Viruses

1. Viruses 0 17

2. Overview: A Borrowed Life A virus is an infectious particle consisting of little more than genes packaged into a protein coat Viruses lead “a kind of borrowed life,” existing in a shady area between life-forms and chemicals

3. Figure 17.1 0.25 m

4. 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

5. 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

6. Figure 17.2 Membranous envelope Capsomere RNA RNA DNA Head DNA Capsid Capsomereof capsid Tailsheath Tailfiber Glycoprotein Glycoproteins 80  225 nm 70–90 nm (diameter) 80–200 nm (diameter) 18  250 nm 20 nm 50 nm 50 nm 50 nm (c) Influenza viruses (a) Tobacco mosaic virus (b) Adenoviruses (d) Bacteriophage T4

7. Figure 17.2a Capsomere RNA DNA Capsomereof capsid Glycoprotein 70–90 nm (diameter) 18  250 nm 20 nm 50 nm (b) Adenoviruses (a) Tobacco mosaic virus

8. Some viruses have membranous envelopes that help them infect hosts These viral envelopes are derived from the host cell’s membrane and contain a combination of viral and host cell molecules

9. Bacteriophages, also called phages, are viruses that infect bacteria They have the most complex capsids found among viruses Phages have an elongated capsid head that encloses their DNA A protein tail piece attaches the phage to the host and injects the phage DNA inside

10. Concept 17.2: Viruses replicate only in host cells Viruses are obligate intracellular parasites, which means they can replicate only within a host cell Each virus has a host range, a limited number of host cells that it can infect

11. 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 These exit from the host cell, usually damaging or destroying it Animation: Simplified Viral Replicative Cycle

12. Figure 17.3 VIRUS DNA Entry anduncoating Transcription andmanufacture ofcapsid proteins 1 3 Capsid Replication 2 HOSTCELL Viral DNA mRNA Capsidproteins Viral DNA 4 Self-assembly ofnew virus particlesand their exit fromthe cell

13. Replicative Cycles of Phages Phages are the best understood of all viruses Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle

14. The Lytic Cycle The lytic cycle is a phage replicative cycle that culminates in the death of the host cell The lytic cycle produces new phages and lyses (breaks open) the host’s cell wall, releasing the progeny viruses A phage that reproduces only by the lytic cycle is called a virulent phage Bacteria have defenses against phages, including restriction enzymes that recognize and cut up certain phage DNA Animation: Phage T4 Lytic Cycle

15. Figure 17.4-1 1 Attachment

16. Figure 17.4-2 1 Attachment 2 Entry of phageDNA anddegradationof host DNA

17. Figure 17.4-3 1 Attachment 2 Entry of phageDNA anddegradationof host DNA 3 Synthesis ofviral genomesand proteins

18. Figure 17.4-4 1 Attachment 2 Entry of phageDNA anddegradationof host DNA Phage assembly 3 4 Synthesis ofviral genomesand proteins Assembly Head Tailfibers Tail

19. Figure 17.4-5 1 Attachment 2 Entry of phageDNA anddegradationof host DNA 5 Release Phage assembly 3 4 Synthesis ofviral genomesand proteins Assembly Head Tailfibers Tail

20. The Lysogenic Cycle The lysogenic cycle replicates the phage genome without destroying the host Phages that use both the lytic and lysogenic cycles are called temperate phages The viral DNA molecule is incorporated into the host cell’s chromosome This integrated viral DNA is known as a prophage

21. Every time the host divides, it copies the phage DNA and passes the copies to daughter cells A single infected cell can give rise to a large population of bacteria carrying the virus in prophage form An environmental signal can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode Animation: Phage  Lysogenic and Lytic Cycles

22. Figure 17.5a PhageDNA The phage injects its DNA. Phage DNAcircularizes. Phage Bacterialchromosome Lytic cycle The cell lyses, releasing phages. Phage DNA and proteins aresynthesized and assembled.

23. Figure 17.5b Daughter cellwith prophage Many celldivisionscreate manyinfectedbacteria. Prophage exitschromosome. Lysogenic cycle Prophage is copiedwith bacterialchromosome. Prophage Phage DNA integrates intobacterial chromosome.

24. Replicative Cycles of Animal Viruses There are two key variables used to classify viruses that infect animals The nature of the viral genome (single- or double- stranded DNA or RNA) The presence or absence of an envelope

25. Viral Envelopes An animal virus with an envelope uses it to enter the host cell The envelope is derived from the plasma membrane of a host cell, although some of the molecules on the envelope are specified by the genome of the virus

26. Figure 17.6 Capsid RNA HOST CELL Envelope (withglycoproteins) Viral genome(RNA) Template mRNA Capsidproteins ER Copy of genome (RNA) Glycoproteins New virus

27. 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)

28. Viral DNA that is integrated into the host genome is called a provirus Unlike a prophage, a provirus is a permanent resident of the host cell The host’s RNA polymerase transcribes the proviral DNA into RNA molecules The RNA molecules function both as mRNA for synthesis of viral proteins and as genomes for new virusesreleased from the cell Animation: HIV Replicative Cycle

