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

Bacteriophage: infecting Escherichia coli

Viral structure n Virus: “poison” (Latin); infectious particles consisting of a nucleic acid in a protein coat – DNA or RNA n Capsids/ Capsomeres n Viral Envelopes; n Bacteriophages (phages)

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)

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

Lysogenic Cycle

Animal Viruses: n Class I: dsDNA n Class II: ssDNA n Class III: dsRNA n Class IV: ssRNA- serves as mRNA n Class V: ssRNA- template for RNA synthesis n Class VI: template for DNA synthesis

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

Retrovirus (HIV)

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

Simple Infectious Agents: n Prions: “Proteinatious infectious particles” – Bovine Spongiform Encephalpathy, Creutzfeldt- Jakob Variant Disease, Scrapie – trigger chain reaction conversions; a transmissible protein

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

Bacterial DNA Replication:

Bacterial DNA-mechanisms of DNA transfer: n Transformation: genotype alteration by the uptake of naked, foreign DNA from the environment – Griffith experiment

Bacterial DNA-mechanisms of DNA transfer: 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

Bacterial DNA-mechanisms of DNA transfer: n Conjugation: – Temporary joining of bacterial cells – One way transfer of DNA – Sex pili

Bacterial Plasmids n Small, circular, self-replicating DNA separate from the bacterial chromosome – Types of Plasmids: n F Plasmids n R Plasmids

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 one location to another in a cell’s genome n chromosome to plasmid, plasmid to plasmid, etc. n jumping genes

Operons n Definition: Unit of genetic function consisting of coordinately related clusters of genes with related functions (transcription unit) – The promoter, operator and genes that code for enzymes

Operons n promoter: RNA polymerase binding site; begins transcription n operator: controls access of RNA polymerase to genes n repressor: protein that binds to operator and prevents attachment of RNA polymerase ~ coded from a regulatory gene n corepressor: a small molecule that cooperates with the repressor protein to switch an operon off

Two Operon Models: n Repressible: – Tryptophan operon model – Repressor activated by the presence of a corepressor n Inducible: – Lactose operon model – Repressor inactivated by inducer