Chapter 18

 Tobacco mosaic virus (TMV) – 1 st identified  Structure ◦ Smallest are only 20nm in diameter ◦ Genome may be double or single stranded DNA or double or single stranded RNA ◦ Enclosed by a protein shell – capsid  Protein subunits – capsomeres  Shape differs with virus ◦ May have a membranous envelope (viral envelope) to help with infecting host  Ex. Influenza virus

 Bacteriophages (phages) – viruses that infect bacteria ◦ Have most complex capsids ◦ Ex. T4 phage – infects E. coli.  May only reproduce within host cells. ◦ Host range – range of cells a virus may infect  Determined by receptor molecules on surface of cells  May be very wide or very narrow  Ex. West Nile – wide; polio – narrow

 Lytic cycle – virulent virus (phage) only goes through this cycle ◦ Ex. Cold, flu, rabies ◦ Results in death of host cell ◦ Steps  Attachment to host cell  Injection of viral DNA  Hydrolyzation – break down of host DNA  Assembly of new viruses  Release – cell lyses (breaks open)

 Lysogenic cycle ◦ Ex. HIV, Herpes ◦ Does not destroy host cell ◦ Temperate phages – may use either type of reproduction ◦ Steps  Attachment to host cell  Injection of viral DNA  Insertion of viral DNA into host DNA – called prophage  Reproduction of cell  Eventually switches to lytic cyle

 Single stranded RNA viruses may act as mRNA in the host cell – automatically producing viral proteins  Retrovirus – RNA viruses that use reverse transcriptase to transcribe DNA from and RNA template ◦ New viral DNA permanently integrates in the hosts DNA ◦ Host cell the produces viral proteins based on the new viral DNA ◦ Ex. HIV

 Vaccines – harmless variants of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen  Emerging viruses ◦ Mutation of existing viruses ◦ Spread of existing viruses across species ◦ Spreading of rare viruses as a population becomes less isolated  Plant viruses may spread in two ways ◦ Horizontal transmission – external source ◦ Vertical transmission – from parent

 Viroids - tiny, naked circular RNA that infect plants ◦ Do not code for proteins, but use cellular enzymes to reproduce ◦ Stunt plant growth  Prions – misfolded, infectious proteins that cause degenerative brain diseases ◦ Ex. mad cow disease ◦ Long incubation periods ◦ Trigger chain reaction conversions of normal proteins

 Structure ◦ Double-stranded circular DNA ◦ Nucleoid – region in bacterium densely packed with DNA (no membrane) ◦ Plasmids – small circles of DNA – in addition to the chromosome

◦ Cell wall~ maintains shape and provides protection – composed of peptidoglycan (sugars & proteins)  Gram stain – used to help id  Gram +: w/peptidoglycan penicillin action breaks down walls  Gram -: little peptidoglycan, have outer membranes w/ lipopolysaccharides; most pathogens; impede drug action ◦ Capsule: adherence; protection ◦ Fimbriae and Pili: adherence and conjugation ◦ Endospore: resistant cells for harsh conditions (250 million years!)

 Domain: Bacteria  Domain: Archaea  Size – 1-5 μm  Shape cocci (sphere) bacilli (rod) helical (spiral)  Combined biomass least 10X all eukaryotes

 1- Flagella – thinner than eukaryotes  2- Helical shape (spirochetes) – rotate internal flagella-like filaments  3- Slime  4-Taxis (movement away or toward a stimulus)

 Asexual – binary fission ◦ Most 1-3 hours, some only 20 min  Gene transfer and recombination ◦ Transduction – phages carry bacterial genes from 1 host cell to another  Generalized – random transfer of host cell chromosome  Specialized – incorporation of prophage DNA into host chromosome

◦ Transformation – genotype alteration by the uptake of naked, foreign DNA from the environment ◦ Conjugation – direct transfer of genetic material  cytoplasmic bridges and pili connect two bacterial cells  sexual reproduction  Rapid reproduction allow beneficial mutations to spread through population quickly

 Plasmids ◦ Small, circular, self-replicating DNA separate from the bacterial chromosome ◦ F (fertility) Plasmid – codes for the production of sex pili  F+ (donor) or F- (recipient)  Hfr cell – F factor is built into chromosome ◦ R (resistance) Plasmid – codes for antibiotic drug resistance ◦ Transposons – transposable genetic element  Piece of DNA that can move from 1 location to another in a cell’s genome (chromosome to plasmid, plasmid to plasmid, etc.)  “jumping genes”

 Bacteria respond to environmental change by regulating gene expression.  Operons – Unit of genetic function consisting of coordinately related clusters of genes with related functions (transcription unit) – includes the operator, the promoter, and the genes they control

 Repressible (trp operon) – operon is usually on but can be inhibited ◦ tryptophan synthesis ◦ promoter – RNA polymerase binding site; begins transcription ◦ operator – controls access of RNA polymerase to genes (if tryptophan not present) ◦ repressor – protein that binds to operator and prevents attachment of RNA polymerase – turns operon off  coded from a regulatory gene (tryptophan present – acts as a corepressor) ◦ transcription is repressed when tryptophan binds to a regulatory protein

 Inducible (lac operon) – operon is usually off but can be turned on (stimulated) ◦ 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