Chapter 8 - Viruses and Their Replication Flashcards
archaea viruses
- great diversity
- major role in oceanic microbiomes
How do archaea viruses differ from eukaryotic and bacterial viruses?
in genome, morphology, architecture, proteins encoded
virus
genetic element that cannot replicate independently of a living host cell
virology
study of viruses
virus particle/virion
extracellular form of a virus
-exists outside host and facilitates transmission from one host cell to another
virion make up
nucleic acid genome surrounded by a protein coat and, in some cases, other layers of material (phospholipid membrane with viral glycoproteins)
characteristics of viruses
- infectious, acellular patheogens
- obligate intracellular parasites with host and cell-type specificity
- DNA of RNA genome; never both
- lack genes for reproduction, requiring exploitation of host-cell genomes to reproduce
Viral genomes are sometimes circular, but most..
linear
RNA viruses that contain ________ can be directly read by ribosomes to synthesize viral proteins.
+ssRNA
Viruses containing _______ must use the _______ template for the synthesis of _______ before viral proteins can be made.
-ssRNA, -ssRNA, +ssRNA
+ssRNA
positive single strand RNA
-ssRNA
negative single strand RNA
Viruses can be classified on the basis of…
the host they infect
New classification schemes call for classification of viruses based on…
nucleic acid similarities
viruses sizes
usually smaller than prokaryotic cells at 0.02 to 0.3 um
-most viral genomes smaller than those of cells
capsid
the protein shell that surrounds the genome of a virus particle
capsid composition
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
enveloped virus
virus that contains additional layers around the nucleocapsid (lipid bilayer with embedded proteins)
-envelop makes initial contact with host cell
arrangement of nucleic acid and protein coat in a simple virus
RNA assumes a helical configuration surrounded by the protein capsid
-center is hollow
In ro-shaped viruses, length is determined by…. and width is determine by….
length: length of nucleic acid
width: size and packaging of protein subunits
icosahedral symmetry
spherical viruses
-most efficient arrangement of subunits in a closed shell
influenza virus envelope
contains “spikes” of hemagglutinin and neuraminidase forming a halo of projections around negatively stained virus particles
ebola virion composition
- helical
- spikes of envelope
- lipid envelope
- membrane associated proteins
- nucleocapsid proteins
If a virion has a ____________, they might infect humans.
lipid envelope
complex viruses composition
several parts, each with separate shapes and symmetries
-icosahedral head and helical tails
Most phages contain _____ genomes
dsDNA (double stranded)
Enzymes in virions critical to infection
- lysozyme-like
- nucleic acid polymerases
- neuraminidases
lysozyme-like enzymes
- make hole in cell wall
- lyses bacterial cell in later stages of infection to release new virions
nucleic acid polymerases
-for replication of the viral genome and for transcription of virus specific RNA
neuraminidases
surface protein
-enzymes that cleave glycosidic bonds and allowing liberation of viruses from cell
What is used to culture human or animal viruses?
flasks
viruses replicate only in…
certain types of CELLS or in WHOLE ORGANISMS
What kind of viruses are the easiest to grow?
bacterial; model systems
titer
number of infectious units per volume of fluid
plaque assay
analogous to the bacterial colony; one way to measure virus infectivity
plaques
clear zones that develop on lawns of host cells
- lawn can be bacterial or tissue culture
- each plaque results form infection by a single virus particle
efficiency of plating
used in quantitative virology
Number of plaque-forming units is almost aways lower that direct count due to…
- inactive virions
- conditions not appropriate for infectivity
phases of viral replication
- attachment/adsorption
- entry/penetration
- synthesis
- assembly/maturation
- release
latent period
eclipse + maturation; newly synthesized virions have not yet appeared outside the cell
burst size
number of virions released
lambda bacteriophage of e.coli
- structure: head and tail
- genome: dsDNA
- linear
- size of genome: 48500
MS2 bacteriophage of e. coli
icosahedral
- ssRNA
- linear
- 3600
T7 and T3 bacteriophage of e. coli
head and tail
- dsDNA
- linear
- 40000
Attachment of virion to host cell is..
highly specific
permissive cell
host cell that allows the complete replication cycle of a virus
T4 attached to cells via..
tail fibers that interact with polysaccharides on the E. coli cell envelope
Attachment of virus to its host cell results in ..
changes to both virus and cell surface that facilitate penetration
mechanisms to diminish viral infections in eukaryotes and prokaryotes
- immune defense mechanisms (RNA interference)
- restriction modification systems
restriction modification systems
- only effective against dsDNA viruses
- modification of hosts’ own DNA at RE recognition sites prevent cleavage of its DNA
mechanisms to diminish viral infections in bacteria
- chemical modification of viral DNA (glycosylation or methylation)
- production of proteins that inhibit host cell RE system
5-hydroxymethylcytosine
DNA resistant to virtually all known restriction enzymes
-modified base in T4 DNA
T4
-dsDNA genome
-circularly permuted
-terminally redundant
(last 2 factors affect genome packaging)
T4 genome can be divided into what 3 parts?
early, middle, and late proteins
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
packaging of T4 genome
- precursor of bacteriophage head is assembled
- packaging motor is assembled
- dsDNA is pupes into head under pressure using ATP
- T4 tail, tail fibers, and other components added
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
lysogeny
state where most virus genes are not expressed and virus genome is replicated in synchrony with host chromosome
prophage
virus genome
lysogen
a bacterium containing a prophage
animal virus infection stages
- attachment
- penetration
- uncoating
- release
- assembly
- biosynthesis
influenza
- influenza A virus
- negative ssRNA
- 13600
acquired immunodeficiency syndrome (AIDS)
- virus: human immunodeficiency virus (HIV)
- ssRNA/dsDNA positive
- 9700
ebola hemorrhagic fever
- ebola virus
- ssRNA neg
- 19000
severe acute respiratory syndrome (SARS)
- SARS virus
- ssRNA pos
- 29800
persistent infections
release of virions from host cell does not result in cell lysis
- can be latent of chronic
- infected cell remains alive and continues to produce virus
latent infections
delay between infection by the virus and lytic event
transformation
conversion of normal cell into tutor cell
cell fusion
two or more cells become one cell with many nuclei
replication of retovirus
- entry and uncaring of retrovirus
- reverse transcriptase
- viral DNA enters nucleus and integrates into host genome
- transcription by host RNA pol forms viral mRNA and genome copies
- translation of mRNA forms viral protein; new nucleocapsids assembled and released by budding
