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T4 bacteriophage infecting an E. coli cell 0.5 µm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Comparing the size of a virus, a bacterium, and an animal cell Virus Bacterium Animal cell Animal cell nucleus 0.25 µm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Infection by tobacco mosaic virus (TMV) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Viruses • Size: 20 to ~200nm • Genomes • – dsDNA, ssDNA, dsRNA, ssDNA – Usually one linear or circular molecule Capsids and envelopes – Capsid: protein coat enclosing viral genome – Protein subunits form into regular shapes • Rods, icosahedrons, polyhedral head with a tail – Viral envelope • Derived from host membrane, with embedded viral proteins Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 18.4 Viral structure Capsomere of capsid RNA Capsomere Membranous envelope DNA Head Capsid Tail sheath RNA DNA Tail fiber Glycoprotein Glycoprotein 18 × 250 mm 20 nm (a) Tobacco mosaic virus 70–90 nm (diameter) 80–200 nm (diameter) 50 nm 50 nm (b) Adenoviruses Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (c) Influenza viruses 80 × 225 nm 50 nm (d) Bacteriophage T4 Viral host range • Viruses are obligate intracellular parasites • Requires appropriate host cell for almost all metabolism • Host range limited by – Type of host – Type of cell • Egs. – Measles, polio, smallpox; humans only – West Nile virus; mosquitos, birds, humans – Cold viruses; upper respiratory tract cells only Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings West Nile Virus • MATERIAL SAFETY DATA SHEET - INFECTIOUS SUBSTANCES • SECTION I - INFECTIOUS AGENT • NAME: West Nile Virus • SYNONYM OR CROSS REFERENCE: West Nile encephalitis virus, West Nile encephalitis, WN virus, WNV, arbovirus, viral encephalitis • CHARACTERISTICS: single stranded, positive sense RNA; lipid-enveloped virion 50 nm diameter; family Flaviviridae, genus Flavivirus (1), Japanese encephalitis antigenic complex which includes the Japanese encephalitis, Murray Valley encephalitis, St. Louis encephalitis, and Kunjin viruses (2) • SECTION II - HEALTH HAZARD • PATHOGENICITY: The disease is characterized by the sudden onset of a febrile "flu-like" illness. Most infections are mild to moderate and symptoms can include malaise, anorexia, nausea, vomiting, eye pain, headache, myalgia, rash and lymphadenopathy (3). More severe infections result in aseptic meningitis or encephalitis and symptoms can include meningismus, mental status changes, occasional seizures, and coma. The overall case fatality rate for WNV ranges from 4% to 11% (3). Risk of severe neurologic disease after infection increases markedly among persons 50 years of age and older (3). • EPIDEMIOLOGY: It is indigenous to Africa, Asia (India and Indonesia), Australia and Europe. In recent years, local epidemics have been reported in North America, Romania, Russia, southern France, Israel and the Cape Province of South Africa (4). In temperate and subtropical zones, cases occur in summer or early fall when mosquitoes are most abundant (4). • HOST RANGE: Mammal, reptile (5,6) and avian hosts (4). Mammals (including humans, horses and squirrels) are considered incidental or dead-end hosts (1,4), however viraemia in avians can be sufficient for transmission of infection (1). • INFECTIOUS DOSE: Unknown. • MODE OF TRANSMISSION: Spread by the bite of an infected mosquito. Evidence has also been found demonstrating indirect transmission from person-to-person through blood transfusions (7) and organ donations (8). West Nile virus can also be transmitted from an infected animal-to-person through punctures and cuts. • INCUBATION PERIOD: Usually 3-14 days, with symptoms lasting 3-6 days (3). • COMMUNICABILITY: Not transmitted directly from person-to-person. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Viral reproduction • Genome enters host cell • Host enzymes and components used to make new viral parts • Viral genome and capsid spontaneously selfassemble • Completed, infectious viruses exit the cell Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Simplified viral reproductive cycle Entry into cell and uncoating of DNA DNA VIRUS Capsid Transcription Replication HOST CELL Viral DNA mRNA Viral DNA Capsid proteins Self-assembly of new virus particles and their exit from cell Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Lytic vs lysogenic • Lytic cycle (virulent phage) – Release of virus burst and kills host cell • Lysogenic cycle (temperate phage) – Viral DNA integrates into host genome • 1 phage protein prevents transcription of other phage genes – Can be transmitted to daughter cells – Can initiate lytic cycle in response to environmental signal Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The lytic cycle of phage T4, a 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. The lytic and lysogenic cycles of phage λ, 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 Many cell divisions produce a large population of bacteria infected with the prophage. Lysogenic cycle Certain factors determine whether The cell lyses, releasing phages. Lytic cycle is induced or New phage DNA and proteins are synthesized and assembled into phages. