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Chapter 18~ Microbial Models: The Genetics of Viruses and Bacteria Viral structure Virus: “poison” (Latin); – – infectious particles nucleic acid in a protein coat (capsid) DNA or RNA Bacteriophages (phages) Some viruses have viral envelopes, membranes cloaking their capsids. Mostly, these envelopes are derived from the membrane of the host cell. They also have some viral proteins and glycoproteins. Fig. 18.2c Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Viral infection Host range: – – Each type of virus can only infect and parasitize a limited range of host cells by a “lock and key” fit between viral proteins and receptors on host cell membrane. May be narrow (1 species) or wide ( several species) In eukaryotes, viruses are tissue specific – – HIV: Helper T cells Cold virus-cells lining URT Viral reproduction: Lytic Cycle Results in death of host cell 1. The lytic cycle: 1- attachment 2- injection 3- hydrolyzation 4- assembly 5- release Virulent virus (phage reproduction only by the lytic cycle) Viral reproduction: Lysogenic Cycle 2.Lysogenic: Genome replicated w/o destroying the host cell Genetic material of virus becomes incorporated into the host cell DNA (prophage DNA). 3. Temperate virus (phages capable of using the lytic and lysogenic cycles) Lysogenic Cycle: various Herpes viruses Herpes Simplex ( cold sores) Herpes Varicella-zoster ( chickenpox & shigles) Epstein Barr ( mononucleosis) QuickTime™ and a Cinepak decompressor are needed to see this picture. RNA viruses: a look at the Retroviruses Retroviruses: transcribe DNA from an RNA template (RNA--->DNA). Reverse transcriptase (catalyzing enzyme) HIV--->AIDS ..\bacteria\Google Image Result for http--trc_ucdavis_edu-biosci10vbis10v-week7-20f-Slide4_gif.mht ..\bacteria\Google Image Result for http--www_learnConjugation and transformation animations -imagesthumbnails-a_infect1_jpg.mht 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 Bacterial genetics Nucleoid: region in bacterium densely packed with DNA (no membrane) Plasmids: small circles of DNA Reproduction: binary fission (asexual) 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: “baxterial sex” direct transfer of genetic material; cytoplasmic bridges; pili; sexual Conjugation transfers genetic material between two bacterial cells that are temporarily joined. One cell (“male”) donates DNA and its “mate” (“female”) receives the genes. A sex pilus from the male initially joins the two cells and creates a cytoplasmic bridge between cells. “Maleness,” the ability to form a sex pilus and donate DNA, results from an F factor as a section of the bacterial chromosome or as a plasmid. Fig. 18.14 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Transduction occurs when a phage carries bacterial genes from one host cell to another. In generalized transduction, a small piece of the host cell’s degraded DNA is packaged within a capsid, rather than the phage genome. – – – When this pages attaches to another bacterium, it will inject this foreign DNA into its new host. Some of this DNA can subsequently replace the homologous region of the second cell. This type of transduction transfers bacterial genes at random. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Specialized transduction occurs via a temperate phage. – – – When the prophage viral genome is excised from the chromosome, it sometimes takes with it a small region of adjacent bacterial DNA. These bacterial genes are injected along with the phage’s genome into the next host cell. Specialized transduction only transfers those genes near the prophage site on the bacterial chromosome. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings 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 ..\bacteria\Google Image Result for http--trc_ucdavis_edu-biosci10vbis10v-week7-20f-Slide4_gif.mht ..\bacteria\Google Image Result for http--www_learnConjugation and transformation animations -imagesthumbnails-a_infect1_jpg.mht Transposons jumping genes” Barbara McClintock 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.); May alter gene expression Operons Unit of genetic function consisting of coordinately related clusters of genes with related functions (transcription unit) In Prokaryotes One Promoter Advantage: one “Off/on” switch controls related genes. Operator: these are the switches. By itself, operator is “on”. Repressor turns it “off” Located between the promoter and the genes Repressible & Inducible Operons Repressors (repressible operons) Or Inducers (inducible operons) Operons Repressible (trp operon): tryptophan (a.a.) synthesis Tryptophan: corepressor 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, II 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 Def: Unit of genetic function consisting of coordinately related clusters of genes with related functions (transcription unit) An Inducer (Allolactose—isomer of lactose) inactivates the repressor (turning on these genes so lactose can be digested) More active in absence of glucose