Chapter 5: Viruses and their multiplication Flashcards
Virus description
genetic element that can multiply only in a living (host) cell
Not living, not found on tree of life
Obligate intracellular parasite: Needs host cell for energy, metabolic intermediates, protein synthesis
Has its own nucleic acid genome
Virion
Virion- extracellular form of a virus
Exists outside host and facilitates transmission from one host cell to another
Replication/reproduction occurs only upon infection (entry into host cell)
Viral surface proteins
Capsid: the protein shell that surrounds the
genome of a virus
Naked viruses (e.g., most bacterial and plant
viruses) have no other layers
Enveloped viruses (e.g., many animal viruses) have
an outer layer consisting of a phospholipid bilayer
(from host cell membrane) and viral proteins
Nucleocapsid: nucleic acid + protein in enveloped
viruses
V surface proteins significance
Virion surface proteins important for host cell attachment and may include enzymes involved in infection/replication
virulent (lytic) infection
replicates and destroys host
Host cell metabolism redirected to support multiplication and virion assembly
lysogenic infection
host cell genetically altered because viral genome becomes part of host genome
Viral genome
either DNA or RNA genomes
single-stranded or double-stranded
usually smaller in size and gene content than cells
Group I:
double-stranded DNA viruses
Group II
single-stranded DNA viruses
Group III
double-stranded RNA viruses
Group IV
positive sense single-stranded RNA viruses
Group V
negative sense single-stranded RNA viruses
Group V
single-stranded RNA viruses with a DNA intermediate in their life cycle
Group VII
double-stranded DNA viruses with an RNA intermediate in their life cycle
beneficial virus examples
e.g., Arabidopsis infected with plum pox virus
increases drought tolerance
e.g., insect densovirus infection of rosy apple aphid
results in decreased size and offspring, but wings
form
e.g., hepatitis G coinfection of HIV patients
decreases HIV replication and infectivity
virus size
Most viruses are smaller than prokaryotic cells; ranging from 0.02 to 0.3 μm
Pandoravirus over 1 μm long
Poliovirus (~28 nanometers) is size of ribosome
Virion Structure
Some viruses only have one capsid protein
because small size of viral genomes restricts
number of proteins (e.g., tobacco mosaic virus)
Capsids can be put together through selfassembly (spontaneous) or may require host
cell folding assistance
Capsomere
individual protein molecules
arranged in a precise and highly repetitive pattern
around the nucleic acid making up the capsid
helical symmetry
: rod-shaped (e.g., tobacco mosaic virus or T M V)
length determined by length of nucleic acid
width determined by size and packaging of capsomeres
icosahedral symmetry
spherical
20 triangular faces, 12 vertices; 5, 3, or 2 identical segments
most efficient arrangement of subunits in a closed shell
requires fewest capsomeres
Highly complex structures of virus
Virion contains several parts with their own shapes and symmetry
Most complex are head-plus-tail bacteriophages (e.g., T4)
Viruses of Acanthamoeba (e.g., Pandoravirus (ovoid with apical pore), Mimivirus (stargate)
Enveloped Viruses characteristics
have lipoprotein membrane surrounding nucleocapsid
most (e.g., Ebola) use outer surface proteins to attach and infect
relatively few enveloped plant or bacterial viruses because of cell walls surrounding cell membrane
Entire virion enters animal cell during infection
Enveloped viruses exit more easily
Envelope + fibrils functions
Specificity and penetration controlled in part
by envelope biochemistry
Fibrils: hairlike polymer structures that attract
amoeba hosts
Apical pore and stargate function as portals to
release genome
Virus enzymes
lysozyme
neuraminidases
nucleic acid polymerases
lysozyme
makes hole in cell wall to allow nucleic acid entry
also lyses bacterial cell to release new virions
neuraminidases (influenza)
destroy glycoproteins and glycolipids
allows liberation of viruses from cell
nucleic acid polymerases
RNA replicases: RNA-dependent RNA polymerases
Reverse transcriptase: RNA-dependent DNA polymerase in retroviruses
Virus culturing
Bacterial viruses are easiest to grow (hosts in liquid medium or spread as “lawns” on agar and
inoculated with virus).
Animal and plant viruses cultivated in tissue cultures (from animal organ in culture medium or hairy
root-based system in liquid medium).
Titre
number of infectious virions per volume of fluid
Plaque assay description
Plaques are clear zones of cell lysis that develop on lawns of host cells where successful viral infection
occurs.
