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VIRUSES   Chapter 19
What is a virus? A virus is a submicroscopic infectious particle composed of a  protein coat (capsid) and a nucleic acid core (either DNA or RNA). Viruses are similar in size to a large protein macromolecule, generally smaller than 200 nm in diameter.
Discovery of Viruses Search for cause of tobacco mosaic disease led to viruses The elusive virus was crystallized in 1935 by Stanley
 
Fig. 19-2 RESULTS 1 2 3 Extracted sap from tobacco plant with tobacco mosaic  disease Passed sap through a porcelain  filter known to trap bacteria Rubbed filtered sap on healthy tobacco plants 4 Healthy plants became infected
Viral Capsids Capsids are built from protein subunits called  capsomeres May be rod-shaped (helical viruses), polyhedral (icosahedral viruses) or more complex Some viruses have membranous envelopes that help them infect hosts (flu virus Bacteriophages, also called phages, are viruses that infect bacteria
Fig. 19-3 RNA Capsomere 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 50 nm 50 nm 20 nm (a) Tobacco mosaic virus (b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4
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
Viral Reproduction Once a viral genome has entered a cell, the cell begins to manufacture viral proteins using the host cell’s materials (enzymes, ribosomes, tRNAs, amino acids, ATP, etc.)
Transcription and manufacture of capsid proteins  Self-assembly of new virus particles and their exit from the cell Entry and uncoating Fig. 19-4 VIRUS 1 2 3 DNA Capsid 4 Replication HOST CELL Viral DNA mRNA Capsid proteins Viral DNA
Phages are the best understood of all viruses Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle
The Lytic Cycle The  lytic cycle  culminates in the death of the host cell by producing 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  virulent phage Bacteria have defenses against phages, including  restriction enzymes  that recognize and cut up certain phage DNA Animation: Phage T4 Lytic Cycle
Fig. 19-5-5 Phage assembly Head Tail Tail fibers Assembly Release Synthesis of viral genomes and proteins Entry of phage DNA and degradation of host DNA Attachment 1 2 4 5 3
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 and is called a  prophage. Every time the host divides, it copies the phage DNA and passes the copies to daughter cells Viruses that can be lysogenic or lytic are called  temperate  phages. Animation: Phage Lambda Lysogenic and Lytic Cycles
Fig. 19-6 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 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.
Animal Viruses Classified as DNA or RNA viruses, single or double-stranded Many have envelopes with glycoproteins that are specific for receptors. The glycoproteins are made by the ER and added to the host cell’s membrane which envelopes the emerging viruses.
Fig. 19-7 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
Table 19-1a
Table 19-1b
RNA Viruses 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 is ex.) 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
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
Evolution of Viruses Since viruses can reproduce only within cells, they probably evolved as bits of cellular nucleic acid Candidates for the source of viral genomes are plasmids and transposons, small mobile DNA segments Mimivirus , a double-stranded DNA virus, is the largest virus yet discovered
mimivirus Which came first, the cell or the mimivirus? The virus was first isolated in 1992 from amoebae growing in a water tower.
 
Viral diseases in animals Symptoms caused by - toxins - body’s defense mechanisms Vaccines – weakened or derivatives of viral particles capable of causing an immune response Antibiotics not effective Some antiviral medications interfere with viral nucleic acid synthesis
Where do new viruses come from? Mutations  of existing viruses The  dissemination  of an existing virus to a more widespread population Or  spread between species Epidemic – general outbreak of a disease Pandemic – global epidemic
Fig. 19-9 (a) The 1918 flu pandemic (b) Influenza A H5N1 virus (c) Vaccinating ducks 0.5 µm
Plant viruses 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 Plant viral disease can spread by vertical transmission from parent plant or by horizontal transmission from an external source.
Fig. 19-10
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 misfolded 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
Viroids in Plants
Misfolding of proteins to form prions Fig. 19-11 Prion Normal protein Original prion New prion Aggregates of prions
Scrapie in sheep
 

