Figure 18-01

Chapter 18 ~ The Genetics of Viruses and Bacteria

Figure 18-03

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 18-4d

80 225 nm

DNAHead

TailsheathTailfiber

Bacteriophage T450 nm

LE 18-4c

Glycoprotein

80–200 nm (diameter)

RNA

Capsid

Influenza viruses50 nm

Membranousenvelope

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

LE 18-9

Capsid

Viral envelopeGlycoprotein

Reversetranscriptase

RNA(two identicalstrands)

HIV

Retrovirus (HIV)

• REMINDER: animation “Flipped

Chapter 18”

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

Viral Infections

(effect on plants)

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 Transduction– GENERALIZED

Bacterial Transduction:– Specialized

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

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

OPERONS

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

Animations

Trp Operon:

http://bcs.whfreeman.com/thelifewire/content/chp13/1302002.html

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

Animations

Lac Operon http://www.sumanasinc.com/webcontent/

animations/content/lacoperon.html Trp Operon: http://bcs.whfreeman.com/thelifewire/

content/chp13/1302002.html CAP http://highered.mcgraw-hill.com/sites/

0072556781/student_view0/chapter12/animation_quiz_4.html