Viral structure Nucleic acid in a protein coat (capsid) Nucleic acid in a protein coat (capsid) sometimes viral envelope (host cell membrane + viral proteins + glycoproteins) sometimes viral envelope (host cell membrane + viral proteins + glycoproteins) Depending on the virus the nucleic acid may be: Depending on the virus the nucleic 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 genes few or many genes

Viral Reproductive Cycles reproduce in host cell reproduce in host cell Host range dependent on proteins Host range dependent on proteins Three patterns of replication Three patterns of replication DNA-->DNA DNA-->DNA RNA -->RNA RNA -->RNA RNA--> DNA-->RNA RNA--> DNA-->RNA Injects genome into host cell Injects genome into host cell Uses host’s nucleic acids, enzymes, ribosomes, amino acids, ATP, etc. Uses host’s nucleic acids, enzymes, ribosomes, amino acids, ATP, etc.

Reproduction of bacteriophages -Bacteriophage: virus which infects bacteria Lytic cycle Lytic cycle Death of host cell Death of host cell Bacterium lyses upon virus exit Bacterium lyses upon virus exit Exclusively lytic cycle = virulent phages Exclusively lytic cycle = virulent phages Bacteria use restriction enzymes to cut viral DNA (pseudo- immune response) Bacteria use restriction enzymes to cut viral DNA (pseudo- immune response)

Reproduction of bacteriophages Lysogenic Cycle Lysogenic Cycle Phage DNA incorporated into host cell’s chromosome (Prophage) Phage DNA incorporated into host cell’s chromosome (Prophage) Phage DNA replicated when bacteria replicates its genome. Phage DNA replicated when bacteria replicates its genome. **Temperate phages- uses both lytic & lysogenic cycles; environmental trigger causes switch to lytic cycle

Reproduction of Animal Cell Viruses Nearly all have envelopes Nearly all have envelopes Helps virus go undetected Helps virus go undetected Virus uses host ER or nucleus to construct envelope Virus uses host ER or nucleus to construct envelope

Reproduction of Animal Cell Viruses Retroviruses Retroviruses Reverse transcriptase transcribes RNA template into DNA within host Reverse transcriptase transcribes RNA template into DNA within host Ex. HIV Ex. HIV Viral DNA integrates into host genome (provirus) becoming a permanent fixture Viral DNA integrates into host genome (provirus) becoming a permanent fixture

Effects of viral infection Host cell damaged by: Host cell damaged by: releasing hydrolytic enzymes from lysosome releasing hydrolytic enzymes from lysosome Some viral proteins are toxic Some viral proteins are toxic Symptoms usually related to body’s attempt to defend itself Symptoms usually related to body’s attempt to defend itself Vaccines stimulate immune system to mount defenses before actual infection Vaccines stimulate immune system to mount defenses before actual infection Effective anti-viral drugs interfere with viral nucleic acid synthesis Effective anti-viral drugs interfere with viral nucleic acid synthesis Viruses mutate rapidly  new outbreaks & new host organisms (emergent viruses) Viruses mutate rapidly  new outbreaks & new host organisms (emergent viruses)

Plant cell viruses Symptoms: bleached or brown spots on leaves & fruits, stunted growth, damaged flowers or roots Symptoms: bleached or brown spots on leaves & fruits, stunted growth, damaged flowers or roots RNA genome RNA genome rod-shaped or polyhedral capsid rod-shaped or polyhedral capsid Spread Spread Horizontal transmission- infection from an external source i.e. insect, farmer Horizontal transmission- infection from an external source i.e. insect, farmer Vertical transmission- inherit virus from parent plant Vertical transmission- inherit virus from parent plant

Viroids & Prions Viroids Viroids Small circular RNA molecules that infect plants Small circular RNA molecules that infect plants NO proteins made  use host cell to replicate its RNA NO proteins made  use host cell to replicate its RNA Cause abnormal development & stunted growth Cause abnormal development & stunted growth Prions Prions Very small infectious proteins in animals Very small infectious proteins in animals Ex. mad cow Ex. mad cow Slow acting with long incubation periods Slow acting with long incubation periods Indestructible; thought to be transmitted through food Indestructible; thought to be transmitted through food

Bacteria Double stranded circular DNA Double stranded circular DNA concentrated in nucleoid concentrated in nucleoid Contain plasmids- small self-replicating circles of DNA Contain plasmids- small self-replicating circles of DNA not a part of the main genome not a part of the main genome Divide rapidly Divide rapidly

Genetic Recombination in Bacteria Transformation Transformation Bacteria uptakes foreign DNA Bacteria uptakes foreign DNA Transduction Transduction Phages carry bacterial genes from one host to another Phages carry bacterial genes from one host to another Conjugation Conjugation Temporary joining of 2 bacterial cells allows for one-way direct transfer of genes using sex-pili Temporary joining of 2 bacterial cells allows for one-way direct transfer of genes using sex-pili Requires F factor Requires F factor

Bacteria: Transposition of Genes Transposable elements Transposable elements DNA in a single bacteria moves within the genome DNA in a single bacteria moves within the genome Gene can be: Gene can be: “copy and paste” “copy and paste” “cut and paste” “cut and paste” Transposons- carry additional genes to new site Transposons- carry additional genes to new site

Bacteria: Regulation of gene expression Feedback inhibition Feedback inhibition Environmental stimuli determine enzyme activity (negative feedback) Environmental stimuli determine enzyme activity (negative feedback) Operons Operons Negative feedback through gene expression Negative feedback through gene expression

Operons Operon= promoter, operator, & the genes they control Operon= promoter, operator, & the genes they control Operator is between promoter and genes being controlled Operator is between promoter and genes being controlled Operator controls the access of RNA polymerase to the genes Operator controls the access of RNA polymerase to the genes Inducible operons: transcription normally off  stimulated by a regulatory protein Inducible operons: transcription normally off  stimulated by a regulatory protein Repressible operons: transcription normally on  inhibited with binding of regulatory protein Repressible operons: transcription normally on  inhibited with binding of regulatory protein

Repressible operon: trp operon RNA polymerase normally can access genes RNA polymerase normally can access genes Regulator gene located away from operon & has own promoter  produces inactive repressor protein Regulator gene located away from operon & has own promoter  produces inactive repressor protein Repressor protein activated by tryptophan Repressor protein activated by tryptophan Trp repressor protein binds to operator preventing RNA polymerase attachment Trp repressor protein binds to operator preventing RNA polymerase attachment

Inducible operons: Lac operon Beta-galactosidase: lactose  glucose + galactose Beta-galactosidase: lactose  glucose + galactose Lac operon  genes for Beta-galactosidase Lac operon  genes for Beta-galactosidase Regulator gene (lacI) produces active repressor protein  blocks transcription Regulator gene (lacI) produces active repressor protein  blocks transcription Allolactose (Inducer) binds to repressor protein  inactivation; lac operon is then turned on Allolactose (Inducer) binds to repressor protein  inactivation; lac operon is then turned on Lac operon also affected by [glucose]; high  operon remains off Lac operon also affected by [glucose]; high  operon remains off