M13: Introduction to Viruses: Viral Growth and Replication Flashcards
Properties of viruses:
Viruses are (obligate / facultative) (intracellular / extracellular) parasites that use the machinery of the host to replicate.
The virus genome is composed either of _ or _ surrounded by a protein _: the capsid of some viruses is surrounded by an _
On its own, a virus may be considered as an inert biochemical complex since it cannot _ outside of a living cell.
Once it has invaded a cell it is able to direct the host cell machinery to _.
obligate
intracellular
DNA or RNA
capsid
envelope
replicate
synthesize new intact infectious virus particles (virions)
Properties of viruses:
Viruses attach to host cells by binding to receptors expressed on the host cell surface; the virus then enters host cells by _ or direct penetration of the host cell membrane.
Viruses lack the genetic information necessary for the generation of _ or for _.
Because viruses are _, they are entirely dependent on _ for chance movement and spread to infect other susceptible cells.
Can be acquired from other _ or from the _.
endocytosis
metabolic energy (ATP) protein synthesis (ribosomes)
non-motile
external physical factors
humans
environment
Properties of viruses:
Infection within the host is controlled primarily by _. Some viruses can escape elimination by the _ (latent or persistent viral infections).
Can be diagnosed by inoculation of , _ staining and identification of virus particles or antigens or by detection of viral nucleic acids ().
Viruses are rapidly and constantly evolving – they have high _ and _ rates. This allows them to evade host defenses and to develop antiviral drug resistance.
There are relatively few effective treatments for viruses.
cell-mediated immunity
immune system
cell cultures
immunocytochemical
PCR
replication
mutation
Virus Structure:
The genome of a virus is either a _ molecule or one or more molecules of _
- The viral genome is always enclosed within a protein shell called a _. Most exhibit one of two kinds of symmetry (2)
- Capsids may or not be composed of _, which are defined as morphologic subunits of the capsid discernible by electron microscopy.
- Capsids are formed by the specific _ of capsid proteins.
- Virus particles come in two forms:
- _ – composed of a viral genome enclosed within a capsid (_).
- _ – composed of a nucleocapsid surrounded by a lipid-containing envelope.
DNA
RNA
capsid
- Helical or rod shaped capsids
- Spherical capsid with icosahedral symmetry
capsomeres
clustering
- Nonenveloped (nucleocapsid).
- Enveloped
Definitions:
Virion - The _
Viral Genome – the viral _
Capsid – The protein _ surrounding the viral _
Capsomere - a clustering of _ discernible by electron microscopy.
Nucleocapsid - The viral _ + the _
Nonenveloped viruses - have only the viral _ and _ (naked nucleocapsids, Adenovirus).
Enveloped viruses – have a _ external to the _
intact, infectious viral particle
nucleic acid (either DNA or RNA)
shell, nucleic acid
capsid proteins
genome, capsid
genome, capsid
lipoprotein membrane, protein capsid
Importance of viral surface proteins:
- The proteins on the viral surface can determine viral _ (_)
- Neutralizing antibodies can recognize and bind to proteins on the viral surface and interfere with the ability of a virus to _, thus neutralizing the infectivity of a virus.
a. For nonenveloped viruses, neutralizing antibodies recognize the _ proteins.
b. For enveloped viruses, neutralizing antibodies recognize the _ proteins, typically _ that stick out from the lipoprotein membrane. - The envelope is an essential part of the infectious virion for _ viruses.
a. Without its envelope, the virus is _.
b. Enveloped viruses are in general (easier / harder) to destroy than nonenveloped viruses.
- tropism (what cell types the virus can infect)
- enter a cell
a. capsid
b. outer envelope, glycoproteins - enveloped
a. noninfectious
b. easier
Symmetry:
Virus capsids demonstrate one of two types of symmetry. (2)
Helical
Icosahedral
Symmetry:
Helical:
_ proteins are arranged in a helical coil.
One can visualize a helix by forming a two-dimensional lattice and rolling it into a cylindrical structure of a diameter that accommodates the viral _.
Animal viruses with helical nucleocapsids also possess _.
Capsid
nucleic acid
envelopes
Symmetry:
Icosahedral:
Capsid proteins are arranged in an icosahedron or _ structure.
The most economical way to build a spherical shell of maximal internal volume is to arrange (symmetric / nonsymmetric) proteins with icosahedral symmetry.
An icosahedron comprises 20 triangular faces organized with characteristic _, _, and _-fold axis of rotational symmetry.
spherical
nonsymmetric
2-, 3-, and 5-fold
Classification of Viruses:
Baltimore Classification System – Six classes based on the nature of the viral genome.
DNA
- dsDNA
- ssDNA
RNA
- dsRNA
- ssRNA
- -> plus strand –> retroviruses
- -> minus strand
DNA Viruses:
- Two classes of DNA viruses – _ and _
- Seven families (7)
- The initials of DNA virus families spells (HHAPPPI)
- All DNA viruses except _ are dsDNA
- All are linear except _ and _ (circular).
