Virus Structures and Classification Flashcards
What are features of viruses that distinguish them from bacteria/toxins?
- Size - Tobacco Mosaic Virus was able to pass through a filter while bacteria could not
- Growth - Showed that titer of TMV increased after infecting a plant proving it was not a toxin
What are a viruses requirements for living?
Obligate intracellular parasites (Are not autopoietic)
What are the two theories of virus origin?
Cellular Origin - Once cellular components but over time separated
What is a nucleocapsid?
RNA or DNA CORE that is protected by a protein coat (CAPSID)
What is the structure of nucleocapsids?
Repeating protein subunits called capsomeres
What is an envelope?
Virus-modified cellular membranes acquired upon exit from host that wraps around the nucleocapsid.
What is the range of virus size?
18-300nm
What is an advantage of being a large virus?
- Can carry more of their own genes/proteins to complete replication cycle.
- Can carry genes which will alter the host immune response
What is a disadvantage of being a large virus?
Longer replication time (more immune system opportunities)
Why are viruses obligate intracellular parasites?
They NEVER convert RNA into protein
What is a replication strategy used by ssDNA?
Hairpin. Repeating ends with ssDNA strand in the middle to which host machinery can bind and copy.
What is a replication strategy used by dsDNA?
Rolling circle. Similar to Bacteria
What is (+)ssRNA?
RNA that is ready to be processed by a ribosome to make a protein.
What is (-)ssRNA?
RNA that must be copied before it can be processed by a ribosome to make a protein.
What are the advantages of having a large genome?
Can carry more genes (modulate host immune responses)
What do most DNA viruses need that RNA viruses do not?
Because DNA is usually replicated in the nucleus, most DNA viruses need access to the nucleus.
What do RNA viruses need that DNA virus do not?
A gene that encodes for an RNA dependent RNA polymerase OR carry a protein in that is capable of RNA polymerase with them into the cell.
What is tropism?
The range of hosts a virus is capable of infecting
What are different pathways that a virus can enter a cell?
- Receptor mediated endocytosis
- Direct penetration (attach and secrete their genome inside)
- Non-enveloped viruses are not well understood
After entering the cytoplasm, what must dsDNA viruses do?
(Nucleocapsid) Attach to a nuclear pore and get their genome into the nucleus
Does dsRNA viruses ever release its genome into the cytoplasm?
NO. dsRNA doesn’t occur in host cells and would trigger host immunity
Where does dsRNA viruses replicate their genome?
Inside themselves and then exported to cytoplasm
What are the different mechanisms of virus replication?
- Adenovirus - empty protein coat imports genome
- Reovirus - RNA packaged during capsid assembly
- Retrovirus - preassembly on a membrane
How are viruses released?
- Lysis - Viral molecules that rupture cellular membrane (bacteriophage)
- Weak lysis - Depends on membrane breakdown after cell death
- Budding - Enveloped viruses use cell membrane as outer coating of virus particle (enveloped only)
What disadvantage is there for a virus to replicate in the nucleus?
The nucleus must be in a replication stage for there to be replication of the virus genome.
What advantage/disadvantage is there with a lytic style of infection?
Advantage: The progeny are able to infect other cells. Disadvantage: The progeny cells are exposed to the immune system.
What is the time for one step growth?
Time from the start of the infection to the beginning of the plateau (cell death)
How do + sense RNA viruses do replication?
+ sense RNA can be used for translation, makes RNA for template for new + sense
How do - sense RNA viruses do replication?
- sense RNA make full-length +ssRNA intermediate for translation and replication
Describe the 4 steps in the genome replication process used by dsRNA viruses
- Virus particle includes viral polymerase
- dsRNA induces innate immune response to genome inside the particle
- mRNA is synthesized in the particle and exported to the cytoplasm
- mRNA serves as the + strand in virus genome, - strand is synthesized during assembly
What 3 things must occur for genome replication of DNA viruses (include one virus that is an exception)?
- DNA virus must gain access to the nucleus (pox viruses are an exception because they already have RNA polymerase in virion)
- DNA virus must prepare the cell for DNA replication (growth plase, dNTP production, replication machinery)
- DNA virus must ensure that ends are copied
Describe the genome, type of particle, and location of replication for the smallpox/vaccinia virus
- Genome: dsDNA
- Particle: Envelope- Brick-shaped
- Site of Replication: Cytoplasm
Describe the genome, type of particle, and location of replication for the Yellow fever/West Nile virus
- Genome: +ssRNA
- Particle: Enveloped - sphericle
- Site of Replication: Cytoplasm
Describe the genome, type of particle, and location of replication for the Influenza virus
- Genome: -ssRNA
- Particle: 8 segments, enveloped
- Site of Replication: nucleus
Describe the genome, type of particle, and location of replication for the poliovirus/rhinovirus
- Genome: +ssRNA
- Particle: Non-enveloped and spherical
- Site of Replication: Cytoplasm
Describe the genome, type of particle, and location of replication for HIV (retroviridae virus)
- Genome: +ssRNA (2 COPIES!)
- Particle: Enveloped and Spherical
- Site of Replication: Nucleus (Reverse Transcriptase)