29. Figure 17.7 Membrane ofwhite blood cell Viral envelope Glycoprotein HIV Capsid RNA (twoidenticalstrands) HOSTCELL HIV Reversetranscriptase Reversetranscriptase Viral RNA RNA-DNAhybrid 0.25 m DNA HIV entering a cell NUCLEUS Provirus ChromosomalDNA RNA genomefor the nextviral generation mRNA New virus New HIV leaving a cell

30. Figure 17.7a Viral envelope Glycoprotein Capsid RNA (twoidenticalstrands) HOSTCELL HIV Reversetranscriptase Reversetranscriptase Viral RNA RNA-DNAhybrid DNA NUCLEUS Provirus ChromosomalDNA RNA genomefor the nextviral generation mRNA New virus

31. Figure 17.7aa Viral envelope Glycoprotein Capsid RNA (twoidenticalstrands) HOSTCELL HIV Reversetranscriptase Reversetranscriptase Viral RNA RNA-DNAhybrid DNA

32. Figure 17.7ab NUCLEUS Provirus ChromosomalDNA RNA genomefor the nextviral generation mRNA New virus

33. Figure 17.7b Membrane ofwhite blood cell HIV 0.25 m HIV entering a cell New HIV leaving a cell

34. Evolution of Viruses Viruses do not fit our definition of living organisms Since viruses can replicate only within cells, they probably evolved after the first cells appeared Candidates for the source of viral genomes are plasmids (circular DNA in bacteria and yeasts) and transposons (small mobile DNA segments) Plasmids, transposons, and viruses are all mobile genetic elements

35. Concept 17.3: Viruses are formidable pathogens in animals and plants Diseases caused by viral infections afflict humans, agricultural crops, and livestock worldwide

36. 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 molecular components such as envelope proteins that are toxic

37. A vaccine isa harmless derivative of a pathogen that stimulates 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

38. Emerging Viruses Viruses that suddenly become apparent are called emerging viruses. HIV is a classic example The West Nile virus appeared in North America first in 1999 and has now spread to all 48 contiguous states

39. In 2009 a general outbreak, or epidemic, of a flu-like illness occurred in Mexico and the United States; the virus responsible was named H1N1 H1N1 spread rapidly, causing a pandemic, or global epidemic

40. Figure 17.8 1 m (a) 2009 pandemic H1N1 influenza A virus (b) 2009 pandemic screening

41. Figure 17.8a 1 m (a) 2009 pandemic H1N1 influenza A virus

42. Three processes contribute to the emergence of viral diseases The mutation of existing viruses, which is especially high in RNA viruses Dissemination of a viral disease from a small, isolated human population, allowing the disease to go unnoticed before it begins to spread Spread of existing viruses from animal populations; about three-quarters of new human diseases originate this way

43. Strains of influenza A are given standardized names The name H1N1 identifies forms of two viral surface proteins, hemagglutinin (H) and neuraminidase (N) There are numerous types of hemagglutinin and neuraminidase, identified by numbers

44. Viral Diseases in Plants More than 2,000 types of viral diseases of plants are known; these have enormous impacts on the agricultural and horticultural industries Plant viruses have the same basic structure and mode of replication as animal viruses Most plant viruses known thus far have an RNA genome and many have a helical capsid

45. Plant viral diseases spread by two major routes Infection from an external source of virus is called horizontal transmission Herbivores, especially insects, pose a double threat because they can both carry a virus and help it get past the plant’s outer layer of cells Inheritance of the virus from a parent is called vertical transmission

46. Figure 17.UN01

47. Figure 17.UN02a Group 1 A/California/07/2009 Group 3 A/Taiwan/1164/2010 Group 6 A/Taiwan/T1773/2009 A/Taiwan/T1338/2009 A/Taiwan/T0724/2009 A/Taiwan/T1821/2009 A/Taiwan/937/2009 A/Taiwan/T1339/2009 Group 7 A/Taiwan/940/2009 A/Taiwan/7418/2009 A/Taiwan/8575/2009 A/Taiwan/4909/2009 A/Taiwan/8542/2009 A/Taiwan/1018/2011 Group 9 A/Taiwan/552/2011 A/Taiwan/2826/2009 A/Taiwan/T0826/2009 A/Taiwan/1017/2009 A/Taiwan/7873/2009 A/Taiwan/11706/2009 Group 8 A/Taiwan/6078/2009 A/Taiwan/6341/2009 A/Taiwan/6200/2009 A/Taiwan/5270/2010 Group 8-1 A/Taiwan/3994/2010 A/Taiwan/2649/2011 Group 10 A/Taiwan/1102/2011 A/Taiwan/4501/2011 A/Taiwan/67/2011 A/Taiwan/1749/2011 A/Taiwan/4611/2011 A/Taiwan/5506/2011 Group 11 A/Taiwan/1150/2011 A/Taiwan/2883/2011 A/Taiwan/842/2010 A/Taiwan/3697/2011

48. Figure 17.UN03 The phage attaches to ahost cell and injects its DNA. PhageDNA Bacterialchromosome Prophage Lytic cycle 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 • Virulent or temperate phage• Destruction of host DNA• Production of new phages• Lysis of host cell causes release of progeny phages