Figure Chapter 18 ~ The Genetics of Viruses and Bacteria

Figure 18-03

LE Young ballet students in Hong Kong wear face masks to protect themselves from the virus causing SARS. The SARS-causing agent is a coronarvirus like this one (colorized TEM), so named for the “corona” of glyco-protein spikes protruding form the envelope.

Viral structure n Virus = “poison” infectious particles = nucleic acid in a protein coat n Capsid = Protein Coat n DNA or RNA – DS-DNA / DS – RNA – SS – DNA / SS - RNA n Bacteriophages = phages – Virus that infect bacteria

LE 18-4d 80  225 nm DNA Head Tail sheath Tail fiber Bacteriophage T4 50 nm

LE 18-4c Glycoprotein 80–200 nm (diameter) RNA Capsid Influenza viruses 50 nm Membranous envelope

LE 18-4 Capsomere of capsid RNA Capsomere 18  250 mm Glycoprotein 70–90 nm (diameter) DNA Glycoprotein 80–200 nm (diameter) 80  225 nm DNA RNA Capsid Influenza viruses Head Tail sheath Tail fiber Tobacco mosaic virus Adenoviruses Bacteriophage T4 50 nm 20 nm Membranous envelope

Viral reproduction: Lytic Cycle n “Fast & Furious” – Immediate death of host n The lytic cycle: – 1- attachment – 2- entry – 3- synthesis – 4- assembly – 5- release n Virulent Virus (deadly) = reproduction only by the lytic cycle n

LYTIC CYCLE Attachment Entry of phage DNA and degradation of host DNA Synthesis of viral genomes and proteins Assembly Release Phage assembly Head Tails Tail fibers

Viral reproduction: Lysogenic Cycle n Genome copied 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 & lysogenic cycles ) n Lysogenic-Cycle Lysogenic-Cycle

LYSOGENIC CYCLE Phage DNA The phage attaches to a host cell and injects its DNA. Phage DNA circularizes Bacterial chromosome Lytic cycle The cell lyses, releasing phages. Lytic cycle is induced or Lysogenic cycle is entered Certain factors determine whether Lysogenic cycle Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells. Prophage Many cell divisions produce a large population of bacteria infected with the prophage. Daughter cell with prophage Phage DNA integrates into the bacterial chromosomes, becoming a prophage. New phage DNA and proteins are synthesized and assembled into phages.

RNA viruses n Retroviruses: transcribe “backwards” = DNA from an RNA template n Reverse transcriptase (enzyme) n HIV--->AIDS

LE 18-9 Capsid Viral envelope Glycoprotein Reverse transcriptase RNA (two identical strands)

HIV

Retrovirus (HIV) REMINDER: animation “Flipped Chapter 18”

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 – change colors. n Prions: “infectious proteins”; “mad cow disease”; trigger chain reaction conversions; a transmissible protein

Viral Infections (effect on plants)

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: (review) genotype alteration by taking naked, foreign DNA from the environment (Griffith experiment)

Bacterial Processes (con’t.) n Transduction: phages that carry bacterial genes from 1 host cell to another – generalized ~ random transfer of host cell chromosome – specialized ~ prophage gets into DNA of host chromosome

n Bacterial Transduction – GENERALIZED

n Bacterial Transduction: – Specialized

Conjugation: direct transfer of genetic material; cytoplasmic bridges; pili; sexual

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: piece of DNA that can move from location to another in a cell’s genome – (chromosome to plasmid, plasmid to plasmid, etc.) n “jumping genes”

LE 18-18_2 F plasmidBacterial chromosome F + cell Mating bridge F + cell Bacterial chromosome F – cell Conjunction and transfer of an F plasmid from and F + donor to an F – recipient F + cell Hfr cell F factor

LE Insertion sequence Transposase gene 5 3 Inverted repeat Inverted repeat Transposon Insertion sequence Insertion sequence Antibiotic resistance gene Transposase gene Inverted repeat

Barbara McClintock’s Discovery n Transposons: – piece of DNA that can move from location to another in a cell’s genome – (chromosome to plasmid, plasmid to plasmid, etc.) – Most of these chromosomes had no telomeres. n “jumping genes” n

Animations n Plasmids: very similar to OUR transformation experiment: n /content/plasmidcloning.html /content/plasmidcloning.html

OPERONS

Important Concepts of the Operon n The Operator Region controls Transcription n The Promoter Region controls Translation of the structural genes

Both operons use a regulatory protein encoded in the DNA – separate from the rest of the operon n In one case the regulatory protein (repressor) protein is active until it is deactivated; n In the other case the regulatory (repressor) protein is inactive until activated.

Operons I - Repressible (trp operon): n Tryptophan n promoter: RNA polymerase 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) transcription is repressed when tryptophan binds to a regulatory protein

LE 18-21b_1 DNA Protein Tryptophan (corepressor) Tryptophan present, repressor active, operon off mRNA Active repressor

LE 18-21b_2 DNA Protein Tryptophan (corepressor) Tryptophan present, repressor active, operon off mRNA Active repressor No RNA made

LE 18-21a Promoter DNA trpR Regulatory gene RNA polymerase mRNA 3 5 Protein Inactive repressor Tryptophan absent, repressor inactive, operon on mRNA 5 trpE trpD trpC trpBtrpA Operator Start codon Stop codon trp operon Genes of operon E Polypeptides that make up enzymes for tryptophan synthesis D C B A

Animations n Trp Operon: n / html / html

Operons II - 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

LE 18-22a DNA lacl Regulatory gene mRNA 5 3 RNA polymerase Protein Active repressor No RNA made lacZ Promoter Operator Lactose absent, repressor active, operon off

LE 18-22b DNAlacl mRNA 5 3 lac operon Lactose present, repressor inactive, operon on lacZ lacYlacA RNA polymerase mRNA 5 Protein Allolactose (inducer) Inactive repressor  -Galactosidase Permease Transacetylase

Lac Operon n nt/animations/content/lacoperon.html nt/animations/content/lacoperon.html gene-machine-lac-operon gene-machine-lac-operon

LE DNA cAMP lacl CAP-binding site Promoter Active CAP Inactive CAP RNA polymerase can bind and transcribe Operator lacZ Inactive lac repressor Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized DNA lacl CAP-binding site Promoter RNA polymerase can’t bind Operator lacZ Inactive lac repressor Inactive CAP Lactose present, glucose present (cAMP level low): little lac mRNA synthesized

LE 18-23a DNA cAMP lacl CAP-binding site Promoter Active CAP Inactive CAP RNA polymerase can bind and transcribe Operator lacZ Inactive lac repressor Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized

LE 18-23b DNA lacl CAP-binding site Promoter RNA polymerase can’t bind Operator lacZ Inactive lac repressor Inactive CAP Lactose present, glucose present (cAMP level low): little lac mRNA synthesized

Animations n Lac Operon n /content/lacoperon.html /content/lacoperon.html n Trp Operon: n / html / html n CAP n hill.com/sites/ /student_view0/c hapter12/animation_quiz_4.html hill.com/sites/ /student_view0/c hapter12/animation_quiz_4.html