1. 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

2. 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 are technically not alive; they need a host to reproduce. Viruses are considered “vectors” – they bounce from host to host, causing disease but also transferring genetic material (process of “transduction”)

3. 0.5 µm Process of Transduction between a virus and bacteria

4. 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 consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope

5. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Viral Genomes Viral genomes may consist of either – Double- or single-stranded DNA, or – Double- or single-stranded RNA Depending on its type of nucleic acid, a virus is called a DNA virus or an RNA virus (more on this later in the lecture)

6. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Capsids and Envelopes A capsid is the protein shell that encloses the viral genome; it is built from protein subunits called capsomeres [explain reason] Some animal viruses have membranous envelopes that help them infect hosts Viral envelopes, which are derived from the host cell’s membrane, contain a combination of viral and host cell molecules Viruses have different structures (next slide)

7. 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

8. 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 they can reproduce only within a host cell Each virus has a host range, a limited number of host cells that it can infect. Phages (bacterial viruses) are broad; animal viruses are specific

9. 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

10. Transcription and manufacture of capsid proteins Self-assembly of new virus particles and their exit from the cell Entry and uncoating Summary of Video VIRUS 1 2 3 DNA Capsid 4 Replication HOST CELL Viral DNA mRNA Capsid proteins Viral DNA

11. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Reproductive Cycles of Phages Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle

12. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Lytic Cycle The lytic cycle is a phage reproductive cycle that culminates in the death of the host cell, producing new phages by digesting the host’s cell wall, releasing the progeny viruses A phage that reproduces only by the lytic cycle is called “virulent” – (infective strain) Bacteria have defenses against phages, including restriction enzymes that recognize and cut up specific sequences of phage DNA [explain why bacterial DNA is undigested] Animation: Phage T4 Lytic Cycle Animation: Phage T4 Lytic Cycle

13. Phage assembly HeadTailTail fibers Assembly Release Synthesis of viral genomes and proteins Entry of phage DNA and degradation of host DNA Attachment 1 2 4 5 3 Summary of Video

14. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 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 cell divides, it copies the phage DNA and passes that copy on to daughter cells, until… An environmental signal triggers 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 Animation: Phage Lambda Lysogenic and Lytic Cycles Animation: Phage Lambda Lysogenic and Lytic Cycles

15. 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. Summary of Video

16. 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 Summary of Cycles

17. 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: – Is the genetic material DNA or RNA? – Is the DNA or RNA single-stranded or double- stranded?

18. (+) sense (-) non-sense special (+) sense DNA RNA

19. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Viral Envelopes Many animal viruses have a membranous envelope Viral glycoproteins on the envelope bind to specific receptor molecules on the surface of a host cell Viral envelopes are also formed, in part, from the host cell’s plasma membrane as the virus exits from the host cell (shear forces tear off a piece of the plasma membrane) [Explain why eukaryotic cells do not lyse] – productive cycle

20. 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

21. 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 animal cells Retroviruses use reverse transcriptase (a.k.a. RNA-dependent DNA polymerase) to copy their RNA genome into cDNA, which is then integrated into the host genome The viral DNA that is integrated into the host genome is called a provirus. Unlike a prophage, a provirus remains a permanent resident of the host cell The host’s RNA polymerase transcribes the proviral DNA into RNA, which function as mRNA for synthesis of viral proteins and as genomes for new virus particles released from the cell HIV (Human Immunodeficiency Virus) is the retrovirus that causes AIDS (Acquired ImmunoDeficiency Syndrome) Animation: HIV Reproductive Cycle Animation: HIV Reproductive Cycle

22. 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 (T 4 cell) Reverse transcriptase DNA NUCLEUS Provirus Chromosomal DNA RNA genome for the next viral generation mRNA New virus New HIV leaving a cell Summary of Video

23. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Evolution of Viruses Since viruses can only reproduce within cells, they probably evolved as bits of cellular nucleic acid Candidates for the source of viral genomes are plasmids (independent circular DNA in bacteria and yeasts) and transposons (small mobile DNA segments within a chromosome) Plasmids, transposons, and viruses are all mobile genetic elements (vectors) The process of DNA transfer between bacterium is called “conjugation”

24. Process of Conjugation between bacteria Sex pilus

25. G-ma got the Nobel Prize…

26. 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

27. 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, which lyse the host cell (and allow the release of progeny virus) Some viruses cause infected cells to produce toxins that lead to disease symptoms. Others have envelope proteins that are toxic to the host, also causing uncomfortable disease symptoms

28. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Vaccines are harmless derivatives of pathogenic microbes that stimulate the immune system to mount an enhanced defense against the pathogen, preventing viral illness [explain primary vs. secondary immune response] Viral infections cannot be treated by antibiotics. Antibiotics target prokaryotes (i.e. bacteria…) Antiviral drugs can help to treat, not cure, viral infections

29. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Emerging Viruses Emerging viruses are those that appear suddenly and come to the attention of scientists (due to the rapid onset of disease) Outbreaks of “new” viral diseases in humans are usually caused by existing viruses that expand their host (species infected) territory Viral strains that jump species can exchange genetic information with other viruses to which humans have no immunity

30. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Epidemics are caused by new strains of a virus to which people have little or no immunity Viral diseases in a small isolated population can emerge and become global, causing a pandemic, or global epidemic. The H1N1 influenza virus caused the 1918 “Spanish Flu” pandemic, killing 40 million people The “avian flu” is a virus that recently appeared in humans and originated in wild birds. The SARS outbreak in Hong Kong was caused by the H5N1 influenza virus. This disease was quelled by the culling of 1.5 million birds and the vaccination of the remainder

31. (a) The 1918 flu pandemic (b) Influenza A H5N1 virus (c) Vaccinating ducks 0.5 µm

32. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Viral Diseases in Plants Most plant viruses possess an RNA genome More than 2,000 types of viral borne plant disease are known, causing spots on leaves and fruits, stunted growth, and damaged flowers or roots Plant viruses spread disease in two major modes: – Horizontal transmission, entering through damaged cell walls (caused by hoeing) – Vertical transmission, inheriting the virus from a parent (caused by reproduction)

34. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Viroids and Prions: The Simplest Infectious Agents Viroids are very 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 (how do you destroy a prion?) Prions propagate by catalyzing the conversion of normal proteins to abnormal prions Scrapie in sheep, mad cow disease, and Jakob-Creutzfeldt disease in humans are all caused by prions

35. Prion Normal protein Original prion New prion Aggregates of prions Conformational change induced by a prion

36. 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