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Microbial Models: The Genetics of Viruses Chapter 18 p. 334-346.

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Presentation on theme: "Microbial Models: The Genetics of Viruses Chapter 18 p. 334-346."— Presentation transcript:

1 Microbial Models: The Genetics of Viruses Chapter 18 p. 334-346

2 Discovery of Viruses Tobacco Mosaic Virus: stunts growth & changes leaf coloration of plants Contagious, but not caused by bacteria (too small) Able to reproduce (in host) Did not dilute when passed plant → plant

3 A Virus is a Genome in a Protective Coat Virus: infectious particles consisting of nucleic acid enclosed in protein coat Some also have membrane envelope Nucleic Acid: DNA or RNA; single- or double-stranded Protein Coat: capsid; composed of protein subunits (capsomeres) Envelope: viral envelope; composed of host cell membrane, proteins, glycoproteins

4 Viral Structures

5 Viral Reproduction: an overview Viruses can ONLY reproduce inside a host Lack ability to make own proteins or metabolize Can only infect certain types of hosts (“Host Range”) May also be tissue-specific in eukaryotes i.e.: cold: upper respiratory; AIDS: WBC’s Viruses use host resources to replicate their genomes

6 Use replication to make copies of viral DNA Use transcription, & translation to produce capsids Genomes & capsids spontaneously recombine to produce several new viral particles

7 The Lytic Cycle Phage reproductive cycle that kills host cells Virulent phage: infects ONLY by Lytic cycle 1) Phage binds to receptor site on bacteria 2) Phage injects DNA into host cell 3) Bacterial DNA hydrolyzed 4) Phage genome & capsid components copied 5) 100-200 phages reassembled, bacteria cell wall destroyed 6) Cell lyses, releasing phages to infect more cells Both bacteria & phages have defenses against each other (constantly evolving)


9 The Lysogenic Cycle Phage reproductive cycle that replicates genome but does not kill host Temperate Phage: uses Lytic & Lysogenic cycles 1) Phage binds to bacteria cell & injects DNA 2) Phage DNA incorporates into bacteria DNA at specific site through crossing over (“Prophage”) 3) Prophage DNA replicated as bacteria replicates 4) Prophage genome “dormant” inside host until triggered to detach & initiate lytic cycle By certain chemicals or radiation


11 Animal Virus Reproduction Have many different modes of infection & reproduction Depends on: Presence of viral envelope Type nucleic acid

12 Viral Envelopes Made of lipid bilayer that fuses w/ host cell membrane Once inside cell, viral genome replicates & directs synthesis of new viral envelopes New viruses “bud off” host cell membrane & spread to infect more cells

13 RNA as Viral Genetic Material 1) Double-stranded 2) ss mRNA: directly translated into viral protein 3) ss mRNA template: viral genome used to make compliment; many copies made 4) ss DNA template (“Retroviruses”): uses reverse transcriptase to make DNA from RNA template DNA incorporates into host animal cell (“provirus”) & remains permanently

14 Cause & Prevention of Animal Viral Diseases Viral symptoms may be caused by: Hydrolytic enzymes (lysosomes) Production of toxins Toxic components (envelope proteins) Degree of damage depends on host cell’s ability to repair/replace themselves Vaccine: harmless form of virus that triggers body to defend itself against actual virus i.e.: smallpox – cowpox virus vaccine

15 Emerging Viruses New viral diseases may emerge by: 1) Mutation of existing virus (esp. RNA) 2) Spread of virus to new host species 3) Spread of virus from small, isolated population to large one Ebola

16 Viruses and Cancer Tumor Viruses: cause cancer in animals i.e.: retroviruses, papovirus, adenovirus Transform healthy cells → cancerous by incorporating viral genome into host DNA Usually require other mutagens i.e.: Hepatitis & liver cancer

17 Plant Viruses are Agricultural Pests Mostly RNA; may stunt plant growth & decrease yields Horizontal Transmission: virus is from external source i.e.: other plants, insects, tools Vertical Transmission: virus is inherited from parent plant Viral infections spread throughout plant through plasmodesmata

18 Microbial Models: The Genetics of Bacteria Chapter 18 p. 346-358

19 Bacteria Have Short Generation Span Prokaryotes: contain small, ds circular DNA in nucleoid region Divide by Binary Fission: DNA replicates, cell grows, divides, produces 2 new identical cells May mutate to form new strains Can reproduce very quickly (E. coli every 20 minutes!)

20 Genetic Recombination DNA from 2 different bacterial strains can recombine to form new strains By crossing-over Leads to genetic diversity Uses transformation, transduction, or conjugation

21 Transformation The alteration of bacterial DNA by incorporating environmental DNA “Genetic Recombination” New alleles replace native alleles May code for pathogens, resistance, new proteins, etc.

22 Transduction Phages carry bacterial genes from one cell to another Generalized Transduction: bacteria host cell DNA is packaged inside capsid; “infects” new bacteria cell, replacing homologous section Specialized Transduction: a temperate phage will take with it small sections of bacteria host cell DNA Only genes near prophage site

23 Conjugation The direct transfer of genetic material between 2 joined bacterial cells “Male” cell extends sex pilus to pull cells together “Maleness” determined by F factor (DNA segment) Once joined, “male” donates portion of DNA to “female” through cytoplasmic bridge Aids in genetic recombination

24 R Plasmids & Conjugation R plasmids carry genes for resistance to antibiotics May code for enzymes that destroy antibiotic Resistant population tends to grow Resistant bacteria may spread resistance through conjugation

25 Transposons Movement within a cell’s genome is result of recombination May “cut-and-paste” or “copy-and-paste” Brings genes for resistance to R plasmid

26 Insertion Sequences Simple, containing only the sequence to be transposed (“Transposase Gene”) Capped at each end by inverted repeats Signal removal of transposase & guides new placement DNA polymerase fills in gaps Results in direct repeats at new location

27 Composite Transposons Include extra genes sandwiched between insertion sequences i.e.: for resistance May help bacteria adapt to new environment by ↑ resistance

28 Metabolic Control of Bacteria Bacteria are able to adjust their metabolism in response to environment 1) Adjusting # enzymes made (gene expression) During transcription 2) Adjust activity of present enzymes By “Feedback Inhibition”

29 Operons: the basic concept Genes involved in same metabolic processes are often grouped together as 1 transcription unit Single “on/off” switch (“Operator”) controls group Operon = operator + promoter + transc. unit “On” unless repressor present, which blocks RNA polymerase (specific!!!) Produced by regulatory gene at separate location May require corepressor (may be molecule itself = Negative Feedback) i.e. trp Operon

30 trp Operon

31 lac Operon Some repressors are always “on” & require an inducer to inactivate -Genes of operon are silenced

32 Repressible vs. Inducible Operons Repressible Operons: trp Operon Usually “On” Anabolic pathways (raw material → product) “Repress” end product when already present Inducible Operons: lac Operon Usually “Off” Catabolic pathways (nutrients → simple molecules) Produce enzymes (“induce”) only when nutrient present

33 Positive Gene Regulation Promotes gene expression when molecule (cAMP) binds to protein (CRP) & activates it Facilitates binding of RNA polymerase to promoter (activates transcription)

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