biology

Astrovirus

Astrovirus is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Astrovirus rather than just read about it. In short: Astroviruses (Astroviridae) are a type of virus that was first discovered in 1975 using electron microscopes following an outbreak of diarrhea in humans. In addition to humans, astroviruses have now been isolated from numerous mammalian animal species (and are classified as genus Mamastrovirus) and from avian species such as ducks, chickens, and turkey poults (classified as genus Avastrovirus).

Key takeaways

Astrovirus belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Astrovirus to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Astrovirus from memory before moving on to harder problems.

Reference excerpt

Astroviruses (Astroviridae) are a type of virus that was first discovered in 1975 using electron microscopes following an outbreak of diarrhea in humans. In addition to humans, astroviruses have now been isolated from numerous mammalian animal species (and are classified as genus Mamastrovirus) and from avian species such as ducks, chickens, and turkey poults (classified as genus Avastrovirus). Human astroviruses have been shown in numerous studies to be an important cause of gastroenteritis in young children worldwide. In animals, Astroviruses also cause infection of the gastrointestinal tract but may also result in encephalitis (humans and cattle), hepatitis (avian) and nephritis (avian). Astroviruses are 28–35 nm diameter, icosahedral viruses that have a characteristic five- or six-pointed star-like surface structure when viewed by electron microscopy. Along with the Picornaviridae and the Caliciviridae, the Astroviridae comprise a third family of nonenveloped viruses whose genome is composed of plus-sense, single-stranded RNA. Astrovirus has a non-segmented, single stranded, positive sense RNA genome within a non-enveloped icosahedral capsid.

Microbiology

Taxonomy This family of viruses consists of two genera, Avastrovirus (AAstV) and Mamastrovirus (MAstV). The International Committee on Taxonomy of Viruses (ICTV) established Astroviridae as a viral family in 1995. There have been over 50 astroviruses reported, although the ICTV officially recognizes 22 species. The genus Avastrovirus comprises three species; Chicken astrovirus (Avian nephritis virus types 1–3), Duck astrovirus (Duck astrovirus C-NGB), and Turkey astrovirus (Turkey astrovirus 1). The genus Mamastrovirus includes Bovine astroviruses 1 and 2, Human astrovirus (types 1–8), Feline astrovirus 1, Porcine astrovirus 1, Mink astrovirus 1 and Ovine astrovirus 1.

Structure

Astroviruses have a star-like appearance with five or six points. Their name is derived from the Greek word "astron" meaning star. They are non-enveloped RNA viruses with cubic capsids, approximately 28–35 nm in diameter with T=3 symmetry. Human astroviruses are part of the Mammastrovirus genus and contains 8 serotypes. The human astrovirus capsid spikes have a distinct structure. The spike domain in particular has a 3-layered beta-sandwiches fold and a core, 6-stranded beta-barrel structure. The beta-barrel has a hydrophobic core. The triple-layered beta-sandwich is packed outside the beta-barrel. The spike also forms a dimer. This unique structure was found to be similar to the protein projections found on the capsid of the hepatitis E virus. The projection domain of the human astrovirus contains a receptor binding site for polysaccharides. The amino acid sequence of the astrovirus capsid protein does not have similar homology to other known viral proteins, but the closest would be hepatitis E virus.

Life cycle Astroviruses infect birds and mammals through the fecal-oral route. They have a tissue tropism for enterocytes. Entry into the host cell is achieved by attachment to host receptors, which mediates endocytosis. Replication follows the positive-strand RNA virus replication model. Astrovirus RNA is infectious and functions as a messenger RNA for ORF1a and ORF1b. A frame-shifting mechanism between these two nonstructural polypeptides translates RNA-dependent RNA polymerase. In replication complexes near intracellular membranes, ORF1a and ORF1b are cleaved to generate individual nonstructural proteins that are involved in replication. The resulting subgenomic RNA contains ORF2 and encodes precursor capsid protein (VP90). VP90 is proteolytically cleaved during packaging and produces immature capsids made of VP70. Following encapsidation, immature capsids are released from the cell without lysis. Extracellular virions are cleaved by Trypsin and form mature infectious virions.

Morphology Astroviruses are 28-30 nm non-enveloped viruses with T-3 icosahedral symmetry. They have spherical shapes and consist of a capsid protein shell. Astroviruses have distinctive five or six pointed star-like projections on 10% of the virions (the other virions have smooth surfaces). The virion capsid is expressed from a subgenomic mRNA and its precursor undergoes multiple cleavages to make the VP70 protein. Capsids that are made of the VP70 protein are cleaved by trypsin to make particles that are very infectious (VP25/26, VP27/29 and VP34). The spikes that create the star-like appearance on the virion surface are made by two structural proteins (VP25 and VP27) while the capsid shell is made from VP34.

Genome

Astroviruses have a genome composed of a single strand of positive sense RNA. The strand has a poly A tail at the 3' end, but no 5' cap but instead is linked to a VPg protein. With the exclusion of polyadenylation at the 3' end, the genome is between 6.8 and 7.9 kb long. The genome is arranged into three open reading frames (ORFs), with an overlap of approximately 70 nucleotides between ORF1a and ORF1b. The remaining ORF is known as ORF2. ORF2 encode the structural proteins, which are -at least- VP26, VP29 and VP32, the most antigenic and immunogenic of these being VP26. This protein is probably involved in the first steps of viral infection, being a key factor in the biological cycle of astroviruses. The human astrovirus genome mutation rate has been estimated to be 3.7×10−3 nucleotide substitutions per site per year with the synonymous changes rate of 2.8×10−3 nucleotide substitutions per site per year. The capability for genetic recombination appears to be present in type-3 and type-4 human astroviruses, and in porcine astrovirus strains.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Astrovirus

Start with the simplest possible case. Write down what Astrovirus claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Astrovirus before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Astrovirus ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Astrovirus

In research: Astrovirus appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Astrovirus in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Astrovirus is common in secondary-school and first-year university syllabi. It links to neighbouring topics Astroviridae, Foodborne illnesses, Gastroenterology, so understanding it makes those chapters shorter.
In everyday life: Look for Astrovirus outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Astrovirus in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Astrovirus means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Astrovirus out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Astrovirus in simple terms?

Why does Astrovirus matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Astrovirus?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Astrovirus.