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WHAT DOES IT MEAN TO BE ALIVE? Are the tiny viruses infecting this E. coli cell alive? 0.5 mm “I’D LIKE TO INFECT YOU WITH MY DNA” (VIRAL REPRODUCTION) ;) VIRAL VIDEO LINKS NPR podcast : how viruses invade your body SciShow: Top 5 Deadliest Disease!! LECTURE PRESENTATIONS For CAMPBELL BIOLOGY, NINTH EDITION Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson Chapter 19 Viruses Lectures by Erin Barley Kathleen Fitzpatrick © 2011 Pearson Education, Inc. Concept 19.1: A virus consists of a nucleic acid surrounded by a protein coat • Viruses were detected indirectly long before they were actually seen © 2011 Pearson Education, Inc. The Discovery of Viruses: Scientific Inquiry • Tobacco mosaic disease stunts growth of tobacco plants and gives their leaves a mosaic coloration • In the late 1800s, researchers hypothesized that a particle smaller than bacteria caused the disease • In 1935, Wendell Stanley confirmed this hypothesis by crystallizing the infectious particle, now known as tobacco mosaic virus (TMV) © 2011 Pearson Education, Inc. Figure 19.2 RESULTS 3 Rubbed filtered 1 Extracted sap 2 Passed sap through a sap on healthy from tobacco porcelain filter tobacco plants plant with known to trap tobacco mosaic bacteria disease 4 Healthy plants became infected Structure of Viruses • Viruses are not cells, they are infectious particles © 2011 Pearson Education, Inc. Capsids and Envelopes • A capsid is the protein shell that encloses the viral genome • Capsids are built from protein subunits called capsomeres • A capsid can have various structures © 2011 Pearson Education, Inc. Examples of viral structure Capsomere RNA DNA Membranous RNA envelope Capsid Head DNA Tail sheath Capsomere of capsid Tail fiber Glycoprotein 18 250 nm 20 nm (a) Tobacco mosaic virus Glycoproteins 70–90 nm (diameter) 80–200 nm (diameter) 50 nm (b) Adenoviruses 80 225 nm 50 nm 50 nm (c) Influenza viruses (d) Bacteriophage T4 Concept 19.2: Viruses replicate only in host cells • Viruses are obligate intracellular parasites, which means they can replicate only within a host cell using the cell’s parts, much like… © 2011 Pearson Education, Inc. General Features of Viral Replicative Cycles • Once a viral genome has entered a cell, the cell begins to manufacture viral proteins • The virus makes use of host enzymes, ribosomes, tRNAs, amino acids, ATP, and other molecules • Viral nucleic acid molecules and capsomeres spontaneously self-assemble into new viruses © 2011 Pearson Education, Inc. Animation: Simplified Viral Reproductive Cycle Right-click slide / select “Play” © 2011 Pearson Education, Inc. 1 Entry and uncoating DNA VIRUS 3 Transcription and manufacture of capsid proteins Capsid 2 Replication HOST CELL Viral DNA mRNA Viral DNA Capsid proteins 4 Self-assembly of new virus particles and their exit from the cell A simplified viral replicative cycle. Replicative Cycles of Phages • Phages are the best understood of all viruses • Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle (see animations that follow) © 2011 Pearson Education, Inc. Animation: Phage T4 Lytic Cycle Right-click slide / select “Play” © 2011 Pearson Education, Inc. Figure 19.5-1 1 Attachment Figure 19.5-2 1 Attachment 2 Entry of phage DNA and degradation of host DNA Figure 19.5-3 1 Attachment 2 Entry of phage DNA and degradation of host DNA 3 Synthesis of viral genomes and proteins Figure 19.5-4 1 Attachment 2 Entry of phage DNA and degradation of host DNA Phage assembly 4 Assembly Head Tail Tail fibers 3 Synthesis of viral genomes and proteins Figure 19.5-5 1 Attachment 2 Entry of phage DNA and degradation of host DNA 5 Release Phage assembly 4 Assembly Head Tail Tail fibers 3 Synthesis of viral genomes and proteins The Lysogenic Cycle • The lysogenic cycle replicates the phage genome without destroying the host • The viral DNA molecule is incorporated