Figure 18-01 Chapter 18 ~ The Genetics of Viruses and Bacteria
Dec 18, 2015
LE 18-11
Young ballet students in HongKong wear face masks toprotect themselves from thevirus causing SARS.
The SARS-causing agent is acoronarvirus like this one(colorized TEM), so named forthe “corona” of glyco-proteinspikes protruding form theenvelope.
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
LE 18-4
Capsomereof capsid
RNACapsomere
18 250 mm
Glycoprotein
70–90 nm (diameter)
DNA
Glycoprotein
80–200 nm (diameter) 80 225 nm
DNARNA
Capsid
Influenza viruses
Head
TailsheathTailfiber
Tobacco mosaic virus Adenoviruses Bacteriophage T450 nm50 nm50 nm20 nm
Membranousenvelope
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
http://www.hhmi.org/biointeractive/viral-lifecycle
LYTIC CYCLEAttachment
Entry of phage DNAand degradation of host DNA
Synthesis of viralgenomes and proteins
Assembly
ReleasePhage assembly
Head Tails Tail fibers
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) http://www.dnatube.com/video/3422/Virus-
Lysogenic-Cycle
LYSOGENIC CYCLE
Phage
Phage DNA
The phage attaches to ahost cell and injects its DNA.
Phage DNAcircularizes
Bacterial chromosome
Lytic cycle
The cell lyses, releasing phages.Lytic cycleis induced
or Lysogenic cycleis entered
Certain factorsdetermine whether
Lysogenic cycle
Occasionally, a prophageexits the bacterial chromosome,initiating a lytic cycle.
The bacterium reproducesnormally, copying the prophageand transmitting it to daughter cells.
Prophage
Many cell divisionsproduce a large population of bacteria infected withthe prophage.
Daughter cellwith prophage
Phage DNA integrates into thebacterial chromosomes, becoming aprophage.
New phage DNA and proteins aresynthesized and assembled into phages.
RNA viruses
Retroviruses: transcribe “backwards” = DNA from an RNA template
Reverse transcriptase (enzyme) HIV--->AIDS
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
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: (review) genotype alteration by taking naked, foreign DNA from the environment (Griffith experiment)
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
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
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”
LE 18-18_2
F plasmid Bacterial chromosome
F+ cellMatingbridge
F+ cell
F+ cellBacterial 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 18-19
Insertion sequence
Transposase gene
53
Invertedrepeat
53
35
35
Invertedrepeat
Transposon
Insertion sequence
Insertion sequence
Antibioticresistance gene
Transposase geneInverted repeat
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” 1940-1950
Animations
Plasmids: very similar to OUR transformation experiment:
http://www.sumanasinc.com/webcontent/animations/content/plasmidcloning.html
Important Concepts of the Operon
The Operator Region controls Transcription
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
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.
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
LE 18-21b_1
DNA
Protein
Tryptophan(corepressor)
Tryptophan present, repressor active, operon off
mRNA
Activerepressor
LE 18-21b_2
DNA
Protein
Tryptophan(corepressor)
Tryptophan present, repressor active, operon off
mRNA
Activerepressor
No RNA made
LE 18-21a
Promoter Promoter
DNA trpR
Regulatorygene
RNApolymerase
mRNA
3
5
Protein Inactiverepressor
Tryptophan absent, repressor inactive, operon on
mRNA 5
trpE trpD trpC trpB trpA
OperatorStart codonStop codon
trp operon
Genes of operon
E
Polypeptides that make upenzymes for tryptophan synthesis
D C B A
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
LE 18-22a
DNA lacl
Regulatorygene
mRNA
5
3
RNApolymerase
ProteinActiverepressor
NoRNAmade
lacZ
Promoter
Operator
Lactose absent, repressor active, operon off
LE 18-22b
DNA lacl
mRNA5
3
lac operon
Lactose present, repressor inactive, operon on
lacZ lacY lacA
RNApolymerase
mRNA 5
Protein
Allolactose(inducer)
Inactiverepressor
-Galactosidase Permease Transacetylase
Lac Operon
http://www.sumanasinc.com/webcontent/animations/content/lacoperon.html
http://phet.colorado.edu/en/simulation/gene-machine-lac-operon
LE 18-23
DNA
cAMP
lacl
CAP-binding site
Promoter
ActiveCAP
InactiveCAP
RNApolymerasecan bindand transcribe
Operator
lacZ
Inactive lacrepressor
Lactose present, glucose scarce (cAMP level high): abundant lacmRNA synthesized
DNA lacl
CAP-binding site
Promoter
RNApolymerasecan’t bind
Operator
lacZ
Inactive lacrepressor
InactiveCAP
Lactose present, glucose present (cAMP level low): little lacmRNA synthesized
LE 18-23a
DNA
cAMP
lacl
CAP-binding site
Promoter
ActiveCAP
InactiveCAP
RNApolymerasecan bindand transcribe
Operator
lacZ
Inactive lacrepressor
Lactose present, glucose scarce (cAMP level high): abundant lacmRNA synthesized
LE 18-23b
DNA lacl
CAP-binding site
Promoter
RNApolymerasecan’t bind
Operator
lacZ
Inactive lacrepressor
InactiveCAP
Lactose present, glucose present (cAMP level low): little lacmRNA synthesized