Calculate titre from # of plaques
Analogous to counting colonies
Similar process for animal viruses
Plants much harder: viruses must be purified and counted microscopically or through viral proteinspecific methods
Plaque assay mechanism
Why is the number of plaque-forming units (pfu) is always lower than direct counts by electron microscopy
efficiency of infection usually much less than 100%
defective virions or conditions inappropriate for infectivity
Useful for estimating appropriate titre to yield plaques
Five steps of viral replication in a permissive (supportive) host
attachment (adsorption) of the virion
penetration (entry, injection) of the virion nucleic acid
synthesis of virus nucleic acid and protein by host cell as redirected by virus
assembly of capsids and packaging of viral genomes into new virions
release of new virions from host cell
The Replication Cycle of a Lytic Bacterial Virus
one-step growth curve phases
virion numbers increase
when cells burst
-Eclipse phase: 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
Most complex penetration mechanisms found intailed bacteriophages (e.g., T4)
Virions attach to cells via tail fibers that interact
with polysaccharides on E. coli LPS layer
Tail fibers retract, and tail pins contact cell wall
T4 lysozyme forms small pore in
peptidoglycan
Tail sheath contracts, and viral DNA enters
cytoplasm similar to syringe injection
Capsid stays outside
Prokaryotes possess mechanisms to diminish viral infections
Genome injection does not ensure infection
toxin-antitoxin molecules
antiviral CRISPR
restriction endonucleases: enzymes that cleave foreign DNA at specific sites
Some viruses have modified genome that is unaffected by restriction enzymes
Production of T4 Virions and Release
Virion synthesis takes <30 minutes and ends in release of new virions from lysed cell
Within 1 minute of entry, host-specific protein synthesis ends and phage-specific protein synthesis starts
T4 genome encodes three major sets of proteins: early, middle, and late proteins.
early proteins: enzymes needed for DNA replication and proteins that modify host enzymes to express
viral genes
middle and late proteins: head and tail proteins and enzymes required to liberate mature phage particles
Time Course of Events in Phage T4 Infection
Production of T4 Virions and Release
Genome is pumped into capsid under pressure using energy-linked packaging motor
Host cell metabolism produces viral proteins and supplies ATP
T4 packaging stages
Empty proheads (bacteriophage head precursors) assembled
Packaging motor assembled at prohead opening and genome pumped into prohead using ATP
Motor discarded and capsid head sealed
T4 after packaging mechanism
After head is filled, T4 tail, tail fibers, and other components are self-assembled
Late enzymes break membrane and peptidoglycan
Lysis occurs, 100+ virions released
Virulent
Viruses always lyse and kill host after infection
Temperate
Viruses establish long-term, stable relationship but are capable of virulence
Lysogeny
Temperate viruses can enter a stage where few viral proteins produced, viral genome is replicated with host chromosome and passed to daughter cells
Lysogen
host cell that harbors temperate virus
- can result in lysogenic conversion with new genetic properties (e.g., virulence in pathogens)
Life cycle of a temperate phage
examples: lambda and P1
in lysogeny, genome is integrated into bacterial
chromosome forming prophage (viral DNA)
lysogeny maintained by phage-encoded repressor
protein
Inactivation of repressor induces lytic stage
(induction)
Viral DNA excised; phage early, middle, and late
proteins produced; virions produced and host lyses
Cell stress (e.g., DNA damage) induces lytic pathway
3 key differences between Eu viruses
Entire virion enters the animal cell
Eukaryotic cells contain a nucleus, the site of replication for many animal viruses
Viroplasms (membrane-bound viral factories) form in some eukaryotic cells to increase virion assembly rate
and protect from host defense
Viral Infection of Animal Cells features (specificity)
Studied in cell culture
Bind specific host cell receptors, typically used for cell-cell contact or immune function (e.g., poliovirus
and HIV receptors)
Viruses often infect only certain tissues because different surface proteins expressed by different
tissues/organs
Viral infection of animal cell mechanisms (entry)
Host cell entry occurs by fusion with
cytoplasmic membrane or endocytosis
Uncoating occurs at the cytoplasmic
membrane or in the cytoplasm
Viral DNA genomes enter nucleus, most viral
RNA genomes are replicated or converted
to DNA within nucleocapsid
animal Virion envelope assembly
After genome packaging, many animal viruses must be enveloped
Occurs during exit through lysis or budding when virus picks up part of cell’s cytoplasmic membrane and uses it as
part of envelope
Animal virion infection outcomes
Virulent infection: lysis of host cell, most common
Latent infection: Viral DNA exists in host genome as provirus (similar to lysogeny) and virions are not produced;
host cell is unharmed unless/until virulent pathway is triggered.
Persistent infections: slow 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
Possible Effects of Animal Virus Infection of Host Cells diagram
Viral Infection of Plant Cells similarity to animal cells
Plant viruses share many animal viral traits (e.g., mostly RNA genomes, complete virion enters cell, viral
factories form)
Major differences for plant viruses
Three major differences:
Wider host range
Mostly not enveloped
Transmission different
Plant viruse overview
Plant cell wall prevents entry by endocytosis and fusion
Viruses enter through wounds or penetration by insects, nematodes, fungi
Vectors: pests that transfer viruses to other host cell types
After entry, capsid removed, genome replicated, (in nucleus) new virions assembled
Movement proteins help viruses travel through plasmodesmata (channels) connecting cells; can infect entire
plant
V
Viral infection of plants mechanism
