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Chapter 18 ~ The Genetics of Viruses and Bacteria
Figure 18-01 Chapter 18 ~ The Genetics of Viruses and Bacteria
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Figure 18-03
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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.
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Viral structure Virus = “poison” infectious particles = nucleic acid in a protein coat Capsid = Protein Coat DNA or RNA DS-DNA / DS – RNA SS – DNA / SS - RNA Bacteriophages = phages – Virus that infect bacteria
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Head DNA Tail sheath Tail fiber 80 225 nm 50 nm Bacteriophage T4
LE 18-4d Head DNA Tail sheath Tail fiber 80 225 nm 50 nm Bacteriophage T4
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Membranous envelope Capsid RNA Glycoprotein 80–200 nm (diameter) 50 nm
LE 18-4c Membranous envelope Capsid RNA Glycoprotein 80–200 nm (diameter) 50 nm Influenza viruses
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Capsomere of capsid Membranous envelope RNA Capsomere Capsid DNA RNA
LE 18-4 Capsomere of capsid Membranous envelope RNA Capsomere Capsid DNA RNA Head DNA Tail sheath Tail fiber Glycoprotein Glycoprotein 18 250 mm 70–90 nm (diameter) 80–200 nm (diameter) 80 225 nm 20 nm 50 nm 50 nm 50 nm Tobacco mosaic virus Adenoviruses Influenza viruses Bacteriophage T4
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Viral reproduction: Lytic Cycle
“Fast & Furious” Immediate death of host The lytic cycle: 1- attachment 2- entry 3- synthesis 4- assembly 5- release Virulent Virus (deadly) = reproduction only by the lytic cycle
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LYTIC CYCLE Attachment Entry of phage DNA and degradation of host DNA
Phage assembly Release Head Tails Tail fibers Assembly Synthesis of viral genomes and proteins
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Viral reproduction: Lysogenic Cycle
Genome copied 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 & lysogenic cycles)
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LYSOGENIC CYCLE Phage DNA The phage attaches to a
host cell and injects its DNA. Daughter cell with prophage Many cell divisions produce a large population of bacteria infected with the prophage. Phage DNA circularizes Phage Bacterial chromosome Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Lytic cycle Lysogenic cycle Certain factors determine whether The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells. The cell lyses, releasing phages. Lytic cycle is induced or Lysogenic cycle is entered Prophage New phage DNA and proteins are synthesized and assembled into phages. Phage DNA integrates into the bacterial chromosomes, becoming a prophage.
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RNA viruses Retroviruses: transcribe “backwards” = DNA from an RNA template Reverse transcriptase (enzyme) HIV--->AIDS
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Glycoprotein Viral envelope Capsid RNA (two identical strands) Reverse
transcriptase
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HIV
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REMINDER: animation “Flipped Chapter 18”
Retrovirus (HIV) REMINDER: animation “Flipped Chapter 18”
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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 – change colors. Prions: “infectious proteins”; “mad cow disease”; trigger chain reaction conversions; a transmissible protein
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Viral Infections (effect on plants)
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Bacterial genetics Nucleoid: Plasmids: Reproduction:
region in bacterium densely packed with DNA (no membrane) Plasmids: small circles of DNA Reproduction: binary fission (asexual)
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Bacterial DNA - transfer processes
Transformation: (review) genotype alteration by taking naked, foreign DNA from the environment (Griffith experiment)
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Bacterial Processes (con’t.)
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
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Bacterial Transduction
GENERALIZED
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Bacterial Transduction:
Specialized
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Conjugation: direct transfer of genetic material; cytoplasmic bridges; pili; sexual
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Bacterial Plasmids “jumping genes”
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: piece of DNA that can move from location to another in a cell’s genome (chromosome to plasmid, plasmid to plasmid, etc.) “jumping genes”
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F plasmid Bacterial chromosome F+ cell F+ cell Mating bridge F– cell
LE 18-18_2 F plasmid Bacterial chromosome F+ cell F+ cell Mating bridge F– cell F+ cell Bacterial chromosome Conjunction and transfer of an F plasmid from and F+ donor to an F– recipient F+ cell Hfr cell F factor
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Inverted repeat Inverted repeat Insertion sequence Insertion sequence
LE 18-19 Insertion sequence 5¢ 3¢ 3¢ 5¢ Inverted repeat Transposase gene Inverted repeat Transposon Insertion sequence Antibiotic resistance gene Insertion sequence 5¢ 3¢ 3¢ 5¢ Inverted repeat Transposase gene
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Barbara McClintock’s Discovery
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. “jumping genes”
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Animations Plasmids: very similar to OUR transformation experiment:
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OPERONS
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Important Concepts of the Operon
The Operator Region controls Transcription The Promoter Region controls Translation of the structural genes
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Both operons use a regulatory protein encoded in the DNA – separate from the rest of the operon
In one case the regulatory protein (repressor) protein is active until it is deactivated; In the other case the regulatory (repressor) protein is inactive until activated.
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Operons I - Repressible (trp operon):
Tryptophan promoter: RNA polymerase 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
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Tryptophan present, repressor active, operon off
LE 18-21b_1 DNA mRNA Protein Active repressor Tryptophan (corepressor) Tryptophan present, repressor active, operon off
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Tryptophan present, repressor active, operon off
LE 18-21b_2 DNA No RNA made mRNA Protein Active repressor Tryptophan (corepressor) Tryptophan present, repressor active, operon off
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Polypeptides that make up enzymes for tryptophan synthesis
LE 18-21a trp operon Promoter Promoter Genes of operon DNA trpR trpE trpD trpC trpB trpA Operator Regulatory gene RNA polymerase Start codon Stop codon 3¢ mRNA 5¢ mRNA 5¢ E D C B A Protein Inactive repressor Polypeptides that make up enzymes for tryptophan synthesis Tryptophan absent, repressor inactive, operon on
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Animations Trp Operon:
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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
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Lactose absent, repressor active, operon off
LE 18-22a Regulatory gene Promoter Operator DNA lacl lacZ No RNA made 3¢ mRNA RNA polymerase 5¢ Active repressor Protein Lactose absent, repressor active, operon off
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Lactose present, repressor inactive, operon on
LE 18-22b lac operon DNA lacl lacZ lacY lacA RNA polymerase 3¢ mRNA mRNA 5¢ 5¢ Permease Transacetylase Protein -Galactosidase Inactive repressor Allolactose (inducer) Lactose present, repressor inactive, operon on
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Lac Operon
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LE 18-23 Promoter DNA lacl lacZ RNA polymerase can bind and transcribe CAP-binding site Operator Active CAP cAMP Inactive lac repressor Inactive CAP Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized Promoter DNA lacl lacZ CAP-binding site Operator RNA polymerase can’t bind Inactive CAP Inactive lac repressor Lactose present, glucose present (cAMP level low): little lac mRNA synthesized
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Lactose present, glucose scarce (cAMP level high): abundant lac
LE 18-23a Promoter DNA lacl lacZ RNA polymerase can bind and transcribe CAP-binding site Operator Active CAP cAMP Inactive lac repressor Inactive CAP Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized
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Lactose present, glucose present (cAMP level low): little lac
LE 18-23b Promoter DNA lacl lacZ CAP-binding site Operator RNA polymerase can’t bind Inactive CAP Inactive lac repressor Lactose present, glucose present (cAMP level low): little lac mRNA synthesized
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Animations Lac Operon Trp Operon: CAP
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