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: D:\bc_campbell_biology_7\0,7052,3117226-,00.html 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) D:\bc_campbell_biology_7\0,7052,3117226-,00.html

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

Youtube excerpts http://www.youtube.com/watch?v=Rpj0emEGShQ Part 7 Understandingvirusespart1 Part2 Ornamentsmartianchristmastree Part4 Part5 part6

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 For 10th edition .Section 27.2 (pages 572-575) 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

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 10th edition, see: section 21.4 (pp 444-447)

Def: Unit of genetic function consisting of coordinately related clusters of genes with related functions (transcription unit) FOUND ONLY IN PROKARYOES!!! (NOT Eukaryotes) 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

Operons, continued campbellbiology ANIMATON! Inducible (lac operon): Def: Unit of genetic function consisting of coordinately related clusters of genes with related functions (transcription unit) 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 campbellbiology ANIMATON!