Virus Properties Flashcards
LO 1. Define the basic properties of viruses.
submicroscopic, obligate intracellular parasites; not alive;
LO 2. List the strategies viruses employ for survival.
- house genetic info w/I capsids
- genome contains all info needed to initiate and complete an infectious cycle
- establish a relationship with a host, from benign to lethal
LO 3. Describe 2 means of classifying viruses.
- the Classical System (grouped by shared physical properties)
- the Baltimore System (by the way they produce mRNA)
LO 4. Understand basic methods for studying viruses.
electron microscopy cell culture animal models sequence analysis serology other molecular techniques
LO 5. Identify the main structural characteristics of virus particles.
capsids, envelopes, genetic info (DNA, RNA, or both)
LO 6. Describe the 7 basic virus genomes, protein expression strategies, and replication strategies of each
- dsDNA (use the - strand as template, RNA pol II, in nucleus or cytoplasm)
- gapped circular dsDNA (uses a protein as part of its pol machinery)
- ssDNA (ds intermediate made in nucleus)
- dsRNA
- ss(+)RNA (RNA-dependent RNA pol (RdRp)
- ss(-)RNA (also use RdRp)
- ss(+)RNA with DNA intermediate (copy their ssRNA into DNA first using reverse transcriptase)
LO 7. Describe a typical, generalized replication cycle of a virus.
- attachment
- entry and uncoating
- viral gene expression
- viral genome replication
- assembly of new viruses and egress of these from the cell
Viruses house their DNA or RNA genomes in small proteinaceous particles called _____.
capsids
Name 2 capsid shapes.
- helical
2. Isosahedral
What is the Classical System?
grouping viruses by their shared physical characteristics (type of genome, capsids, naked or enveloped, dimensions)
What is the Baltimore System?
categorizes viruses based on how they produce mRNA
What is the (+) strand?
mRNA that has an open reading frame- it’s ribosome ready and able to be translated into protein
What is the (-) strand?
complementary mRNA seq that is not directly translated to mRNA, but is used to make DNA
What are the 2 key principles of viral genomes?
- Genomes serve as the template for synthesis of progeny genome (small, finite # of nucleic acid copying strategies)
- viral genomes have to provide mechanisms for making mRNA
What are the 2 key principles of viral genomes?
- Genomes serve as the template for synthesis of progeny genome (small, finite # of nucleic acid copying strategies)
- viral genomes have to provide mechanisms for making mRNA
What is a helical capsid?
has rotational symmetry; arranges irregularly-shaped proteins in a circle to form a disk
What is an icosahedral capsid?
proteins arranged to form a hollow, quasi-spherical structure with genome inside it; 20 triangular faces
What is an envelope?
lipid bilayers with viral glycoproteins embedded in the membrane that viruses acquire by budding thru a host membrane
Why are envelopes important?
enveloped viruses don’t necessarily kill their host cells during replication, but non-enveloped viruses almost always do
Why are viral glycoproteins important?
they mediate entry and to specific hosts, assembly and egress, and evasion from immune systems
Describe the viral growth curve.
- eclipse period (0-12 hours; no detectable infection inside or out of the cell)
- latent period (16 hours; time it takes from initial infection to release of new viral particles)
- burst (sudden increase of PFUs)
What happens during the latent period?
attachment, entry, uncoating, gene expression, genome replication, assembly, and egress
What is attachment?
binding of viral-attachment protein to a cellular receptor, which can be protein (specific) or carbohydrate (less-specific)
What is entry?
penetration of the virus into the target cell. Energy dependent- cell must be metabolically active. Endocytosis or viral envelope fusion.
How do enveloped viruses gain entry?
viral envelope fuses with the plasma membrane or endocytic membranes; requires the presence of a specific fusion protein
What is uncoating?
once inside the cell, the viral particle loses its capsid and viral genome exposed
How do DNA viruses express their genes?
- fill (-) strand DNA using host’s DNA pol 2 in the nucleus
- transcribe normal mRNA
* ** exception: poxviruses replicate in cytoplasm and encode their own RNA pol
How do RNA viruses express their genes?
- fill (-) strand DNA using host’s DNA pol 2 in the nucleus
- transcribe normal mRNA
* ** exception: poxviruses replicate in cytoplasm and encode their own RNA pol
How do RNA viruses express their genes?
all have their own RNA-dependent RNA pol (RdRp) to produce mRNA and replicate RNA genomes
How do (+) stranded RNA viruses express their genes?
direct translation by cellular ribosomes; amplification via RdRp
How do (-) stranded RNA viruses and ds RNA viruses express their genes?
must transcribe (+) sense mRNA first by using RdRp
How do retroviruses express their genes?
first copy ssRNA into dsDNA using reverse transcriptase; integrate new dsDNA into host genome; transcribe mRNA using host RNA pol 2
What is assembly?
packaging of the genome into capsids
Name 2 ways icosahedral capsids can package their genomes.
- capsid assembles around the genome
2. genome is fed into preformed capsids
How do helical capsids package their DNA?
genome is coated with nucleocapsid protein during its synthesis
What does CPE stand for?
cytopathic effect
What is CPE?
capsid proteins produced in such high amounts that they are visible as cytoplasmic or nuclear inclusions on light microscopy; size and location characteristic of particular virus
What is egress?
exit from the infected cell
How do enveloped viruses egress?
budding (from plasma membrane = into extracellular environment; from Golgi or ER = secreted from cell)
How do naked viruses egress?
cell lysis