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Ap Viruses

  • 1. VIRUSES Chapter 19
  • 2. What is a virus? A virus is a submicroscopic infectious particle composed of a protein coat (capsid) and a nucleic acid core (either DNA or RNA). Viruses are similar in size to a large protein macromolecule, generally smaller than 200 nm in diameter.
  • 3. Discovery of Viruses Search for cause of tobacco mosaic disease led to viruses The elusive virus was crystallized in 1935 by Stanley
  • 4.  
  • 5. Fig. 19-2 RESULTS 1 2 3 Extracted sap from tobacco plant with tobacco mosaic disease Passed sap through a porcelain filter known to trap bacteria Rubbed filtered sap on healthy tobacco plants 4 Healthy plants became infected
  • 6. Viral Capsids Capsids are built from protein subunits called capsomeres May be rod-shaped (helical viruses), polyhedral (icosahedral viruses) or more complex Some viruses have membranous envelopes that help them infect hosts (flu virus Bacteriophages, also called phages, are viruses that infect bacteria
  • 7. Fig. 19-3 RNA Capsomere 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 50 nm 50 nm 20 nm (a) Tobacco mosaic virus (b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4
  • 8. 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
  • 9. Viral Reproduction Once a viral genome has entered a cell, the cell begins to manufacture viral proteins using the host cell’s materials (enzymes, ribosomes, tRNAs, amino acids, ATP, etc.)
  • 10. Transcription and manufacture of capsid proteins Self-assembly of new virus particles and their exit from the cell Entry and uncoating Fig. 19-4 VIRUS 1 2 3 DNA Capsid 4 Replication HOST CELL Viral DNA mRNA Capsid proteins Viral DNA
  • 11. Phages are the best understood of all viruses Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle
  • 12. The Lytic Cycle The lytic cycle culminates in the death of the host cell by producing 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 virulent phage Bacteria have defenses against phages, including restriction enzymes that recognize and cut up certain phage DNA Animation: Phage T4 Lytic Cycle
  • 13. Fig. 19-5-5 Phage assembly Head Tail Tail fibers Assembly Release Synthesis of viral genomes and proteins Entry of phage DNA and degradation of host DNA Attachment 1 2 4 5 3
  • 14. 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 and is called a prophage. Every time the host divides, it copies the phage DNA and passes the copies to daughter cells Viruses that can be lysogenic or lytic are called temperate phages. Animation: Phage Lambda Lysogenic and Lytic Cycles
  • 15. Fig. 19-6 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 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.
  • 16. Animal Viruses Classified as DNA or RNA viruses, single or double-stranded Many have envelopes with glycoproteins that are specific for receptors. The glycoproteins are made by the ER and added to the host cell’s membrane which envelopes the emerging viruses.
  • 17. Fig. 19-7 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
  • 20. RNA Viruses 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 is ex.) 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
  • 21. 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
  • 22. Fig. 19-8b HIV Membrane of white blood cell HIV entering a cell 0.25 µm New HIV leaving a cell
  • 23. Evolution of Viruses Since viruses can reproduce only within cells, they probably evolved as bits of cellular nucleic acid Candidates for the source of viral genomes are plasmids and transposons, small mobile DNA segments Mimivirus , a double-stranded DNA virus, is the largest virus yet discovered
  • 24. mimivirus Which came first, the cell or the mimivirus? The virus was first isolated in 1992 from amoebae growing in a water tower.
  • 25.  
  • 26. Viral diseases in animals Symptoms caused by - toxins - body’s defense mechanisms Vaccines – weakened or derivatives of viral particles capable of causing an immune response Antibiotics not effective Some antiviral medications interfere with viral nucleic acid synthesis
  • 27. Where do new viruses come from? Mutations of existing viruses The dissemination of an existing virus to a more widespread population Or spread between species Epidemic – general outbreak of a disease Pandemic – global epidemic
  • 28. Fig. 19-9 (a) The 1918 flu pandemic (b) Influenza A H5N1 virus (c) Vaccinating ducks 0.5 µm
  • 29. Plant viruses 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 Plant viral disease can spread by vertical transmission from parent plant or by horizontal transmission from an external source.
  • 31. 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 misfolded 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
  • 33. Misfolding of proteins to form prions Fig. 19-11 Prion Normal protein Original prion New prion Aggregates of prions
  • 35.