- All have icosahedral capsid except _
- All replicate in the nucleus except _
- Enveloped (4)
- Naked (noneveloped) (3)
dsDNA and ssDNA
hepadna, herpes, adeno, pox, papova, parvo, irido
parvoviruses
papovaviruses and hepadnaviruses
pox
pox
hepadna, pox, herpes, irido
parvo, adeno, papova
DNA Viruses:
ssDNA
–> nonenveloped (1)
dsDNA
- -> linear
- -> –> icosahedral capsid, replicate in nucleus
- -> –> –> enveloped (2)
- -> –> –> nonenveloped (1)
- -> –> nonicosahedral capsid, doesn’t replicate in nucleus, enveloped (1)
- -> circular
- -> –> enveloped (1)
- -> –> nonenveloped (1)
PARVOviridae
HERPESviridae
IRIDOviridae
ADENOviridae
POXviridae
HEPADNAviridae
PAPOVAviridae
RNA Viruses:
Four classes of RNA viruses containing thirteen medically relevant families.
- ss(+)RNA - (#) families
- ss(-)RNA - (#) families
- segmented ss(-)RNA - (#) families
- segmented dsRNA – _
six
three
three
reoviruses
RNA Viruses:
dsRNA
–> nonenveloped (1)
ssRNA(-)
–> enveloped (6)
ssRNA(+)
- -> enveloped (4)
- -> nonenveloped (2)
- segmented genomes
REOviridae*
ARENAviridae* BUNYAviridae* FILOviridae ORTHOMYXOviridae* PARAMYXOviridae RHABDOviridae
TOGAviridae
CORONAviridae
FLAVIviridae
RETROviridae
PICORNAviridae
CALCIviridae
RNA Viruses:
Mnemonic
Courageous - corona Bunyan - bunya* Fighter - filo Pilots - picorna Can - calici Fly - flavi Over - orthomyxo* Paraguay - paramyxo And - arena* Rhapidly - rhabdo* Return - retro To - toga Reo - reo*
The Life cycle of Viruses: Initiation Phase (3)
Attachment
Penetration
Uncoating
The Life cycle of Viruses:
Initiation Phase:
Attachment:
Attachment of the viral particle to the cell occurs via binding of a protein on the virion _ to a cell-surface protein that acts as the viral _.
The influenza virus _ protein binds to sialic acid, found as antennary sugars on most complex oligosaccharides found on cell surface proteins and lipids. Both _ and entry are mediated by this binding.
Penetration:
Movement of the complete _ or the _ through the cell plasma membrane
- _ between the viral lipid envelope and the cell plasma membrane - this can occur for (enveloped / unenveloped) viruses only.
- Receptor-mediated _ – enveloped or unenveloped viruses.
- Some viruses enter cells by poorly understood mechanisms.
Uncoating:
_ of the viral genome from the viral capsid.
surface
receptor
Hemagglutinin (HA)
attachment
viral particle
nucleocapsid
- Fusion, enveloped
- endocytosis
Release
The Life cycle of Viruses: Replication Phase (2)
Gene expression
Genome Replication
The Life cycle of Viruses:
Replication Phase:
Gene expression - synthesis of _.
Genome Replication - synthesis of _. Hundreds to thousands of copies of the viral genome are produced.
NOTE: Some viruses (e.g. _ viruses) have early and late gene expression phases
viral proteins
viral nucleic acid
DNA
The Life cycle of Viruses: Release Phase (2)
Assembly
Release/Egress
The Life cycle of Viruses:
Release Phase:
Assembly - viral particles are assembled = the viral _ + the viral _ + other virion-associated _.
NOTE: _ are cellular structures or “factories” where viral replication and assembly takes place (can be nuclear or cytoplasmic, depending on the virus).
Release/Egress – movement to the _ and release of _.
- _ of the infected cell may or may not occur.
- Enveloped viruses exit from the infected cell either by
• _ through the plasma membrane
• _ of secretory vesicles containing virus particles with the plasma membrane. - All enveloped viruses derive their envelopes from the _ (e.g. _, or _).
- Nonenveloped viruses usually exit by _ of the cell, but can also exit by unknown mechanisms that do not cause cell _.
Once the progeny virions have been released, they can initiate _. Hundreds to thousands of infectious virions are produced in each infected cell.
capsid
genome
proteins
Inclusion bodies
cell surface
infectious virus particles
- Death
- budding, fusion
- cellular membrane, (plasma or nuclear membranes)
- lysis, lysis
a new round of infection and virus replication in new cells
Replication Strategies of Viruses:
Although all viruses need _ in order to synthesize viral proteins, most viruses _ to replicate the viral genome (viral _).
EXCEPTION: _ (the only single-stranded DNA viruses) use the host-cell DNA polymerase to replicate their DNA.
host cell enzymes
encode their own enzymes
polymerases
parvoviruses
Replication Strategies of Viruses:
Sites of replication:
- Most DNA viruses replicate their nucleic acid and assemble into nucleocapsid complexes in the _.
EXCEPTION: poxviruses are DNA viruses that replicate entirely in the _; they encode their own _, their own _ for transcription of mRNA, as well as enzymes for RNA _ and _.
- Most RNA viruses replicate and assemble entirely in the _.
EXCEPTION: Influenza viruses are unusual segmented negative-sense RNA virus that replicate their RNA genomes and make mRNAs in the _.
- nucleus
cytoplasm DNA polymerase RNA polymerase capping polyadenylation
- cytoplasm
nucleus
Replication Strategies of Viruses:
Viral replication is associated with a (high / low) mutation rate. This is an important mechanism of _ and anti-viral _.
Viral polymerases are _ – especially the RNA-dependent RNA polymerases (RNA viruses) which lack _ ability.
high
immune evasion
drug resistance
error-prone
proofreading
