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Chapter 18~ Microbial Models: The Genetics of Viruses and Bacteria

Chapter 18~ 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.

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Chapter 18~ Microbial Models: The Genetics of Viruses and Bacteria

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  1. Chapter 18~ 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 • 4- assembly • 5- release • Results in death of host cell • Virulent virus (phage reproduction only by the lytic cycle)

  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) • Temperate virus (phages capable of using the lytic and lysogenic cycles) • May give rise to lytic cycle

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

  6. Retrovirus

  7. 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

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

  9. Bacterial DNA-transfer processes • Transformation: genotype alteration by the uptake of naked, foreign DNA from the environment (Griffith expt.) • 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; pili; sexual

  10. 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.); “jumping genes”

  11. Operons: Type I Def: Unit of genetic function consisting of coordinately related clusters of genes with related functions (transcription unit) • 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

  12. Operons: Type II 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

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  14. 19.1-Chromatin Structure in Eukaryotic Genomes Chromatin - complex of DNA and proteins

  15. DNA Packing or Organization • Histone protein • + charged amino acids binds to • - charged phosphates of DNA • Nucleosome • - DNA wrapped around the histone • - “beads on a string” • - basic unit of DNA packing • Heterochromatin •highly condensed interphase DNA (can not be transcribed) • Euchromatin •less compacted interphase DNA (can be transcribed)

  16. Molecular Biology of Cancer • Oncogene •cancer-causing genes • Proto-oncogene •normal cellular genes • How? • 1-movement of DNA; chromosome fragments that have rejoined incorrectly • 2-amplification; increases the number of copies of proto-oncogenes • 3-proto-oncogene point mutation; protein product more active or more resistant to degradation • Tumor-suppressor genes •changes in genes that prevent uncontrolled cell growth (cancer growth stimulated by the absence of suppression)

  17. RNAi Discussion - Nova online • How can turning individual genes off help scientists better understand normal growth and development? • Some diseases involve an overproduction of proteins or the production of defective proteins. How might RNAi be used to control the overproduction of proteins or eliminate the production of defective proteins, thereby helping cure or control such diseases. • Which genetic diseases might not be able to be helped by RNAi? 18

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