Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings PowerPoint ® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Chapter 19 Viruses

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Overview: A Borrowed Life Viruses lead “a kind of borrowed life” between life-forms and chemicals Viruses have a genome but can only reproduce within a host cell The origins of molecular biology lie in early studies of viruses that infect bacteria

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Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Concept 19.1: A virus consists of a nucleic acid surrounded by a protein coat Viruses are not cells Viruses are very small infectious particles Viruses are classified according to: – ___________________________________

Fig RNA Capsomere of capsid DNA Glycoprotein 18  250 nm 70–90 nm (diameter) Glycoproteins 80–200 nm (diameter) 80  225 nm Membranous envelope RNA Capsid Head DNA Tail sheath Tail fiber 50 nm 20 nm (a) Tobacco mosaic virus (b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Concept 19.2: Viruses reproduce only in host cells Viruses are obligate intracellular parasites, which means __________________________ _____________________________________ Each virus has a host range, a limited number of host cells that it can infect

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings General Features of Viral Reproductive 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 Animation: Simplified Viral Reproductive Cycle Animation: Simplified Viral Reproductive Cycle

Transcription and manufacture of capsid proteins Self-assembly of new virus particles and their exit from the cell Entry and uncoating Fig VIRUS DNA Capsid 4 Replication HOST CELL Viral DNA mRNA Capsid proteins Viral DNA

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Reproductive Cycles of Phages Phages are the best understood of all viruses Phages have two reproductive mechanisms: – _________________________

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Lytic Cycle _____________________________________ The lytic cycle produces new phages and digests the host’s cell wall, releasing the progeny viruses A phage that reproduces only by the lytic cycle is called a ______________________ Animation: Phage T4 Lytic Cycle Animation: Phage T4 Lytic Cycle

Fig Attachment 1

Fig Entry of phage DNA and degradation of host DNA Attachment 1 2

Fig Synthesis of viral genomes and proteins Entry of phage DNA and degradation of host DNA Attachment 1 2 3

Fig Phage assembly Assembly Synthesis of viral genomes and proteins Entry of phage DNA and degradation of host DNA Attachment HeadTailTail fibers 3

Fig Phage assembly HeadTailTail fibers Assembly Release Synthesis of viral genomes and proteins Entry of phage DNA and degradation of host DNA Attachment

Why aren’t all bacteria exterminated by bacteriophages?

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Lysogenic Cycle _____________________________________ The viral DNA molecule is incorporated into the host cell’s chromosome This integrated viral DNA is known as a ________________ Every time the host divides, it copies the phage DNA and passes the copies to daughter cells Animation: Phage Lambda Lysogenic and Lytic Cycles Animation: Phage Lambda Lysogenic and Lytic Cycles

Fig Phage DNA Phage The phage injects its DNA. Bacterial chromosome Phage DNA circularizes. Daughter cell with prophage Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Cell divisions produce population of bacteria infected with the prophage. The cell lyses, releasing phages. Lytic cycle is induced or Lysogenic cycle is entered Lysogenic cycle Prophage The bacterium reproduces, copying the prophage and transmitting it to daughter cells. Phage DNA integrates into the bacterial chromosome, becoming a prophage. New phage DNA and proteins are synthesized and assembled into phages.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Reproductive Cycles of Animal Viruses There are two key variables used to classify viruses that infect animals: – DNA or RNA? – Single-stranded or double-stranded?

Table 19-1a

Table 19-1b

Fig Capsid RNA Envelope (with glycoproteins) Capsid and viral genome enter the cell HOST CELL Viral genome (RNA) Template mRNA ER Glyco- proteins Capsid proteins Copy of genome (RNA) New virus Many viruses that infect animals have a membranous viral envelope

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings RNA as Viral Genetic Material The broadest variety of RNA genomes is found in viruses that infect animals Retroviruses: _____________________________________ _____________________________________

Fig Glycoprotein Viral envelope Capsid RNA (two identical strands) Reverse transcriptase HIV Membrane of white blood cell HIV entering a cell 0.25 µm Viral RNA RNA-DNA hybrid HOST CELL Reverse transcriptase DNA NUCLEUS Provirus Chromosomal DNA RNA genome for the next viral generation mRNA New virus New HIV leaving a cell

Fig. 19-8a Glycoprotein Reverse transcriptase HIV RNA (two identical strands) Capsid Viral envelope HOST CELL Reverse transcriptase Viral RNA RNA-DNA hybrid DNA NUCLEUS Provirus Chromosomal DNA RNA genome for the next viral generation mRNA New virus

Fig. 19-8b HIV Membrane of white blood cell HIV entering a cell 0.25 µm New HIV leaving a cell

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Concept 19.3: Viruses, viroids, and prions are formidable pathogens in animals and plants Diseases caused by viral infections affect humans, agricultural crops, and livestock worldwide Smaller, less complex entities called viroids and prions also cause disease in plants and animals, respectively

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Viral Diseases in Animals 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 envelope proteins that are toxic

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Vaccines are harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen Vaccines can prevent viral illness but once a disease occurs there is little that can be done to cure it

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Emerging Viruses Emerging viruses are those that appear suddenly or suddenly come to the attention of scientists Examples?

Fig. 19-9a (a) The 1918 flu pandemic

Fig. 19-9b (b) Influenza A H5N1 virus 0.5 µm

Fig. 19-9c (c) Vaccinating ducks

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Viral Diseases in Plants More than 2,000 types of viral diseases of plants are known and cause spots on leaves and fruits, stunted growth, and damaged flowers or roots Most plant viruses have an RNA genome

Fig

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Plant viruses spread disease in two major modes: – ___________________________________ ___________________________________

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Viroids and Prions: The Simplest Infectious Agents Viroids are 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 Creutzfeldt-Jakob disease in humans are all caused by prions

Fig Prion Normal protein Original prion New prion Aggregates of prions

Fig. 19-UN1 Phage DNA Bacterial chromosome The phage attaches to a host cell and injects its DNA Prophage Lysogenic cycle Temperate phage only Genome integrates into bacterial chromosome as prophage, which (1) is replicated and passed on to daughter cells and (2) can be induced to leave the chromosome and initiate a lytic cycle Lytic cycle Virulent or temperate phage Destruction of host DNA Production of new phages Lysis of host cell causes release of progeny phages

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings You should now be able to: 1.Explain how capsids and envelopes are formed 2.Distinguish between the lytic and lysogenic reproductive cycles 3.Explain why viruses are obligate intracellular parasites 4.Describe the reproductive cycle of an HIV retrovirus 5.Describe three processes that lead to the emergence of new diseases 6.Describe viroids and prions