directs production of phage proteins and copies of the phage genome by host enzymes, using components within the cell. QuickTime™ and a Cinepak decompressor are needed to see this picture. Life Cycles B. Lysogenic cycle Incorporate viral DNA into bacteria genome (propahge) Phage genome is replicated (for free!) w/o destroying the host Temperate phages capable of using both cycles The lytic and lysogenic cycles of phage , a temperate phage Phage DNA The phage attaches to a host cell and injects its DNA. Phage DNA circularizes Phage Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Bacterial chromosome Lytic cycle The cell lyses, releasing phages. Lysogenic cycle Certain factors determine whether Lytic cycle is induced Figure 18.7 Many cell divisions produce a large population of bacteria infected with the prophage. New phage DNA and proteins are synthesized and assembled into phages. or Lysogenic cycle is entered Prophage The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells. Phage DNA integrates into the bacterial chromosome, becoming a prophage. QuickTime™ and a Cinepak decompressor are needed to see this picture. III: Life Cycles 2. RNA viruses typically infect animals Retroviruses (HIV), use reverse transcriptase to make cDNA Integrated into genome, provirus QuickTime™ and a Cinepak decompressor are needed to see this picture. • The reproductive cycle of HIV, a retrovirus HIV Membrane of white blood cell 1 The virus fuses with the cell’s plasma membrane. The capsid proteins are removed, releasing the viral proteins and RNA. 2 Reverse transcriptase catalyzes the synthesis of a DNA strand complementary to the viral RNA. HOST CELL 3 Reverse transcriptase catalyzes the synthesis of a second DNA strand complementary to the first. Reverse transcriptase Viral RNA RNA-DNA hybrid 4 The double-stranded DNA is incorporated as a provirus into the cell’s DNA. 0.25 µm HIV entering a cell DNA NUCLEUS Chromosomal DNA RNA genome for the next viral generation Provirus mRNA 5 Proviral genes are transcribed into RNA molecules, which serve as genomes for the next viral generation and as mRNAs for translation into viral proteins. 6 The viral proteins include capsid proteins and reverse transcriptase (made in the cytosol) and envelope glycoproteins (made in the ER). Figure 18.10 New HIV leaving a cell 9 New viruses bud off from the host cell. 8 Capsids are assembled around viral genomes and reverse transcriptase molecules. 7 Vesicles transport the glycoproteins from the ER to the cell’s plasma membrane. III: Life Cycles Life after infection Viruses may damage or kill cells Tissue damage Toxins that lead to disease symptoms may be produced Asymptomatic Sources http://www.aapsj.org/ http://www.stanford.edu/group/virus/1999/jchow/rep .html http://pathmicro.med.sc.edu/mhunt/RNA-HO.htm http://www.brooklyn.cuny.edu/bc/ahp/LAD/C5/C5_V iruses.html http://www.biology.com Campbell Reece Mitchell. Biology, Prentice Hall 1999, 2001 Sherris. Medical Microbiology: An introduction to infectious disease, Appleton and Lange,1990 http://teachers.eastern.k12.nj. us/nabi/biology/index.html Good links