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)