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The largest virus is equal in size to the smallest bacteria.
False
Viruses can be seen under an ordinary microscope.
False
The diameter of viruses varies from 20-300 micrometers.
False
The smallest animal virus is known as FMD and measures between 20 and 28 nm.
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Ultracentrifugation is necessary to sediment viruses due to their heavy weight compared to bacteria.
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Phosphotungistate is used in negative staining to make viral particles appear opaque.
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Poxvirus has a bullet shape.
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The average pore diameter of filters determines whether a virus can pass through.
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All viruses contain both DNA and RNA.
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Viruses range in size from 1 nanometer to 100 nanometers.
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The genome of all DNA viruses is segmented.
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All RNA viruses are single-stranded except for Reoviruses and Birnaviruses.
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Herpesvirus has a hexagonal shape.
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The molecular weight of viruses can range from $1 imes 10^6$ Daltons to $2 imes 10^8$ Daltons.
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A and T are held together by three hydrogen bonds.
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The ratio $(A + T)/(G + C)$ is the same across all virus groups.
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Positive sense RNA must be converted into mRNA before translation into proteins.
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The capsid is primarily composed of nucleic acid.
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Ambisense viruses contain RNA segments that are either entirely positive or entirely negative.
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Reoviruses are unique because they possess only one capsid.
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Icosahedral symmetry is characterized by the arrangement of capsomers around a central axis to form cubes.
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All animal viruses with helical nucleocapsid are non-enveloped.
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Viruses classified under Bunyaviridae carry their own polymerase enzyme.
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The molecular weight of viral RNA can range from $2 imes 10^6$ to $15 imes 10^6$ Dalton.
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Poxvirus has a helical structure.
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The capsid of a virus participates in the attachment to susceptible cells.
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Viral envelopes are formed during the virus's replication inside the host cell.
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Glycoprotein spikes on the viral envelope can vary in shape and size.
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The essential structure of a viral envelope consists of a protein bilayer embedded with carbohydrates.
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Study Notes
Physical Properties of Viruses
- Viruses are the smallest infectious agents, smaller than bacteria.
- The largest virus is half the size of the smallest bacterium, and the smallest virus is about the size of a large protein molecule.
- Viruses can be measured in nanometers (nm).
- Viruses range in size from 20-300 nm in diameter.
- The smallest animal virus is Foot and Mouth Disease Virus (FMD) (20-28 nm).
- The largest is Poxvirus (300 x 250 nm).
- Electron microscope, ultrafiltration, and ultracentrifugation are used to estimate the size of viruses.
Electron Microscope
- Negative staining technique uses electron-dense phosphotungistate to visualize the virus.
- The virus particles appear translucent against the opaque background.
- The positive staining technique stains viral nucleic acid and proteins for clearer visualization.
Ultrafiltration
- Uses cellulose acetate membrane filters with different pore sizes.
- Viruses are size-estimated by comparing the pore sizes of the membranes that let and block the virus.
Ultracentrifugation
- Viruses are separated from bacteria by ultracentrifugation, as they are lighter.
- Centrifugal force separates viruses based on density and size, with heavier viruses moving faster.
Shapes of Viruses
- Viruses have various shapes under the electron microscope (EM):
- Poxvirus: Brick shape
- Rabiesvirus: Bullet shape
- Coronaviruses: Crown shape
- Adenovirus: Large spheres
- Orthomyxoviruses: Thread form
- Bacteriophages: Spermatic form
- Herpesvirus: Hexagonal shape
Molecular Weight of Viruses
- Each virus has a definite molecular weight, measured in Daltons (Da).
- Rate of migration in liquids determines molecular weight.
Chemical Properties of Viruses
- Viruses consist of a viral genome (nucleic acid) enclosed within a protective coat (capsid).
- Some viruses have a nucleocapsid surrounded by a lipoprotein envelope.
- The three main components of a virus are nucleic acid, capsid, and envelope.
Viral Nucleic Acid
- Viruses contain either DNA or RNA, never both.
- DNA consists of two complementary strands of polynucleotides forming a double helix.
- The strands run in opposite directions, one in a 5'–3' direction and the other in a 3'–5' direction.
- The strands are held together by hydrogen bonds (two between adenine (A) and thymine (T), and three between guanine (G) and cytosine (C)).
- The ratio of (A + T)/(G + C) varies between virus groups.
DNA Viral Genome
- All DNA viruses are double-stranded (ds) except Parvoviruses and Circoviruses, which are single-stranded (ss).
