phages Flashcards
virus
genetic element containing either RNA or DNA that replicates in cells but is characterised by having extracellular state; single stranded or ds; small
T/F - viruses contain organelles
false
virus structure
inert; nucleic acid surrounded by protein capsid; sometimes other macromolecular components (envelope)
naked virus vs enveloped virus
naked - capsid composed of capsomeres is outermost layer
enveloped - has capsid, but surrounding that is macromolecular envelope
most prevalent classification system and how it works:
baltimore classification: class (I-VII), description (dsDNA or ssDNA). based on type of genome virus possesses
importance of bacteria-phage interaction
potential controller of microbial pop. size
phage infection may influence phenotype of host (rather than killing it)
important for molecular bio (eg phage can carry coding vector)
method of bacteriophage quantification & steps involved
plaque assay
- small vol. of (known) host cells to micture
- nutrient agar added for host cell to grow
- plate on top of nutrient agar
- incubate at best env. for host
- bacterial lawns will form - wherever this is a virus particle, a bacteria will be infect, the bacteria will be killed, and then new viruses will be released. see lawn of bacterial cells and see plaques.
what defines size of plaque? what is the unit of measurement for plaques?
size of virus
plaque-forming units (bc size of plaque isn’t indication of useful tings)
problems with plaques and how to solve
some bacterial lawns are really thick and you like…can’t see any real information from it. solved by dilution
virus replication
- attachment of viral protein and SPECIFIC receptor (carb/glycoprotein/etc) on host
- virus penetrated to host cell - usually complicated w phages and plant viruses. easier for animal viruses bc no cell wall. often fusion to animal cell membrane, like a vesicle, enters via endocytosis. viral genome is “uncoated” from capsid
- transcription & translation, relying on host machinery
permissive cells
allow virus multiplcation to occur
burst size
amount of new viral particles
methods of virus resistance
host cell lacks surface receptor
host restriction endonucleases destroy injected phage DNA
why does t4 have lysozyme
will lyse cell wall, releasing new viral particle
t4 time of replication
25 min
is t4 lytic or lysogenic
lytic
entry mechanism of bacteriophage
- T4 comes down, tail fibres will interact w receptors in outer membrane (polysacc)
- tail piece will undergo conformation change, contact w host cell surface
- core gets pushed down through cell surface, lysozyme allows it to go through peptidoglycan layers
- pore forms, genome enters
temperate bacteriophages + example
lysogenic - phage dna is injected into host cell cytoplasm, can either go directly into lytic cycle or can undergo lysogeny
some will integrate genome into host genome, or genome will just exist separately as a plasmid in host, can be carried for many generation, most of the viral genes are not expressed
lambda phage is temperate that infects E. coli
prophage
integrated/maintained phage genome
lysogen
bacterial cell carrying prophage (phage genome)
lysis
replication and release of mature virus
lysogeny
integration of viral DNA into host genome or maintenance as plasmid
transduction
transfer of host genes from one cell to another by virus
two types: generalized and specialised
generalized transduction
host bacterial gene accidentally packaged into lytic phage and transferred to new cell
specialised transduction
specific bacterial gene adjacent to site where lysogenic phage integrates into host genome is accidentally packaged and transferred to new host cell
ecological importance of lysogeny
most bacteria isolated from nature are lysogens
integrated viral genes can confer virulence factors on disease-causing bacteria
viruses are the most abundant bio. entity in aquatic ecosystems - influence on nutrient cycling, bacterial and algal diversity, gene transfer
lambda phage
infects e. coli
temperate phage
