Viruses Flashcards
What is the Lytic Cycle
Attachment: Phage attaches by tail fibers to host cell Penetration: Phage lysozyme opens cell wall; tail sheath contracts to force tail core and DNA into cell Biosynthesis: Production of phage DNA and proteins Maturation: Assembly of phage particles Release: Phage lysozyme breaks cell wall
What is Lysogenic Phase
following penetration not all viruses enter
the lytic phase
phage DNA inserted into host cell
chromosome
inserted prophage DNA is latent (no new
virions made and host cell remains healthy)
as bacterium multiplies, so does virus
Results of Lysogeny
Cells immune to reinfection Cells exhibit new properties (phage conversion) Specialized transduction can occur ◦ (bacterial genes transferred)
Pathogenic Conversion
Two mechanism by which naive bug can acquire virulence factors
◦ Conjugation in gram negative bacteria
◦ Lysogenic conversion in viruses
Important virulence factors are glycoprotein
spikes that contain two glycoproten
molecules
hemagglutinin (H)
neuraminidase (N)
neuraminidase (N)
breaks down mucus of
respiratory tract
hemagglutinin (H)
binds to host cell
receptors of nasal epithelium (lytic phase
leads to cell death of ciliated epithelial cells)
Hemagglutinin
HA
spikes used
for attachment to
host cells/
recognition
Neuraminidase
NA
spikes used
to release virus
from cell
Antigenic drift
viral nucleic acids exhibit
frequent mutations that alter surface antigens
(aids in evasion of immune responses),
Antigenic shift
swapping of genes from different
strains increases virulence
Differences between bacteriophages and Animal Viruses
Lysogeny replaced by latency, slow infections,
cancer
Enveloped viruses bud out, non enveloped
rupture plasma membrane
Adsorption
attaches to host cell receptors via
spikes/capsid protein
presence of host cell receptor required for
invasion
Penetration
a) endocytosis (naked or enveloped) – naked virus
must penetrate in this fashion (except phage)
b) fusion of envelope (enveloped only) with host
cell membrane
Uncoating
– envelope/capsid degraded and
nucleic acid liberated to cytoplasm
Synthesis
host cell enzymes (incuding
ribosomes) are hijacked to make more nucleic
acid (replication) and capsomers (protein
synthesis)
Assembly
capsid is constructed and nucleic acid
inserted (nucleocapsid formed)
in enveloped viruses, spikes are inserted into
cell membrane to be picked up during the release
phase
Release
a) nonenveloped – lysis (rupture) of cell releases
virions
b) enveloped – bud from surface and acquire
envelope
Prions
Noncellular infectious agents Protein only, no nucleic acid multiply by converting normal prion protein to pathogenic form are transmissible (sometimes across species) cause chronic neurodegenerative diseases (called encephalopathies) in humans and animals long latency from infection to first symptoms
Viruses
Outside of host cell virus inactive Inside of host cell active or inactive 20-200 nm standard sterilization filter is 0.22 m (220 nm) a) bacteria (larger than 0.22 m) are retained b) viruses (filterable) are not
capsid
protein shell
nucleocapsid
nucleic acid + capsid
virion
fully formed virus capable of infecting
host cell
naked virus
slang for a nonenveloped virus
