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Chapter 18

Chapter 18. The Genetics of Viruses and Bacteria. Overview: Microbial Model Systems. __________ called __________ can infect and set in motion a genetic takeover of __________, 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 • __________ called __________ can infect and set in motion a genetic takeover of __________, such as Escherichia coli • E. coli and its viruses are called __________ because of their frequent use by researchers in studies that reveal broad biological principles • Beyond their value as model systems, viruses and bacteria have unique genetic mechanisms that are interesting in their own right

  3. Bacteria are prokaryotes with cells much smaller and more simply organized than those of eukaryotes • Viruses are smaller and simpler than bacteria

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

  5. Structure of Viruses • Viruses are not cells • Viruses are very small infectious particles consisting of __________ enclosed in a __________ and, in some cases, a _______________

  6. Viral Genomes • Viral genomes may consist of • Double- or single-stranded _____ • Double- or single-stranded _____ • Depending on its type of __________, a virus is called a DNA virus or an RNA virus

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

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

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

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

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

  12. Bacteriophages, also called __________, 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 tailpiece attaches the phage to the host and injects the phage DNA inside

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

  14. General Features of Viral Reproductive Cycles • Viruses are ____________________, which means they can reproduce only within a host cell • Each virus has a __________, 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

  15. 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

  16. Reproductive Cycles of Phages • Phages are the best understood of all viruses • Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle

  17. The Lytic Cycle • The __________ is a phage reproductive cycle that culminates in the death of the host cell • 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 __________ phage • Bacteria have defenses against phages, including _______________ that recognize and cut up certain phage DNA

  18. 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

  19. The Lysogenic Cycle • The __________ replicates the phage genome without destroying the host • The viral DNA molecule is __________ by genetic recombination 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 • Phages that use both the lytic and lysogenic cycles are called __________ phages

  20. 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.

  21. Reproductive Cycles of Animal Viruses • Two key variables in classifying viruses that infect animals: • DNA or RNA? • Single-stranded or double-stranded?

  22. 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

  23. 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

  24. RNA as Viral Genetic Material • The broadest variety of RNA genomes is found in viruses that infect animals • Retroviruses use _______________ to copy their RNA genome into DNA • HIV is the retrovirus that causes AIDS

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

  26. The viral DNA that is integrated into the host genome is called a __________ • Unlike a prophage, a provirus remains a __________ 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 virus particles released from the cell

  27. 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

  28. 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

  29. __________ are harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen • Vaccines can prevent certain viral illnesses

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

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

  32. 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”

  33. The Bacterial Genome and Its Replication • The bacterial chromosome is usually a circular DNA molecule with few associated proteins • Many bacteria also have __________, smaller circular DNA molecules that can replicate independently of the chromosome • Bacterial cells divide by binary fission, which is preceded by replication of the chromosome

  34. LE 18-14 Replication fork Origin of replication Termination of replication

  35. Mutation and Genetic Recombination as Sources of Genetic Variation • Since bacteria can reproduce rapidly, new mutations quickly increase genetic diversity • More genetic diversity arises by recombination of DNA from two different bacterial cells

  36. Mechanisms of Gene Transfer and Genetic Recombination in Bacteria • Three processes bring bacterial DNA from different individuals together: • Transformation • Transduction • Conjugation

  37. Transformation • __________ is the alteration of a bacterial cell’s genotype and phenotype by the uptake of naked, foreign DNA from the surrounding environment • For example, harmless Streptococcus pneumoniae bacteria can be transformed to pneumonia-causing cells

  38. Transduction • In the process known as __________, phages carry bacterial genes from one host cell to another

  39. LE 18-16 Phage DNA A+ B+ A+ B+ Donor cell A+ Crossing over A+ A– B– Recipient cell A+ B– Recombinant cell

  40. Conjugation and Plasmids • __________ is the direct transfer of genetic material between bacterial cells that are temporarily joined • The transfer is one-way: One cell (“male”) donates DNA, and its “mate” (“female”) receives the genes

  41. “Maleness,” the ability to form a sex pilus and donate DNA, results from an _______________ as part of the chromosome or as a plasmid • Plasmids, including the __________, are small, circular, self-replicating DNA molecules

  42. LE 18-17 Sex pilus 5 µm

  43. The F Plasmid and Conjugation • Cells containing the F plasmid, designated F+ cells, function as DNA donors during conjugation • F+ cells transfer DNA to an F recipient cell • Chromosomal genes can be transferred during conjugation when the donor cell’s F factor is integrated into the chromosome • A cell with a built-in F factor is called an _____ cell • The F factor of an Hfr cell brings some chromosomal DNA along when transferred to an F– cell

  44. LE 18-18_1 F plasmid Bacterial chromosome F+ cell F+ cell Mating bridge F– cell F+ cell Bacterial chromosome Conjunction and transfer of an F plasmid from and F+ donor to an F– recipient

  45. LE 18-18_2 F plasmid Bacterial chromosome F+ cell F+ cell Mating bridge F– cell F+ cell Bacterial chromosome Conjunction and transfer of an F plasmid from and F+ donor to an F– recipient Hfr cell F+ cell F factor

  46. LE 18-18_3 F plasmid Bacterial chromosome F+ cell F+ cell Mating bridge F– cell F+ cell Bacterial chromosome Conjunction and transfer of an F plasmid from and F+ donor to an F– recipient Hfr cell F+ cell F factor Hfr cell F– cell

  47. LE 18-18_4 F plasmid Bacterial chromosome F+ cell F+ cell Mating bridge F– cell F+ cell Bacterial chromosome Conjunction and transfer of an F plasmid from and F+ donor to an F– recipient Hfr cell F+ cell F factor Hfr cell F– cell Temporary partial diploid Recombinant F– bacterium Conjugation and transfer of part of the bacterial chromosome from an Hfr donor to an F– recipient, resulting in recombiination

  48. R plasmids and Antibiotic Resistance • __________ confer resistance to various antibiotics • When a bacterial population is exposed to an antibiotic, individuals with the R plasmid will survive and increase in the overall population

  49. Transposition of Genetic Elements • The DNA of a cell can also undergo recombination due to movement of transposable elements within the cell’s genome • Transposable elements, often called “jumping genes,” contribute to genetic shuffling in bacteria

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