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

Viral structure n Virus: “poison” (Latin); – infectious particles – nucleic acid in a protein coat (capsid) n DNA or RNA n Bacteriophages (phages)

n Some viruses have viral envelopes, membranes cloaking their capsids. n Mostly, these envelopes are derived from the membrane of the host cell. n They also have some viral proteins and glycoproteins. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 18.2c

n 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) n In eukaryotes, viruses are tissue specific – HIV: Helper T cells – Cold virus-cells lining URT Viral infection

Viral reproduction: Lytic Cycle n Results in death of host cell n 1. The lytic cycle: 1- attachment 2- injection 3- hydrolyzation 4- assembly 5- release n Virulent virus (phage reproduction only by the lytic cycle)

Viral reproduction: Lysogenic Cycle n 2.Lysogenic: Genome replicated w/o destroying the host cell n Genetic material of virus becomes incorporated into the host cell DNA (prophage DNA). n 3. Temperate virus (phages capable of using the lytic and lysogenic cycles) n May give rise to lytic cycle

Lysogenic Cycle: various Herpes viruses Herpes Simplex ( cold sores) Herpes Varicella-zoster ( chickenpox & shigles) Epstein Barr ( mononucleosis)

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

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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

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-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: “baxterial sex” n direct transfer of genetic material; cytoplasmic bridges; pili; sexual

n Conjugation transfers genetic material between two bacterial cells that are temporarily joined. n One cell (“male”) donates DNA and its “mate” (“female”) receives the genes. n A sex pilus from the male initially joins the two cells and creates a cytoplasmic bridge between cells. n “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. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig

n Transduction occurs when a phage carries bacterial genes from one host cell to another. n 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

n 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 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

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Transposons n jumping genes” n 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 n Unit of genetic function consisting of coordinately related clusters of genes with related functions (transcription unit) n In Prokaryotes n One Promoter n Advantage: one “Off/on” switch controls related genes. n Operator: these are the switches. By itself, operator is “on”. Repressor turns it “off” n Located between the promoter and the genes n Repressible & Inducible Operons n Repressors (repressible operons) n Or n Inducers (inducible operons)

Operons n Repressible (trp operon): n tryptophan (a.a.) synthesis n Tryptophan: corepressor 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

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)

n An Inducer (Allolactose—isomer of lactose) inactivates the repressor (turning on these genes so lactose can be digested) n More active in absence of glucose