-
Virus:
genetic element that cannot replicateindependently of a living (host) cell
-
Virology:
the study of viruses
-
Virus particle (virion):
- extracellular form of a virus
- – Exists outside host and facilitates transmission from one host cell to another
- – Contains nucleic acid genome surrounded by a protein coat and, in some cases, other layers of material
-
Viral Genomes (Figure 9.1)
- – Either DNA or RNA genomes
- – Some circular, but most linear
-
– Most viruses are smaller than prokaryotic cells; range from ___ to ___
0.02 to 0.3 μm
-
-Most viral genomes are ___er than those of cells
smaller
-
Capsid:
- the protein shell that surrounds the genome of a virus particle (Figure 9.2)
- • Composed of a number of protein molecules arranged in a precise and highly repetitive pattern around the nucleic acid
-
Capsomere:
- -subunit of the capsid
- • Smallest morphological unit visible with an electron microscope
-
Nucleocapsid:
complete complex of nucleic acid and protein packaged in the virion (Figure 9.3)
-
Enveloped virus:
- – Have membrane surrounding nucleocapsid
- • Lipid bilayer with embedded proteins
- – Envelope makes initial contact with host cell
-
Helical symmetry:
length defined by:
width defined by:
- rod-shaped viruses (e.g.,tobacco mosaic virus)
- • Length of virus determined by length of nucleic acid
- • Width of virus determined by size and packaging of protein subunits
-
Complex Viruses (Figure 9.5b)
- – Virions composed of several parts, each with separate shapes and symmetries
- – Bacterial viruses contain complicated structures
- • Icosahedral heads and helical tails
-
The Virus Host
Viruses replicate only in certain types of cells or in whole organisms and have varying degrees of difficulty to grow
Bacterial:
Animal:
Plant:
Bacterial viruses are easiest to grow; modelsystems
- Animal viruses (and some plant viruses) can becultivated in tissue or cell cultures
- Plant viruses typically are most difficult because study often requires growth of whole plant
-
The icosohedral shape has ___ triangular sides with a minumum of __ protein units per side
-
Titer:
number of infectious units per volume of fluid
-
Plaque assay:
analogous to the bacterial colony; one way to measure virus infectivity (Figure 9.6)
-
Plaques
- -clear zones that develop on lawns of host cells
- • Lawn can be bacterial or tissue culture (Figure 9.7)
- • Each plaque results from infection by a single virus particle
-
cytopathic effects
- -degenerative changes in cells associated with the multiplication of certain viruses.
- -lysis is not observed
-
Efficiency of plating
- -used in quantitative virology
- – The number of plaque-forming units is almost always lower than direct counts by electron microscopy due to
- • Inactive virions
- • Conditions not appropriate for infectivity
-
Intact Animal Methods
- – Some viruses do not show recognizable changes in cell cultures yet cause death or disease in whole animals
- – Virus is diluted
- – Animals are infected with viral dilution
- – End point is calculated (LD50 or ID50)
-
Phases of Viral Replication
(5)
– Attachment (adsorption) of the virus to asusceptible host cell
– Entry (penetration) of the virion or its nucleic acid
– Synthesis of virus nucleic acid and protein by cellmetabolism as redirected by virus
– Assembly of capsids and packaging of viralgenomes into new virions (maturation)
– Release of mature virions from host cell
-
Virus replication
- typically characterized by aone-step growth curve (Figure 9.9)
- ? Latent period: eclipse + maturation
-
Burst size:
number of virions released
-
Attachment of virion to host cell is highly specific
(3points)
- – Requires complementary receptors on thesurface of a susceptible host and its infectingvirus
- – Receptors on host cell carry out normal functionsfor cell (e.g., uptake proteins, cell to cellinteraction)
- – Receptors include proteins, carbohydrates,glycoproteins, lipids, lipoproteins, or complexes
-
What effects does the attachment of a virus to its host cell have on both parties
The attachment of a virus to its host cell results in changes to both virus and cellsurface that facilitate penetration
-
Bacteriophage T4:
(4steps)
- -virus of E. coli; one of the mostc omplex penetration mechanisms (Figure 9.10)
- – Virions attach to cells via tail fibers that interact with polysaccharides on E. coli cell envelope
- – Tail fibers retract and tail core makes contact withE. coli cell wall
- – Lysozyme-like enzyme forms small pore inpeptidoglycan
- – Tail sheath contracts and viral DNA passes intocytoplasm
-
Restriction-modification systems
– DNA destruction system; only effective against double-stranded DNA viruses
-
– Restriction enzymes (restriction endonucleases)
- -cleave DNA at specific sequences
- – Modification of host’s own DNA at restriction enzyme recognition sites prevents cleavage of own DNA
-
Viral mechanisms to evade bacterial restriction systems
(2)
- – Chemical modification of viral DNA (glycosylation or methylation)
- – Production of proteins that inhibit host cell restriction system
-
Bacteriophage T4 glucosylates its cytosines converting them to _-___-___
why?