- The genome of all DNA viruses is one molecule (non-segmented).
- The genome can be linear or circular.
- The G+C content range from 35% to 74%.
- Molecular weight ranges from 1 x 106 Da in smaller viruses to 200 x 106 Da in larger viruses.
RNA Viral Genome
- All RNA viruses are ss except Reoviruses and Birnaviruses, which are ds.
- ss RNA is either positive or negative strand.
- All RNA viruses have linear genomes.
- Viral RNA can be a single molecule or segmented.
- Examples of segmented RNA viruses include:
- Influenza A virus (8 segments)
- Bunyaviridae (3 segments)
- Birnaviridae (2 segments)
- Molecular weight ranges from 2 x 106 to 15 x 106 Da.
RNA Viral Genome Classification
- RNA viral genomes are classified by their sense as positive (+), negative (-), or ambisense.
Positive Sense RNA (+ strand)
- RNA acts as messenger RNA (mRNA) because it has identical nucleotide sequences to mRNA.
- It is directly translated into proteins by the cell's ribosomes without the need for transcription.
- This genome is infectious.
- Examples include: Caliciviridae, Picornaviridae, Flaviviridae, Togaviridae, Coronaviridae, and Retroviridae.
Negative Sense RNA (- strand)
- RNA has nucleotide sequences complementary to those of mRNA.
- This negative strand must first be converted into mRNA by a transcriptase (polymerase) enzyme before translation into proteins.
- Viruses either carry this enzyme or have genes for it, allowing the host cell to synthesize it.
- This genome is not infectious by itself.
- Examples include: Bunyaviridae, Paramyxoviridae, Orthomyxoviridae, Filoviridae, and Rabdoviridae.
Ambisense
- Ambisense viruses contain at least one ambisense RNA segment, which is partly positive and partly negative polarity.
- An example is Arenaviridae.
Viral Capsid
- The protein coat that encloses the nucleic acid.
- The capsid and the enclosed nucleic acid together are called the nucleocapsid.
- Most viruses have one capsid, while Reoviruses have two.
- The capsid is made up of morphological units called capsomeres.
- Capsomeres are held together by specific hydrophobic bonds, forming regular arrangements.
- These arrangements determine the shape of the virus, while the quantity determines the size.
Symmetry of the Viral Capsid
- The capsomers are arranged in specific relationships within the virion, giving the capsid one of three types of symmetry: helical, icosahedral, or complex.
Helical Symmetry (Tubular Capsid)
- Capsomeres are arranged spirally around the genome, forming a hollow cylinder.
- All animal viruses with a helical nucleocapsid are enveloped.
Icosahedral Symmetry (Cubical Capsid)
- Capsomeres are arranged around a central axis, forming cubes.
Complex Symmetry
- Some viruses do not have a clear, identifiable capsid symmetry.
- They are neither helical nor icosahedral, but have several coats around the genome.
- An example is Poxvirus.
Function of the Capsid
- Protects the enclosed nucleic acid from cellular nucleases.
- Participates in the attachment of the virus to the susceptible cell.
- Provides structural symmetry to the virus.
Viral Envelope
- Acquired during maturation of the virus by the budding process.
- Budding occurs through host cell membranes, plasma membrane, nuclear membrane, endoplasmic reticulum, and Golgi apparatus.
- The essential structure of the envelope is a phospholipid bilayer embedded with specific membrane proteins called matrix proteins, which provide rigidity.
- Lipids of the viral envelope are derived from the host cell, but the proteins are virus-coded.
Viral Envelope Glycoprotein Peplomers or Spikes
- Project from the surface of the envelope, differing in shape and size.
- Examples of glycoprotein types include:
- Rod-like, like Hemagglutinin (HA) in Orthomyxoviridae.
- Mushroom-like, like Neuraminidase (N) in Orthomyxoviridae.
- Club-shaped, like those in Coronaviridae.
- Some viruses have no peplomers (smooth surface).
Function of the Viral Envelope
- Protects the nucleocapsid from adverse environmental conditions.
- Facilitates attachment of the virus to the host cell.
- Contains antigenic determinants.
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Description
This quiz focuses on the physical properties of viruses, including their size, measurement techniques, and visualization methods. Learn about the smallest and largest viruses, as well as the tools used to study them, such as electron microscopes and ultrafiltration. Test your knowledge of these microscopic infectious agents.
