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Chapter 18

Chapter 18. The Genetics of Viruses and Bacteria. Overview: Microbial Model Systems. Viruses called bacteriophages can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli

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Chapter 18

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  1. Chapter 18 The Genetics of Virusesand Bacteria

  2. Overview: Microbial Model Systems • Viruses called bacteriophages can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli • E. coli and its viruses are called model systems b/c of their frequent use by researchers in studies that reveal broad biological principles • Beyond their value as model systems, viruses & bacteria have unique genetic mechanisms that are interesting in their own right

  3. Bacteria are prok’s with cells much smaller and more simply organized than those of euk’s • Viruses are smaller & simpler than bacteria

  4. LE 18-2 Virus Bacterium Animal cell Animal cell nucleus 0.25 µm

  5. Concept 18.1: A virus has a genome but can reproduce only within a host cell • Scientists detected viruses indirectly long before they could see them • The story of how viruses were discovered begins in the late 1800s

  6. The Discovery of Viruses: Scientific Inquiry • Tobacco mosaic disease stunts growth of tobacco plants & 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)

  7. Structure of Viruses • Viruses are not cells • Viruses- very small infectious particles consisting of nucleic acid enclosed in a protein coat &, in some cases, a membranous envelope

  8. Viral Genomes • Viral genomes may consist of • ds or ss DNA • ds or ss RNA • Depending on its type of nucleic acid, a virus is called a DNA virus or an RNA virus

  9. Capsids and Envelopes • A capsid is the protein shell that encloses the viral genome • A capsid can have various structures

  10. LE 18-4a Capsomere of capsid RNA 18  250 mm 20 nm Tobacco mosaic virus

  11. LE 18-4b Capsomere DNA Glycoprotein 70–90 nm (diameter) 50 nm Adenoviruses

  12. Some viruses have membranous envelopes that help them infect hosts • These viral envelopes surround the capsids of influenza viruses & many other viruses found in animals • Viral envelopes, which are derived from the host cell’s membrane, contain a combination of viral and host cell molecules

  13. LE 18-4c Membranous envelope Capsid RNA Glycoprotein 80–200 nm (diameter) 50 nm Influenza viruses

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

  15. LE 18-4d Head DNA Tail sheath Tail fiber 80  225 nm 50 nm Bacteriophage T4

  16. General Features of Viral Reproductive Cycles • Viruses-- 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 • Viruses use enzymes, ribosomes, and small host molecules to synthesize progeny viruses Animation: Simplified Viral Reproductive Cycle

  17. LE 18-5 VIRUS Entry into cell and uncoating of DNA DNA Capsid Transcription Replication HOST CELL Viral DNA mRNA Viral DNA Capsid proteins Self-assembly of new virus particles and their exit from cell

  18. Reproductive Cycles of Phages • Phages are the best understood of all viruses • Phages--2 reproductive mechanisms: • lytic cycle & • lysogenic cycle

  19. The Lytic Cycle • lytic cycle- phage reproductive cycle that ends in the death of the host cell • makes new phages & digests the host’s cell wall, releasing baby viruses • Virulent phage- reproduces only by the lytic cycle • Bacteria defenses against phages- restriction enzymes recognize & cut up certain phage DNA

  20. LE 18-6 Attachment Entry of phage DNA and degradation of host DNA Phage assembly Release Head Tail fibers Tails Synthesis of viral genomes and proteins Assembly

  21. Animation: Phage T4 Lytic Cycle

  22. The Lysogenic Cycle • lysogenic cycle- replicates phage genome w/o destroying the host • Prophage– the integrated viral DNA incorporated by recombination into the host cell’s chromo • Every time the host divides, it copies the phage DNA & passes it to daughter cells • temperate phages-- use both the lytic & lysogenic cycles Animation: Phage Lambda Lysogenic and Lytic Cycles

  23. LE 18-7 Phage DNA The phage attaches to a host cell and injects its DNA. Daughter cell with prophage Many cell divisions produce a large 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 Lysogenic cycle The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells. Certain factors determine whether The cell lyses, releasing phages. Lytic cycle is induced Lysogenic cycle is entered or Prophage Phage DNA integrates into the bacterial chromosomes, becoming a prophage. New phage DNA and proteins are synthesized and assembled into phages.