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Lysogenic cycle is entered Prophage Phage DNA integrates into the bacterial chromosome, becoming a prophage. The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells. Classes of Animal Viruses, FYI only Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Animal viruses • Viral envelopes – Derived from plasma membrane of previous host cell – Used to enter new host cell – Viral proteins studded in envelope • Bind to specific receptors on target cell Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 18.8 The reproductive cycle of an enveloped RNA virus 1 Glycoproteins on the viral envelope bind to specific receptor molecules (not shown) on the host cell, promoting viral entry into the cell. Capsid RNA Envelope (with glycoproteins) 2 Capsid and viral genome enter cell HOST CELL Viral genome (RNA) Template 5 Complementary RNA strands also function as mRNA, which is translated into both capsid proteins (in the cytosol) and glycoproteins for the viral envelope (in the ER). 3 The viral genome (red) functions as a template for synthesis of complementary RNA strands (pink) by a viral enzyme. mRNA Capsid proteins ER Glycoproteins Copy of genome (RNA) 4 New copies of viral genome RNA are made using complementary RNA strands as templates. 6 Vesicles transport envelope glycoproteins to the plasma membrane. 8 New virus 7 A capsid assembles around each viral genome molecule. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The structure of HIV, the retrovirus that causes AIDS Glycoprotein Viral envelope Capsid Reverse transcriptase Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings RNA (two identical strands) Retroviruses • RNA virus with a reverse transcriptase – Enzyme transcribes RNA to DNA • Viral DNA enters host DNA (provirus) – Integrates and becomes permanent in DNA • Releases new virus from host cell – Eventually host cell dies from viral reproduction effects or apoptosis Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Provirus 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. RNA genome for the next viral generation mRNA 6 The viral proteins include capsid proteins and reverse transcriptase (made in the cytosol) and envelope glycoproteins (made in the ER). New HIV leaving a cell 9 New viruses bud off from the host cell. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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. AIDS • Infects helper T-cells (TH), a critical component of immune system • TH cells facilitate communication between immune cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings AIDS • Immune dysfunction leads to opportunistic infections • Viral infections usually controlled by vaccines – Antibiotics are NOT effective – There are some new antiviral medications but…. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings SARS (severe acute respiratory syndrome), a recently emerging viral disease (NOT on exam) (a) Young ballet students in Hong Kong wear face masks to protect themselves from the virus causing SARS. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (b) The SARS-causing agent is a coronavirus like this one (colorized TEM), so named for the “corona” of glycoprotein spikes protruding from the envelope. Model for how prions propagate (NOT on exam) Prion Original prion Many prions Normal protein New prion Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 18.15 Can a bacterial cell acquire genes from another bacterial cell? (NOT on exam) EXPERIMENT Researchers had two mutant strains, one that could make arginine but not tryptophan (arg+ trp–) and one that could make tryptophan but not arginine (arg– trp+). Each mutant strain and a mixture of both strains were grown in a liquid medium containing all the required amino acids. Samples from each liquid culture were spread on plates containing a solution of glucose and inorganic salts (minimal medium), solidified with agar. Mixture Mutant strain arg+ trp– Mutant strain arg− trp+ RESULTS Only the samples from the mixed culture, contained cells that gave rise to colonies on minimal medium, which lacks amino acids. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (NOT on exam) Mixture Mutant strain arg+ trp– Mutant strain arg– trp+ No colonies (control) Colonies grew No colonies (control) CONCLUSION Because only cells that can make both arginine and tryptophan (arg+ trp+ cells) can grow into colonies on minimal medium, the lack of colonies on the two control plates showed that no further mutations had occurred restoring this ability to cells of the mutant strains. Thus, each cell from the mixture that formed a colony on the minimal medium must have acquired one or more genes from a cell of the other strain by genetic recombination. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Bacterial conjugation (NOT on exam) Sex pilus Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 1 µm