ANSC20003 Chap.4 Virus Structure, Replication and Detection
Virus Structure, Replication and Detection
Classified by build and by copying strategy
Viral taxonomy runs from realm down to species and is maintained by an international committee. Classification uses five criteria: the chemistry of the genome and whether it runs as one strand or two; the sequence itself; how the virus goes about copying itself; the symmetry of the coat; and whether an envelope is there. Two properties do most of the practical work.
A virus passes through a filter that stops bacteria, and it can only copy itself inside a living cell, so growing, detecting and controlling them are all questions about the host cell.
Genome chemistry sets the rate of change
DNA viruses copy their genomes with a high-fidelity polymerase and change slowly.
RNA viruses use a polymerase with an error rate several orders of magnitude higher, which is why they throw serotypes and strains, escape immune pressure and force vaccines to be updated.
That single difference explains why one vaccine is reformulated repeatedly and another is not.
The envelope decides how a virus travels
An enveloped virus carries a lipid layer with proteins and glycoproteins, is labile, and is sensitive to detergent, acid, drying and heat. It needs moist conditions and usually close contact to transmit, and it does not survive the gastrointestinal tract.
A non-enveloped virus is a protein capsid only, is stable and resistant to all four of those insults, builds up in the environment, transmits on contaminated surfaces and equipment, remains infective after drying, and survives passage through the gut.
Replication, and the difference between infection and disease
Growth requires a host: the natural host, laboratory animals, embryonated eggs, tissue or organ culture, or cell culture.
The cycle runs attachment, entry, uncoating, genome replication with transcription and translation, then assembly and exit. Infected cells may show visible change or none, so an unchanged culture has not proved absence.
Infection is not disease: outcomes range from exposure without infection through subclinical infection to death, and where an animal lands depends on agent factors of virulence, immune evasion and dose, and host factors of susceptibility, barriers, immune status and environment.
What this chapter covers
- 01
Viral taxonomy and the five criteria used to classify a virus
- 02
Genome chemistry, polymerase fidelity and why some viruses need vaccine updates
- 03
Enveloped against non-enveloped viruses across stability, transmission and control
- 04
Growth substrates, and why there is no broth for viruses
- 05
The five stages of the replication cycle and what each decides
- 06
Cytopathic effect, non-cytopathic infection, persistence and latency
- 07
Pathogenesis stages, sampling decisions, and the three routes to detection
Choosing a detection method a fortnight after recovery
- 1Explain why direct detection may fail at this timepoint.
- 1Name the method that fits and say why.
- 1State the limit of the result and one way to strengthen it.
Key terms
- Obligate intracellular parasite
- An organism that can only replicate inside a host cell, which is why virology has no equivalent of a nutrient broth.
- Nucleocapsid
- The viral genome together with the protein capsid enclosing it, arranged with helical or icosahedral symmetry.
- Cytopathic effect
- Visible change in infected cells in culture, such as fusion into multinucleated cells or the appearance of inclusion bodies.
- Latency
- A state in which viral genome persists in a host cell without producing infectious virus or clinical signs.
- Host range
- The set of species a virus can infect, which determines whether managing one species can control it.
Virus Structure, Replication and Detection FAQ
Which is more fragile, an enveloped or a non-enveloped virus?
The enveloped one. The envelope is a lipid layer, and lipid is what detergent, acid, drying and heat attack, so an enveloped virus is labile and needs moist conditions and close contact. A virus still infective after drying on a surface is telling you it has no envelope.
Why do RNA viruses need their vaccines updated more often?
Their polymerase has an error rate several orders of magnitude higher than that of a DNA virus, so their genomes vary far more. That variation produces serotypes and strains and allows escape from immune pressure, which is what forces continual reformulation.
Does an unchanged cell culture prove that a sample contained no virus?
No. Some infections are non-cytopathic and leave the monolayer looking normal, and the outcomes at cell level include persistence and latency as well as lysis. A culture with no visible change is therefore uninformative on its own rather than negative.
Why does the stage of pathogenesis affect which sample to take?
Because entry, spread, damage to target tissue and shedding happen in different places at different times. The right sample at the wrong stage returns a negative result from an infected animal, which is why the pathogenesis of the agent has to be considered before the sample is chosen.
Exam move
Rebuild the enveloped and non-enveloped table from memory until you can also run it backwards, from a described transmission pattern to a structural prediction, because that direction is what longer questions ask for. Learn the five replication stages as an ordered list and attach one consequence to each.
For infection outcomes, practise writing the sentence that separates infection from disease, since a question about detection or control often turns on it.
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