Ch 8: Virus Structure and Function (Bio 286 - Microbiology) Flashcards
lytic or virulent virus
virus that kills its host
virus
NOT A LIVING CELL (infects living host to replicate); depends on host metabolism (energy, materials, and enzymes); always has a small GENOME (DNA or RNA, single or double stranded); always has a PROTEIN COAT (CAPSID) that protects the genome; infect all forms of living cells; viral genomes contain information for taking over host cell and for making viral proteins (capsid, genome replication process); may carry “captured’ host genes (toxins, genomes, etc)
viruses always have
genome and capsid
scale of viruses
range from about 20-200 nm (0.02 - 0.2 micrometers)
viruses do not have
ribosomes
capsids
protein shell of viruses; ICOSAHEDRAL, FILAMENTOUS (helical), or COMPLEX, or irregular
icosahedral capsids
each triangle made up at least 3 identical capsid proteins; allows small protein to cover a large volume; structure for many animal viruses
filamentous (helical) capsids
long tube of protein with genome inside; tube made up of hundred of identical protein subunits where the tube length reflects size of viral genome
complex capsids
mixture of icosahedral and filamentous shapes; structure for many bacteriophages
asymmetrical (irregular) capsids
irregular shapes arranged without symmetry; tend to be larger viruses such as poxviruses
viral envelope
allows fusion to host cell and organelle membrane; lipid composition is stolen from host; embedded proteins may be encoded by virus; coats viral capsid as the virus leaves the cell or organelle
naked virus
virus without an envelope, typically bacteriophages
enveloped virus
lipid bilayer membranes with virus-specific proteins surround the virus
viral genome
variation in genetic material: can be either DNA or RNA, either single or double stranded, either linear or circular; includes genes encoding viral proteins (capsid, envelope proteins if virus is enveloped, any polymerase not found in host cell)
cells always store genetic material
using DNA
Group I virus
dsDNA
Group I virus: dsDNA
poxvirus
Group II virus
ssDNA
Group II virus: ssDNA
parvovirus
Group III virus
dsRNA
Group III virus: dsRNA
epizootic hemorrhagic disease
Group IV virus
(+)ssRNA
Group IV virus: (+)ssRNA
poliovirus, Covid-19
Group V virus
(-)ssRNA
Group V virus: (-)ssRNA
influenza virus
Group VI virus
RNA retrovirus
Group VI virus: RNA retrovirus
HIV [(+)ssRNA]
Group VII virus
DNA pararetrovirus
Group VII virus: DNA pararetrovirus
hepatitis B [dsDNA]
(+) RNA
can be directly used to make viral proteins
(-) RNA
needs to be translated to have a complementary (+) RNA made in order to make viral proteins
reverse transcriptase (of retroviruses)
uses viral RNA as a template for DNA synthesis
reverse transcriptase
RNA dependent DNA polymerase
retrovirus
An RNA virus that reproduces by transcribing its RNA into DNA and then inserting the DNA into a cellular chromosome; an important class of cancer-causing viruses.
pararetrovirus
Also called DNA reverse-transcribing virus. A virus with a double-stranded DNA genome that generates an RNA intermediate and thus requires reverse transcriptase to generate progeny DNA genomes.
