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Questions and Answers
What is the shape of icosahedral viruses upon closer examination using electron microscopy?
How many faces does an icosahedral virus have?
What are the vertices of an icosahedral virus composed of?
Which type of virus has a helical nucleocapsid supercoiled into a hollow sphere surrounded by an icosahedral shell and further enclosed by an envelope?
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What is a mature virus called?
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Which of the following is an example of a pox virus that has a complex structure and is neither icosahedral nor helical?
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Which type of genetic material is never found in a virus?
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Which of the following is most often seen in viruses?
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Which of the following does not affect viruses?
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How are the proteins and nucleic acid of viruses typically assembled?
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What does a virus contain?
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Which of the following is responsible for dangerous diseases?
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What is a virion?
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Which disease is a chronic disease caused by viruses?
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What describes a retrovirus?
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Which epidemic diseases weakened the Roman Empire?
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Which type of virus is often resistant to bile?
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How was early control of smallpox infection achieved in Turkey?
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How are viruses classified based on their structure?
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Who first used cowpox to protect against smallpox, and in what year?
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What suffix is used for naming virus families?
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When was the tobacco mosaic virus crystallized?
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What significant discovery did Peyton Rous make in 1911?
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What method of transmission is common for enveloped viruses?
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How are viruses typically grown now?
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Which of the following viruses is classified as non-enveloped?
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What is the typical size range of viruses?
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Which structure do viruses lack that makes them unable to grow on culture media?
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How are viruses primarily classified?
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Which characteristic is unique to RNA viruses?
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Why are viruses considered obligate intracellular parasites?
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What is the main role of nucleic acid in viruses?
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Study Notes
Icosahedral Viruses
- Appear spherical but have a 20-faced, equilateral triangle shape under electron microscopy
- Composed of hexons and 12 vertices/corners made up of pentons
- Entire structure consists of protein subunits
Unusual Viruses
- HIV has a helical nucleocapsid supercoiled into a hollow sphere, surrounded by an icosahedral shell and an envelope
- Pox viruses, such as Orf virus, have an ovoid or brick-shaped complex structure
- Neither icosahedral nor helical
History of Virology
- Viral infections have played a significant role in medical history
- Understanding of viruses has only recently developed
- Measles and smallpox epidemics weakened the Roman Empire
- Smallpox brought by Spanish conquerors aided in the conquest of Mexico
- Early control of viral infection came in the 18th century
- Jenner used cowpox to protect against smallpox in 1796
History of Viruses
- Leaf extracts from plants could still produce disease after filtration, showing that viruses are not bacteria
- Tobacco mosaic virus only multiplies in living plants but can survive in a dried state
- Hoof and mouth disease and yellow fever were found not to be bacterial
- Peyton Rous discovered a virus that causes cancer in chickens in 1911
- Tobacco mosaic virus was crystallized in 1935
- By the late 1930s, researchers concluded that viruses are complexes of nucleic acid and proteins only able to reproduce in living cells
Distinguishing Features of Viruses
- Proteins and nucleic acid are synthesized independently and assembled into identical mature virus particles
- Viruses can aggregate and be seen in infected cells as inclusion bodies
- Not affected by antibiotics and other chemotherapeutic drugs
- Inhibited by interferon
Viral Structure
- Mature virus is called a virion
- Genetic material may be in the form of RNA or DNA (never both)
- Most common forms are dsDNA and ssRNA
- Single continuous piece of nucleic acid, with some exceptions
Terminology
- Virus: a genetic element containing either RNA or DNA that replicates in cells but has an extracellular state
- Virion: the complete virus particle, including nucleic acid surrounded by a protein coat and sometimes other material
- Retrovirus: a virus whose RNA genome has a DNA intermediate as part of its replication cycle
Viruses
- Responsible for many important acute and chronic diseases
- Examples include childhood illnesses, HIV, and dangerous diseases like yellow fever and Ebola
Size and Filtration
- Extremely small size, ranging from 20-300nm
- Pass through bacteria-retaining filters
Visibility and Growth
- Invisible by light microscope
- Impossible to grow on culture media
Structure
- Simple structure: nucleic acid + protein + some have lipid outer membrane
Intracellular Features
- Lack intracellular structures, such as ribosomes
- Some surrounded by an envelope but no outer wall
- Genetic information carried in nucleic acid
- Classified on the basis of the hosts they infect, such as animal, plant, or bacterial viruses
Nucleic Acid
- Each type of virus has one type of nucleic acid, which provides a basis for classification
- RNA viruses are the only organisms to have their genome in this form
- Sometimes the RNA genome consists of several segments
Replication
- Lack of protein synthesis, apparatus, and energy-producing systems makes them obligate intracellular parasites
- Incapable of binary fission, unlike bacteria
- Intracellular release of nucleic acid directs viral protein production and its own replication
Enveloped or Non-Enveloped
- Non-enveloped viruses are more resistant to the environment and can infect via the intestine
- Examples include rotavirus, norovirus, hepatitis A, and poliovirus
- Enveloped viruses are less hardy in the environment and spread by droplets
- Examples include influenza, EBV, and HIV
Classification of Viruses
- Classified on the basis of a small number of characteristics
- Structure: icosahedral, helical, or complex
- Enveloped or non-enveloped
- Nature of the nucleic acid: DNA, RNA, ss or ds
Nomenclature
- Family with viridae as suffix, such as Herpesviridae or Poxviridae
- Genus name with suffix virus, such as Herpes Simplex virus
- Species, often named after specific disease or numbered, such as HSV1 or Herpes zoster
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Description
This quiz covers the structure and characteristics of icosahedral viruses, as well as unusual viruses with unique shapes and compositions. Learn about the different types of viruses and their properties.
