1. Microbial Models
The Genetics of Viruses and Bacteria

2. Viral structure
Virus: “poison” (Latin); infectious particles consisting of a nucleic acid in a protein coat
Capsid; (viral envelopes); DNA or RNA
Bacteriophages (phages)

3. 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!!

4. 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

5. Lytic & Lysogenic Cycles:
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Other methods of infection:

6. 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.

7. Retrovirus (HIV)
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8. 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)

9. 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)

10. 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

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

12. 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.

13. Bacterial DNA-transfer processes
For 10th edition .Section 27.2 (pages )
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.
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

14. 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”

15. 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 )

16. 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

17. 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!