Chapter 8: Viruses Flashcards
Virus
genetic element that cannot replicate independently of a living (host) cell
Virus particle
virion
extracellular form of a virus
exists outside host and facilitates transmission from one host cell to another
infection
entry of viral DNA into host cell
Capsid
the protein shell that surrounds the genome of a virus particle
Naked virus
have no other layers than a capsid
- ie most bacterial viruses
Enveloped virus
have outer layer consisting of a phospholipid bilayer (from host cell membrane) and viral proteins
- ie many animal viruses
Nucleocapsid
nucleic acid + protein capsid
virulent infection
lytic
- replicates and destroys host
lysogenic infection
host cell genetically altered because viral genome becomes part of host genome
capsomere
individual protein molecules arranged in a precise and highly repetitive pattern around the nucleic acid making up the capsid
icosahedral
spherical viruses
most efficient arrangement of subunits in a closed shell
requires fewest capsomeres
- ie. Ebola, human papillomavirus
Helical
rod-shaped viruses
length of virus determined by length of nucleic acid
width of virus determined by size and packaging of capsomeres
- ie. tobacco mosaic virus, Human rhinovirus
complex
ie. bacteriophage
lysozyme
makes holes in cell wall to allow nucleic acid entry
also lyses bacterial cell to release new virions
neuraminidases
destroy glycoproteins and glycolipids
allows liberation of viruses from cell
nucleic acid polymerases
RNA-replicases + reverse transcriptase
Phases of viral replication in a permissive host
attachment - penetration - synthesis - assembly - release
virus replication growth curve:
one-step growth curve
- numbers increase when cell bursts
Eclipse
genome replicated and proteins translated
maturation
packaging of nucleic acids in capsids
latent period
eclipse + maturation
release
cell lysis, budding, or excretion
burst size
number of virions released
lawns
hosts in liquid medium or spread on agar and inoculated with viruses
- best for bacterial viruses
tissue culture
animal organ in culture medium
- used for animal viruses
titer
number of infectious units per volume
plaque assay
plaques are clear zones that develop on lawns of host cells where successful viral infection occurs
(where cells are being lysed)
plating efficiency
number of plaque-forming units is always lower than direct counts by electron microscopy
- efficiency of infection is usually much lower than 100%
Attachment
requires complementary receptors on the surface of a susceptible host for its infecting virus
- proteins, carbs, glycoproteins, lipids, lipoproteins, or other cell structures
penetration
capsid left outside cell, viral genome and viral proteins (some viruses) enter host
T4 penetration
-attach via tail fibers interactions with E. coli LPS layer
-fibers retract and tail pins contact cell wall
-T4 lysozyme forms small pore in peptidoglycan
-tail sheath contracts, viral DNA enters cytoplasm
-capsid stays outside
restriction endonucleases
enzymes that cleave foreign DNA at specific sites
-specific for dsDNA (ssDNA/RNA unaffected)
modification
modification of cells own DNA at restriction enzyme recognition sites prevents cleavage of own DNA
- ie base substitution
circular permutation
same genes arranged in different orders
- features of tmany virus genomes
terminally redundant
some DNA sequences duplicated on both ends
early proteins
enzymes needed for DNA replication and transcription
middle and late proteins
head and tail proteins and enzymes required to liberate mature phage particles
virulent
viruses always lyse and kill host after infection
temperate
viruses replicate their genome in tandem with host genome and without killing the host, establishing long-term, stable relationship
lysogen
host cell that harbors temperate virus
lysogeny
most viral genes are not transcribed
- viral genome is replicated with host chromosome and passed to daughter cells
prophage
viral DNA
transduction
packaging of host chromosomal genes and transfer to new host
cl protein
the lambda repressor
- causes repression of lambda lytic events
cro repressor
controls activation of lytic events
latent infection
viral DNA exists in host genome and virions are not produced; host cell is unharmed unless/until virulence is triggered
persistent infection
release of virions from host cell by budding does not result in cell lysis
- infected cell remains alive and continues to produce virus
transformation
conversion of normal cell into tumor cell
retroviruses
RNA virus that replicate through a DNA intermediate
-ie HIV
gag
encode structural proteins in retrovirus
pol
encode reverse transcriptase and integrase in retrovirus
env
encode envelope proteins in retrovirus
