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  1. 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 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

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

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

  4. 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

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

  6. Structure of Viruses • 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

  7. Viral Genomes • Viral genomes may consist of • Double- or single-stranded DNA • Double- or single-stranded RNA • Depending on its type of nucleic acid, a virus is called a DNA virus or an RNA virus • A capsid is the protein shell that encloses the viral genome • A capsid can have various structures

  8. 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 tailpiece attaches the phage to the host and injects the phage DNA inside

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

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

  11. 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

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

  13. 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

  14. 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

  15. 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

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

  17. 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 molecules • The RNA molecules function both as mRNA for synthesis of viral proteins and as genomes for new virus particles released from the cell

  18. 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

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

  20. 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 and prions also cause disease in plants and animals

  21. 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

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

  23. 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

  24. 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.

  25. 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

  26. Plant viruses spread disease in two major modes: • Horizontal transmission, entering through damaged cell walls • Vertical transmission, inheriting the virus from a parent

  27. 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”

  28. The Bacterial Genome and Its Replication • The bacterial chromosome is usually a circular DNA molecule with few associated proteins • Many bacteria also have plasmids, 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

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

  30. 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

  31. LE 18-15 Mixture Mutant strain arg+trp– Mutant strain arg–trp+ Mixture Mutant strain arg+trp– Mutant strain arg–trp+ No colonies (control) No colonies (control) Colonies grew

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

  33. Transformation • 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

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

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

  36. Conjugation and Plasmids • Conjugation 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

  37. R plasmids and Antibiotic Resistance • R plasmids 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

  38. 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

  39. Insertion Sequences • The simplest transposable elements, called insertion sequences, exist only in bacteria • An insertion sequence has a single gene for transposase, an enzyme catalyzing movement of the insertion sequence from one site to another within the genome

  40. Transposons • Transposable elements called transposons are longer and more complex than insertion sequences • In addition to DNA required for transposition, transposons have extra genes that “go along for the ride,” such as genes for antibiotic resistance

  41. Concept 18.4: Individual bacteria respond to environmental change by regulating their gene expression • A bacterium can tune its metabolism to the changing environment and food sources • This metabolic control occurs on two levels: • Adjusting activity of metabolic enzymes • Regulating genes that encode metabolic enzymes

  42. LE 18-20 Regulation of enzyme production Regulation of enzyme activity Precursor Feedback inhibition Enzyme 1 Gene 1 Gene 2 Enzyme 2 Regulation of gene expression Gene 3 Enzyme 3 Enzyme 4 Gene 4 Gene 5 Enzyme 5 Tryptophan

  43. Operons: The Basic Concept • In bacteria, genes are often clustered into operons, composed of • An operator, an “on-off” switch • A promoter • Genes for metabolic enzymes • An operon can be switched off by a protein called a repressor • A corepressor is a small molecule that cooperates with a repressor to switch an operon off

  44. LE 18-21a trp operon Promoter Promoter Genes of operon DNA trpB trpA trpE trpC trpD trpR Operator Stop codon RNA polymerase Regulatory gene Start codon 3¢ mRNA 5¢ mRNA 5¢ D B E C A Inactive repressor Protein Polypeptides that make up enzymes for tryptophan synthesis Tryptophan absent, repressor inactive, operon on