Viral structureViral structure Nucleic acid in a protein coat (capsid)Nucleic acid in a protein coat (capsid)

sometimes viral envelope (host cell sometimes viral envelope (host cell membrane + viral proteins + membrane + viral proteins + glycoproteins)glycoproteins)

Depending on the virus the nucleic Depending on the virus the nucleic acid may be:acid may be: double or single stranded DNA or RNA double or single stranded DNA or RNA linear or circular linear or circular few or many genesfew or many genes

Viral Reproductive CyclesViral Reproductive Cycles reproduce in host cellreproduce in host cell

Host range dependent on proteinsHost range dependent on proteins Three patterns of replicationThree patterns of replication

DNA-->DNADNA-->DNA RNA -->RNARNA -->RNA RNA--> DNA-->RNARNA--> DNA-->RNA

Injects genome into host cellInjects genome into host cell Uses host’s nucleic acids, enzymes, Uses host’s nucleic acids, enzymes,

ribosomes, amino acids, ATP, etc. ribosomes, amino acids, ATP, etc.

Reproduction of Reproduction of bacteriophagesbacteriophages

-Bacteriophage: virus which infects bacteria-Bacteriophage: virus which infects bacteria Lytic cycleLytic cycle Death of host cellDeath of host cell Bacterium lyses upon virus exitBacterium lyses upon virus exit Exclusively lytic cycle = virulent phagesExclusively lytic cycle = virulent phages Bacteria use restriction enzymes to cut Bacteria use restriction enzymes to cut

viral DNA (pseudo- immune response)viral DNA (pseudo- immune response)

Reproduction of Reproduction of bacteriophagesbacteriophages

Lysogenic CycleLysogenic Cycle Phage DNA incorporated into host cell’s Phage DNA incorporated into host cell’s

chromosome (Prophage)chromosome (Prophage) Phage DNA replicated when bacteria Phage DNA replicated when bacteria

replicates its genome. replicates its genome.

**Temperate phages- uses both lytic & **Temperate phages- uses both lytic & lysogenic cycles; environmental trigger lysogenic cycles; environmental trigger causes switch to lytic cyclecauses switch to lytic cycle

Reproduction of Animal Cell Reproduction of Animal Cell VirusesViruses

Nearly all have envelopesNearly all have envelopes Helps virus go undetectedHelps virus go undetected Virus uses host ER or nucleus to Virus uses host ER or nucleus to

construct envelope construct envelope

Reproduction of Animal Cell Reproduction of Animal Cell VirusesViruses

RetrovirusesRetroviruses Reverse transcriptase transcribes RNA Reverse transcriptase transcribes RNA

template into DNA within hosttemplate into DNA within host Ex. HIVEx. HIV Viral DNA integrates into host genome Viral DNA integrates into host genome

(provirus) becoming a permanent fixture(provirus) becoming a permanent fixture

Effects of viral infectionEffects of viral infection Host cell damaged by:Host cell damaged by:

releasing hydrolytic enzymes from lysosomereleasing hydrolytic enzymes from lysosome Some viral proteins are toxicSome viral proteins are toxic

Symptoms usually related to body’s attempt to Symptoms usually related to body’s attempt to defend itselfdefend itself

Vaccines stimulate immune system to mount Vaccines stimulate immune system to mount defenses before actual infectiondefenses before actual infection

Effective anti-viral drugs interfere with viral Effective anti-viral drugs interfere with viral nucleic acid synthesisnucleic acid synthesis

Viruses mutate rapidlyViruses mutate rapidly new outbreaks & new new outbreaks & new host organisms (emergent viruses)host organisms (emergent viruses)

Plant cell virusesPlant cell viruses Symptoms: bleached or brown spots Symptoms: bleached or brown spots

on leaves & fruits, stunted growth, on leaves & fruits, stunted growth, damaged flowers or rootsdamaged flowers or roots

RNA genomeRNA genome rod-shaped or polyhedral capsidrod-shaped or polyhedral capsid SpreadSpread