into the host cell’s chromosome • This integrated viral DNA is known as a prophage • Every time the host divides, it copies the phage DNA and passes the copies to daughter cells © 2011 Pearson Education, Inc. Animation: Phage Lambda Lysogenic and Lytic Cycles Right-click slide / select “Play” © 2011 Pearson Education, Inc. • An environmental signal can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode • Phages that use both the lytic and lysogenic cycles are called temperate phages © 2011 Pearson Education, Inc. Figure 19.6 Phage DNA Daughter cell with prophage The phage injects its DNA. Cell divisions produce a population of bacteria infected with the prophage. Phage DNA circularizes. Phage Bacterial chromosome Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Lytic cycle The cell lyses, releasing phages. Lysogenic cycle Certain factors determine whether lytic cycle is induced New phage DNA and proteins are synthesized and assembled into phages. or lysogenic cycle is entered Prophage The bacterium reproduces, copying the prophage and transmitting it to daughter cells. Phage DNA integrates into the bacterial chromosome, becoming a prophage. Capsid Capsid and viral genome enter the cell RNA Envelope (with glycoproteins) HOST CELL Viral genome (RNA) Template mRNA ER Capsid proteins Copy of genome (RNA) Glycoproteins The replicative cycle of an enveloped RNA virus. New virus RNA as Viral Genetic Material • The broadest variety of RNA genomes is found in viruses that infect animals • Retroviruses use reverse transcriptase to copy their RNA genome into DNA • HIV (human immunodeficiency virus) is the retrovirus that causes AIDS (acquired immunodeficiency syndrome) © 2011 Pearson Education, Inc. Glycoprotein Viral envelope HIV Capsid Reverse transcriptase HIV RNA (two identical strands) Membrane of white blood cell HOST CELL Reverse transcriptase Viral RNA RNA-DNA hybrid 0.25 m DNA HIV entering a cell NUCLEUS Provirus Chromosomal DNA RNA genome for the next viral generation mRNA New virus HIV: a retrovirus New HIV leaving a cell Animation: HIV Reproductive Cycle Right-click slide / select “Play” © 2011 Pearson Education, Inc. Concept 19.3: Viruses, viroids, and prions are formidable pathogens in animals and plants © 2011 Pearson Education, Inc. Why viruses cause disease in animals, other than by lysis • Viruses may damage or kill cells by causing the release of hydrolytic enzymes from lysosomes • Some viruses cause infected cells to produce toxins that lead to disease symptoms • Others have molecular components such as envelope proteins that are toxic © 2011 Pearson Education, Inc. • Vaccines are harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the harmful pathogen – Vaccines can prevent certain viral illnesses • Viral infections cannot be treated by antibiotics • Antiviral drugs can help to treat, though not cure, viral infections © 2011 Pearson Education, Inc. Emerging Viruses • Mostly spread from animals or massively mutated pre-existing viruses – The 2009 flu pandemic was likely passed to humans from pigs; for this reason it was originally called the “swine flu” © 2011 Pearson Education, Inc. Figure 19.9 1 m (a) 2009 pandemic H1N1 (b) 2009 pandemic screening influenza A virus (c) 1918 flu pandemic Viroids and Prions: The Simplest Infectious Agents • Viroids are small circular RNA molecules that infect plants and disrupt their growth • Prions are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals – Prions propagate by converting normal proteins into the prion version – Scrapie in sheep, mad cow disease, and CreutzfeldtJakob disease in humans are all caused by prions © 2011 Pearson Education, Inc. Figure 19.11 Prion Normal protein Original prion New prion Aggregates of prions Test your understanding A Time Number of viruses Number of bacteria Test Your Understanding, question 8 B Time Test Your Understanding, question 6