Microbial Models: The Genetics of Viruses
Chapter 18
p. 334-346
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
A Virus is a Genome in a Protective Coat
Virus: infectious particles consisting of nucleic acid enclosed in protein coatSome also have membrane envelopeNucleic Acid: DNA or RNA; single- or
double-strandedProtein Coat: capsid; composed of protein
subunits (capsomeres)Envelope: viral envelope; composed of
host cell membrane, proteins, glycoproteins
Viral Structures
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
Viral Reproduction: an overview
Use replication to make copies of viral DNA
Use transcription, & translation to produce capsids
Genomes & capsids spontaneously recombine to produce several new viral particles
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)
The Lytic Cycle
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
The Lysogenic Cycle
Animal Virus Reproduction
Have many different modes of infection & reproduction
Depends on: Presence of viral
envelope Type nucleic acid
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
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
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
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
Viruses and Cancer
Tumor Viruses: cause cancer in animals i.e.: retroviruses, papovirus, adenovirusTransform healthy cells → cancerous by
incorporating viral genome into host DNAUsually require other mutagens i.e.: Hepatitis & liver cancer
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
Microbial Models: The Genetics of Bacteria
Chapter 18
p. 346-358
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!)
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
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.
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
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
R Plasmids & Conjugation
R plasmids carry genes for resistance to antibioticsMay code for enzymes that destroy
antibioticResistant population tends to growResistant bacteria may spread resistance
through conjugation
Transposons
Movement within a cell’s genome is result of recombinationMay “cut-and-paste” or “copy-and-paste”Brings genes for resistance to R plasmid
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
Composite Transposons
Include extra genes sandwiched between insertion sequences i.e.: for resistance
May help bacteria adapt to new environment by ↑ resistance
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”
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
trp Operon
lac
Op
ero
nSome repressors are always “on” & require an inducer to inactivate-Genes of operon are silenced
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
Positive Gene Regulation
Promotes gene expression when molecule (cAMP) binds to protein (CRP) & activates it Facilitates binding of
RNA polymerase to promoter (activates transcription)