Microbial Models

The Genetics of Viruses and Bacteria

Viral structure

Virus: “poison” (Latin); infectious particles consisting of a nucleic acid in a protein coat

Capsid; (viral envelopes); DNA or RNA

Bacteriophages (phages)

Viral reproduction: Lytic Cycle Host range: infection of a limited

range of host cells (receptor molecules on the surface of

cells)

The Lytic cycle: 1- attachment 2- injection 3- hydrolyzation of Host DNA 4- assembly 5- release (bursts from osmotic swelling)

Results in death of host cell Virulent virus (phage reproduction

only by the lytic cycle) May take only 30 minutes at 370

Bacteria have defenses: Restriction Enzymes!!

Viral reproduction: Lysogenic Cycle

Genome replicated w/o destroying the host cell

Genetic material of virus becomes incorporated into the host cell DNA (prophage* DNA)

* Called a “provirus” in Eukaryotes

Prophage usually codes for repressors which temporarily turn off virulent properties.

Temperate virus (phages capable of using the lytic and lysogenic cycles)

May give rise to lytic cycle

Lytic & Lysogenic Cycles:

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Other methods of infection:

http://www.blackwellpublishing.com/

RNA viruses Retroviruses: transcribe DNA from an

RNA template (RNA makes DNA!) RNA is double stranded but only one is

a coding strand (the “+” strand) Reverse transcriptase (catalyzing

enzyme) Viral DNA integrates itself as a

“provirus” in t-lymphocyte cell. HIV can lead to AIDS if it goes into

lytic cycle or sufficiently taxes the t-lymphocytes.

Retrovirus (HIV)

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Vaccines:

Harmless variants or derivatives pathogenic microbes that mobilize the immune system.

Sometimes use “attenuated viruses”: ones that are just modified or weakened versions.

Work best for viruses that don’t mutate as quickly or easily. (Double stranded)

Emerging Viruses

Ones that “seem” to make a sudden appearance.

In reality, they not likely to be “new”, but have simply expanded their host territory.

Some examples: May evolve or mutate continuously May spread from one host species to

another. May have disseminated from a small

isolated population May be “released” by environmental

disturbances (Ex.- Global Warming)

Viroids and prions

Viroids: tiny, naked circular RNA that infect plants; do not code for proteins, but use cellular enzymes to reproduce; stunt plant growth

Prions: “infectious proteins”; “mad cow disease”; trigger chain reaction conversions; a transmissible protein

Bacterial genetics Nucleoid: region in bacterium densely

packed with DNA (no membrane) Usually circular.

Contain 1/1000 the DNA of Eukaryotes. Plasmids: small circles of DNA separate

from the main ring. Plasmids replicate along with other DNA

or sometimes just on its own. Episome- a plasmid that can reversibly

incorporate itself into the regular genome of the bacteria.

Reproduction: binary fission (asexual) Mutations are rare (as usual) but impact

bacterial variation more because of such rapid rates of reproduction.

Bacterial DNA-transfer processes Transformation: genotype alteration by the

uptake of naked, foreign DNA from the environment . Some can be”made competent” by successive heating & cooling or adding calcium ions.http://www.learner.org/

Transduction: phages that carry bacterial genes from 1 host cell to another

•generalized~ random transfer of host cell chromosome •specialized~ incorporation of prophage DNA into host chromosome

Conjugation: direct transfer of genetic material; cytoplasmic bridges; sexual pili http://www.learner.org

For 8th edition .Section 27.2 (pages 561-564)

Bacterial Plasmids Small, circular, self-replicating DNA separate from the bacterial chromosome F (fertility) Plasmid: codes for the production of sex pili (F+ or F-) R (resistance) Plasmid: codes for antibiotic drug resistance Transposons: transposable genetic element; piece of DNA that can move from

location to another in a cell’s genome (chromosome to plasmid, plasmid to plasmid,

etc.); also called “jumping genes” http://highered.mcgraw-hill.com/sites/dl/free/0072556781/192785/anim0039.swf

Transposition in Detail: Transposase recognizes the inverted repeats

Targeted inverted repeats are cut, and the target is cut, then the transposon is inserted

For 8th edition, see: section 21.3 & 21.4 (pp 432-438)

Operons: Repressible (trp operon): tryptophan (a.a.) synthesis promoter: RNA polymerase binding

site; begins transcription operator: controls access of RNA

polymerase to genes (tryptophan not present)

repressor: protein that binds to

operator and prevents attachment of RNA polymerase ~ coded from a regulatory gene (tryptophan present ~ acts as a corepressor)

transcription is repressed when tryptophan binds to a regulatory protein

Def: Unit of genetic function consisting of coordinately related clusters of genes with related functions (transcription unit)

FOUND ONLY IN PROKARYOES!!! (NOT Eukaryotes)

Operons, continued

Inducible (lac operon): lactose metabolism lactose not present:

repressor active, operon off; no transcription for lactose

enzymes lactose present:

repressor inactive, operon on; inducer molecule inactivates protein repressor (allolactose)

transcription is stimulated when inducer binds to a regulatory protein

Def: Unit of genetic function consisting of coordinately related clusters of genes with related functions (transcription unit)

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