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Viral structure Nucleic acid in a protein coat (capsid) sometimes viral envelope (host cell membrane + viral proteins + glycoproteins) Depending on the virus the nucleic acid may be: double or single stranded DNA or RNA linear or circular few or many genes Viral Reproductive Cycles reproduce in host cell Three patterns of replication Host range dependent on proteins DNA-->DNA RNA -->RNA RNA--> DNA-->RNA Injects genome into host cell Uses host’s nucleic acids, enzymes, ribosomes, amino acids, ATP, etc. Reproduction of bacteriophages -Bacteriophage: virus which infects bacteria Lytic cycle Death of host cell Bacterium lyses upon virus exit Exclusively lytic cycle = virulent phages Bacteria use restriction enzymes to cut viral DNA (pseudo- immune response) Reproduction of bacteriophages Lysogenic Cycle Phage DNA incorporated into host cell’s chromosome (Prophage) 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 Helps virus go undetected Virus uses host ER or nucleus to construct envelope Reproduction of Animal Cell Viruses Retroviruses Reverse transcriptase transcribes RNA template into DNA within host Ex. HIV Viral DNA integrates into host genome (provirus) becoming a permanent fixture Effects of viral infection Host cell damaged by: releasing hydrolytic enzymes from lysosome Some viral proteins are toxic Symptoms usually related to body’s attempt to defend itself Vaccines stimulate immune system to mount defenses before actual infection Effective anti-viral drugs interfere with viral nucleic acid synthesis 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 RNA genome rod-shaped or polyhedral capsid Spread Horizontal transmission- infection from an external source i.e. insect, farmer Vertical transmission- inherit virus from parent plant Viroids & Prions Viroids Small circular RNA molecules that infect plants NO proteins made use host cell to replicate its RNA Cause abnormal development & stunted growth Prions Very small infectious proteins in animals Ex. mad cow Slow acting with long incubation periods Indestructible; thought to be transmitted through food Bacteria Double stranded circular DNA concentrated in nucleoid Contain plasmids- small self-replicating circles of DNA not a part of the main genome Divide rapidly Genetic Recombination in Bacteria Transformation Transduction Bacteria uptakes foreign DNA Phages carry bacterial genes from one host to another Conjugation Temporary joining of 2 bacterial cells allows for one-way direct transfer of genes using sex-pili Requires F factor Bacteria: Transposition of Genes Transposable elements DNA in a single bacteria moves within the genome Gene can be: “copy and paste” “cut and paste” Transposons- carry additional genes to new site Bacteria: Regulation of gene expression Feedback inhibition Environmental stimuli determine enzyme activity (negative feedback) Operons Negative feedback through gene expression Operons Operon= promoter, operator, & the genes they control Operator is between promoter and genes being controlled Operator controls the access of RNA polymerase to the genes Inducible operons: transcription normally off stimulated by a regulatory protein Repressible operons: transcription normally on inhibited with binding of regulatory protein Repressible operon: trp operon RNA polymerase normally can access genes Regulator gene located away from operon & has own promoter produces inactive repressor protein Repressor protein activated by tryptophan Trp repressor protein binds to operator preventing RNA polymerase attachment Inducible operons: Lac operon Beta-galactosidase: lactoseglucose + galactose Lac operon genes for Beta-galactosidase Regulator gene (lacI) produces active repressor protein blocks transcription Allolactose (Inducer) binds to repressor protein inactivation; lac operon is then turned on Lac operon also affected by [glucose]; high operon remains off