- 5-hydroxymethyl-cytosine
- to avoid digestion by bacteria
-
David Baltimore, Howard Temin, and Renato Dulbecco
- discovered retroviruses andreverse transcriptase
- – Shared 1975 Nobel Prize for Physiology or Medicine
- -Baltimore developed classification scheme for viruses based on relationship of viral genome to its mRNA
-
The Baltimore Classification Scheme (Figure 9.11)
(6) d s ds ss+ ss- retro dsD
- – Class I are double-stranded (ds) DNA viruses
- – Class II are single-stranded (ss) DNA viruses
- – Class III are dsRNA
- – Class IV and V are ssRNA (+ or )
- – Class VI are retroviruses
- – Class VII are dsDNA viruses that replicate through an RNA intermediate
-
Class 2 expression
converted to dsDna and then uses Rna polymerase to synthesize mRna
-
Class 3 expression
use of RNA dependant RNA polymerase to replicate and create mRNA
-
-
Once a host has been infected, new copies of the viral ____ must be made and virus-specific ______ synthesized in order for the virus toreplicate
-
Production of Viral Nucleic Acid and Protein
Generation of ___ ___ occurs first
messenger RNA (mRNA)
-
Viral genome serves as template for viral _____
mRNA
-
In some RNA viruses, ___ ___ itself is the mRNA
viral RNA
-
In some cases essential transcriptional enzymes are contained in the _____
virion
-
Nomenclature used to describe mRNA is used to describe the:
configuration of the genome of a single-stranded DNA or RNA virus (mRNA is said to be in plus (+) configuration; its complement is in minus () configuration)
-
Positive-strand RNA virus:
single-stranded RNAgenome with same orientation as its mRNA
-
Negative-strand RNA virus:
single-stranded RNA genome with orientation complementary to its mRNA
-
Viral Proteins
– Production follows synthesis of viral _____
mRNA
-
• Early proteins
(4points)
- – synthesized soon after infection
- – necessary for replication of virus nucleic acid– typically act catalytically
- – synthesized in smaller amounts
-
Late proteins
- • Synthesized later
- • Include proteins of virus coat
- • Typically structural components
- • Synthesized in larger amounts
-
Best-studied bacteriophages infect ____
- entericbacteria
- – Examples of hosts: E. coli, Salmonella enterica
-
Most phages contain _____ genomes
dsDNA
-
Viral Life Cycles
Virulent mode:
viruses lyse host cells after infection
-
Temperate mode:
- viruses replicate their genomes in tandem with host genome and without killing host
- • Virus can also be lytic
-
Virulent Bacteriophages
- – First viruses studied in detail contained linear, dsDNA genomes that infect enteric bacteria
- – Examples include T1, T2, …, T7
- • T2, T4, and T6 are closely related viruses; T4 most extensively studied
-
T4
- has a dsDNA genome that is circularly permuted and terminally redundant (Figure 9.13)
- – Both factors affect genome packagingDNA contains the modified base5-hydroxymethylcytosine (Figure 9.14)
- – DNA is resistant to virtually all knownrestriction enzymes
-
T4 genome can be divided into three parts:___, ____ and ___ proteins
early, middle, and late
-
– Early and middle proteins:
enzymes needed for DNA replication and transcription
-
– Late proteins:
head and tail proteins and enzymes required to liberate mature phage particles
-
Temperate viruses:
- can undergo a stable genetic relationship within the host (Figure 9.16)
- – But can also kill cells through lytic cycle
-
Lysogeny:
state where most virus genes are not expressed and virus genome (prophage) isreplicated in synchrony with host chromosome
-
Lysogen:
- - a bacterium containing a prophage
- - Under certain conditions lysogenic viruses may revert to the lytic pathway and begin to produce virions
-
Bacteriophage lambda (Figure 9.17)
(5points)
- – Linear, dsDNA genome
- – Complementary, single-stranded regions 12 nucleotides long at the 5' terminus of each strand(Figure 9.18)
- – Upon penetration, DNA ends base-pair, forming the cos site, and the DNA ligates and forms double-stranded circle
- – When lambda is lysogenic, its DNA integrates into E. coli chromosome at the lambda attachment site (att
 ) - – When it enters lytic pathway, lambdasynthesizes long, linear concatamers of DNAby rolling circle replication (Figure 9.19)
-
Regulation of lytic vs. lysogenic events inlambda is controlled by a complex genetic switch (Figure 9.20)
– Key elements are two repressor proteins:
- • cI protein (the lambda repressor): causes repression of lambda lytic events
- • Cro repressor: controls activation of lytic events
-
Entire virion ___ the animal cell, unlike in prokaryotes
enters
-
Eukaryotic cells contain a ___, the site of replication for many animal viruses
nucleus
-
Animal viruses contain all known ___ of viral genome replication (Figure 9.21)
modes
-
Many more kinds of enveloped ___ viruses than enveloped ___ viruses exist
- animal
- bacterial
- – As animal viruses leave host cell, they canremove part of host cell’s lipid bilayer for theirenvelope
-
Persistent infections:
- release of virions from host cell does not result in cell lysis
- • Infected cell remains alive and continues to produce virus
-
Latent infections:
delay between infection by the virus and lytic events
-
Transformation:
conversion of normal cell into tumor cell
-
Cell fusion:
two or more cells become one cell with many nuclei
-
Retroviruses:
- RNA viruses that replicatethrough a DNA intermediate
- – Enveloped viruses (Figure 9.23a)
- – Contain a reverse transcriptase (copiesinformation from its RNA genome into DNA),integrase, and protease
- – Virion contains specific tRNA molecules
-
Retroviruses have a unique genome
– Two identical ssRNA molecules of the plus (+)orientation
-
Retroviruses Contain 3 specific genes
- • gag: encode structural proteins
- • pol: encode reverse transcriptase andintegrase
- • env: encode envelope proteins
-
Process of Replication of a Retrovirus (Figure9.24)
- – Entrance into the cell
- – Removal of virion envelope at the membrane
- – Reverse transcription of one of the two RNAgenomes
- – Integration of retroviral DNA into host genome
- – Transcription of retroviral DNA
- – Assembly and packaging of genomic RNA
- – Budding of enveloped virions; release from cell
|
|