  24. Reproductive Cycles of Animal Viruses • 2 key things in classifying viruses that infect animals: • DNA or RNA? • ss or ds?

  25. Viral Envelopes • Many viruses that infect animals have a membranous envelope • Viral glycoproteins on the envelope bind to specific receptor molecules on the surface of a host cell

  26. LE 18-8 Capsid Capsid and viral genome enter cell RNA HOST CELL Envelope (with glycoproteins) Viral genome (RNA) Template mRNA Capsid proteins ER Glyco- proteins Copy of genome (RNA) 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 is the retrovirus that causes AIDS

  28. LE 18-9 Viral envelope Glycoprotein Capsid RNA (two identical strands) Reverse transcriptase

  29. Provirus-- viral DNA that is integrated into the host genome Unlike a prophage, a provirus remains a permanent resident of the host cell • host’s RNA pol transcribes proviral DNA into RNA molecules • RNA molecules function both as mRNA for synthesis of viral proteins & as genomes for new virus particles released from the cell

  30. LE 18-10 Membrane of white blood cell HIV HOST CELL Reverse transcription Viral RNA RNA-DNA hybrid 0.25 µm HIV entering a cell DNA NUCLEUS Provirus Chromosomal DNA RNA genome for the next viral generation mRNA New HIV leaving a cell

  31. Animation: HIV Reproductive Cycle

  32. Evolution of Viruses • Viruses do not fit our definition of living organisms • Since viruses can reproduce only w/in cells, they probably evolved as bits of cellular nucleic acid

  33. Concept 18.2: 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 & prions also cause disease in plants and animals

  34. Viral Diseases in Animals • Viruses damage or kill cells by: • releasing hydrolytic enzymes from lysosomes or • making toxins that cause disease symptoms

  35. Vaccines- harmless derivatives of microbes that stimulate the immune system to mount defenses against the pathogen • Vaccines can prevent certain viral illnesses

  36. Emerging Viruses • Emerging viruses are those that appear suddenly or suddenly come to the attention of scientists • Severe acute respiratory syndrome (SARS) recently appeared in China • Outbreaks of “new” viral diseases in humans are usually caused by existing viruses that expand their host territory

  37. LE 18-11 The SARS-causing agent is a coronavirus like this one (colorized TEM), so named for the “corona” of glyco-protein spikes protruding form the envelope. Young ballet students in Hong Kong wear face masks to protect themselves from the virus causing SARS.

  38. Viral Diseases in Plants • More than 2,000 types of viral diseases of plants are known • Some symptoms are spots on leaves and fruits, stunted growth, and damaged flowers or roots

  39. Plant viruses spread disease in 2 ways: • Horizontal transmission- through damaged cell walls • Vertical transmission- inheriting virus from a parent

  40. Viroids and Prions: The Simplest Infectious Agents • Viroids- circular RNA that infect plants & disrupts growth • Prions- slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals • Prions propagate by converting normal proteins into the prion version

  41. LE 18-13 Original prion Prion Many prions New prion Normal protein

  42. Concept 18.3: Rapid reproduction, mutation, and genetic recombination contribute to the genetic diversity of bacteria • Bacteria allow researchers to investigate molecular genetics in the simplest true organisms • The well-studied intestinal bacterium Escherichia coli(E. coli) is “the laboratory rat of molecular biology”

  43. The Bacterial Genome and Its Replication • Bacterial chromo- usually a circular DNA molecule w/ few associated proteins • Many bacteria also have plasmids, smaller circular DNA molecules that can replicate independently of the chromo • Bacterial cells divide by binary fission, which is preceded by replication • Plasmids-- including the F plasmid, are small, circular, self-replicating DNA molecules

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