Horizontal transmission- infection from Horizontal transmission- infection from an external source i.e. insect, farmeran external source i.e. insect, farmer

Vertical transmission- inherit virus from Vertical transmission- inherit virus from parent plantparent plant

Viroids & PrionsViroids & Prions ViroidsViroids

Small circular RNA molecules that infect plantsSmall circular RNA molecules that infect plants NO proteins made NO proteins made use host cell to replicate use host cell to replicate

its RNAits RNA Cause abnormal development & stunted Cause abnormal development & stunted

growthgrowth PrionsPrions

Very small infectious proteins in animalsVery small infectious proteins in animals Ex. mad cowEx. mad cow Slow acting with long incubation periodsSlow acting with long incubation periods Indestructible; thought to be transmitted Indestructible; thought to be transmitted

through foodthrough food

BacteriaBacteria Double stranded circular DNADouble stranded circular DNA concentrated in nucleoidconcentrated in nucleoid Contain plasmids- small self-Contain plasmids- small self-

replicating circles of DNA replicating circles of DNA not a part of the main genomenot a part of the main genome

Divide rapidlyDivide rapidly

Genetic Recombination in Genetic Recombination in BacteriaBacteria

TransformationTransformation Bacteria uptakes foreign DNABacteria uptakes foreign DNA

TransductionTransduction Phages carry bacterial genes from one Phages carry bacterial genes from one

host to another host to another ConjugationConjugation

Temporary joining of 2 bacterial cells Temporary joining of 2 bacterial cells allows for one-way direct transfer of allows for one-way direct transfer of genes using sex-piligenes using sex-pili

Requires F factorRequires F factor

Bacteria: Transposition of Bacteria: Transposition of GenesGenes Transposable elementsTransposable elements

DNA in a single bacteria moves within DNA in a single bacteria moves within the genomethe genome

Gene can be:Gene can be: “ “copy and paste”copy and paste” ““cut and paste”cut and paste”

Transposons- carry additional genes to Transposons- carry additional genes to new sitenew site

Bacteria: Regulation of gene Bacteria: Regulation of gene expression expression

Feedback inhibitionFeedback inhibition Environmental stimuli determine Environmental stimuli determine

enzyme activity (negative feedback)enzyme activity (negative feedback) OperonsOperons

Negative feedback through gene Negative feedback through gene expressionexpression

OperonsOperons Operon= promoter, operator, & the genes Operon= promoter, operator, & the genes

they controlthey control Operator is between promoter and genes Operator is between promoter and genes

being controlledbeing controlled Operator controls the access of RNA Operator controls the access of RNA

polymerase to the genespolymerase to the genes Inducible operons: transcription normally Inducible operons: transcription normally

off off stimulated by a regulatory proteinstimulated by a regulatory protein Repressible operons: transcription Repressible operons: transcription

normally onnormally on inhibited with binding of inhibited with binding of regulatory proteinregulatory protein

Repressible operon: trp Repressible operon: trp operonoperon RNA polymerase normally can access RNA polymerase normally can access

genesgenes Regulator gene located away from Regulator gene located away from

operon & has own promoteroperon & has own promoter produces inactive repressor proteinproduces inactive repressor protein

Repressor protein activated by Repressor protein activated by tryptophan tryptophan

Trp repressor protein binds to Trp repressor protein binds to operator preventing RNA polymerase operator preventing RNA polymerase attachmentattachment

Inducible operons: Lac Inducible operons: Lac operonoperon

Beta-galactosidase: lactoseBeta-galactosidase: lactoseglucose + glucose + galactosegalactose

Lac operonLac operon genes for Beta-galactosidase genes for Beta-galactosidase Regulator gene (lacI) produces active Regulator gene (lacI) produces active

repressor proteinrepressor protein blocks transcription blocks transcription Allolactose (Inducer) binds to repressor Allolactose (Inducer) binds to repressor

proteinprotein inactivation; lac operon is then inactivation; lac operon is then turned onturned on

Lac operon also affected by [glucose]; Lac operon also affected by [glucose]; highhigh operon remains off operon remains off