1. Lecture #8Date _________ n Chapter 18~ Microbial Models: The Genetics of Viruses and Bacteria

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

3. Viral reproduction: Lytic Cycle n Host range: infection of a limited range of host cells (receptor molecules on the surface of cells) n The lytic cycle: 1- attachment 2- injection 3- hydrolyzation 4- assembly 5- release n Results in death of host cell n Virulent virus (phage reproduction only by the lytic cycle)

4. Viral reproduction: Lysogenic Cycle n Genome replicated w/o destroying the host cell n Genetic material of virus becomes incorporated into the host cell DNA (prophage DNA) n Temperate virus (phages capable of using the lytic and lysogenic cycles) n May give rise to lytic cycle

5. Lysogenic Cycle

6. RNA viruses n Retroviruses: transcribe DNA from an RNA template (RNA--->DNA) n Reverse transcriptase (catalyzing enzyme) n HIV--->AIDS

7. Retrovirus (HIV)

8. Viroids and prions n Viroids: tiny, naked circular RNA that infect plants; do not code for proteins, but use cellular enzymes to reproduce; stunt plant growth n Prions: “infectious proteins”; “mad cow disease”; trigger chain reaction conversions; a transmissible protein

9. Bacterial genetics n Nucleoid: region in bacterium densely packed with DNA (no membrane) n Plasmids: small circles of DNA n Reproduction: binary fission (asexual)

10. Bacterial DNA-transfer processes n Transformation: genotype alteration by the uptake of naked, foreign DNA from the environment (Griffith expt.) n 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 n Conjugation: direct transfer of genetic material; cytoplasmic bridges; pili; sexual

11. Bacterial Plasmids n Small, circular, self-replicating DNA separate from the bacterial chromosome n F (fertility) Plasmid: codes for the production of sex pili (F+ or F-) n R (resistance) Plasmid: codes for antibiotic drug resistance n 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.); “jumping genes”

12. Operons, I n Repressible (trp operon): n tryptophan (a.a.) synthesis n promoter: RNA polymerase binding site; begins transcription n operator: controls access of RNA polymerase to genes (tryptophan not present) n repressor: protein that binds to operator and prevents attachment of RNA polymerase ~ coded from a regulatory gene (tryptophan present ~ acts as a corepressor) n 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)

13. Operons, II n Inducible (lac operon): n lactose metabolism n lactose not present: repressor active, operon off; no transcription for lactose enzymes n lactose present: repressor inactive, operon on; inducer molecule inactivates protein repressor (allolactose) n 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)

14. Operons, II continued n the lac operon and trp operon are examples of negative control n the lac operon is also under positive control, cAMP receptor protein (CRP) n cAMP binds allostericly to CRP switching it to an active conformation, bins upstream of the promoter n this enhances RNA polymerase’s affinity for the promoter n when glucose levels are high, cAMP is scarce, slowed transcription n when glucose levels are low, cAMP is abundent, efficient transcription