retrovirus virion
contains reverse transcriptase (which is NOT found in bacteriophage)
viral replication occurs
intracellularly
viral life cycles
- attach to host cell… 2. get viral genome into host cell… 3. replicate genome… 4. make viral proteins… 5. assemble capsids… 6. release progeny viruses from host cell
prophage replicates along with its host
while lytic genes are not expressed
permissive host
cell allowing complete replication cycle of a virus to occur
bacteriophage life cycles
- ATTACH to host cell receptor proteins… 2. INJECT genome through cell wall to cytoplasm… 3. REPLICATE genome (lytic or lysogenic)… 4. SYNTHESIZE capsid proteins… 5. ASSEMBLE progeny phase… 6. LYSE cell wall to release progeny phage
(bacteriophage) attachment to host cell proteins
bacterial cell receptors normally used for bacterial purposes (ex: sugar uptake, iron uptake, conjugation), so viruses will take advantage of host proteins
(bacteriophage) injects genome through cell wall to cytoplasm
capsid part stays outside of host cell while only the genome is forcefully injected into the host cell
Hershey-Chase experiment
(supporting injection) proves that viral DNA (not protein) enters host cells and directs the assembly of new viruses; radioactively labeled PROTEIN with 35S methionine and DNA with 32P phosphate… infected E.coli then sheared off phage with a blender… followed the isotopes in the infected bacteria to see that the 32P entered the cells during infection but not the 35S
lytic cycle
phage quickly replicates and kills host cell; generally occurs when host cell conditions are good or very bad; bacteria divide quickly but the phage replicates even faster
lysogenic cycle
phage is quiescent: may integrate into host cell genome and replicate only when host genome divides; generally occurs in moderate cell conditions; phage can reactivate to become lytic and kill host
lysogenic conversion
when a bacterium acquires a new trait from its temperate phage; virus assumes a host cell toxin and the virus can thus produce toxins
(bacteriophage) synthesize capsids
using host cell components
(bacteriophage) exit from host cell
lysis or slow release
lysis
makes protein to depolymerize peptidoglycan and bursts host cell to release progeny phage
slow release
filamentous phages can extrude individual progeny through cell envelope
eukaryotic virus life cycles
- ATTACHMENT to host cell receptor… 2. ENTRY into cell… 3. genome REPLICATION… 4. ALL VIRUSES MAKE PROTEINS WITH HOST RIBOSOMES… 5. ASSEMBLY of new viruses… 6. RELEASE of progeny viruses from host cell
(eukaryotic virus) entry into cell
injects genome directly; or taken up via endocytosis (VIROPEXIS): brought into cell in an endosome; or fuses envelope to plasma: releases capsid into cytoplasm
viropexis
Virus penetration/entry into cell by way of phagocytotic engulfment of viral particle into endosome via endocytosis
entry into a cell of a naked eukaryotic virus
can only occur through viropexis
entry into a cell of an enveloped eukaryotic virus
can occur through viropexis or fusion of envelope to plasma membrane of host cell
(eukaryotic virus) genome replication
DNA VIRUSES MUST GO TO CELL NUCLEUS (to use host polymerase or replicate in cytoplasm with viral polymerase); RNA VIRUSES MUST ENCODE A VIRAL POLYMERASE (tend to stay in cytoplasm with ribosomes because host cells cannot read RNA to make more RNA); ONLY (+)ssRNA CAN BE USED AS mRNA [dsRNA and (+)ssRNA genome can be translated, but (-)ssRNA and retrovirus genomes must be replicated to be translated]
(eukaryotic viruses) all viruses make proteins with host ribosomes
translation occurs in cytoplasm
(eukaryotic viruses) assembly of new viruses
capsid and genome; assembly may occur in cytoplasm or in nucleus; capsid proteins must move into the nucleus and envelope proteins are inserted into host membrane
(eukaryotic viruses) release of progeny viruses from host cell
lysis of cell (similar to bacteria) or BUDDING
budding
virus passes through membrane; membrane lipids surround capsid to form envelope
all enveloped viruses
bud from a membrane
culturing viruses - general amount of phages/mL at each phase
- VIRUSES BOUND TO HOST – free virus concentration drops… 2. ECLIPSE PERIOD – viruses making proteins, genomes, assembly; free virus concentration remains the same… 3. RAPID RISE PERIOD – burst of bacteriophage in bacterial lysis or rapid release of eukaryotic viruses increases free virus concentration
culturing viruses on plates
viruses grown with host cells as food (bacteria in culture, animal cells in tissue culture); host cells form confluent lawn (viruses from PLAQUES where host cells are killed)
plaque
“colony” of viruses; the hole in the confluent lawn where all cells died from virus replication “eating” cells
viral ecology
viruses present in ALL ENVIRONMENTS often at very HIGH DENSITIES; viruses limit population host density; viruses increase host diversity (strong selection for virus-resistant strains); people susceptible if exposed to a new virus (West Nile Virus, smallpox virus in new world)
restriction
general host mechanism to prevent invasion from viral DNA through destruction of foreign DNA by cleaving with enzymes
viroids
JUST NUCLEIC ACID: NAKED (single stranded) RNA; plant pathogens; circles and very small (100-300 nucleotides); a subviral particle that is only genetic material
prions
JUST PROTEIN; animal pathogens; “slow viruses” – spongiform encephalopathy, kuru, mad cow disease, CREUTZFELD-JAKOB DISEASE, DEFECTIVELY FOLDED HOST PROTEIN; subviral particle that is only protein shell
spongiform encephalopathy
degeneration of neurons – abnormal folding causes longer neurotransmitter trafficking, causing neurons to die and leave holes in neural material
