Untitled Quiz

Virus Physical Properties

• Viruses exhibit diverse shapes, including brick-shaped (Poxviridae), spherical (Retroviridae, Paramyxovirdae, Orthomyxoviridae, Arenaviridae, Flaviviridae), icosahedral (Adenoviridae, Papovaviridae, Parvoviridae), sperm-shaped (bacteriophages), bullet-shaped (Rhabdoviridae), slender rigid (Rod) shaped, and polyhedral (plant viruses, TMV, Birnaviridae, Reoviridae).
• Viral morphology shapes vary within and between virus families.
• Enveloped viruses (e.g., rhabdoviruses) can have bullet, cone, or bacilli shapes, and poxviruses may have an oval or brick shape.
• Influenza viruses generally have a spherical shape, although some filamentous or thread-like forms exist.
• The shape flexibility is sometimes due to structure weaknesses in protein bonds between molecules in the virus capsid.
• Stanley was the first to crystallize viruses.
• Viral shape is determined by the arrangement of repeating subunits called capsomers.

Molecular Weight of Viruses (Mol. Wt.)

• Dalton is the mass of a hydrogen atom, equaling 1.67x10^-24 g.
• Molecular weight of a virus is distinguishable by measuring sedimentation rates using an analytical ultracentrifuge.
• The molecular weight of nucleic acids is related to the virus's molecular weight.
• DNA viruses have molecular weights ranging from 1.5x10⁶ to 160-200x10⁶.
• RNA viruses have molecular weights ranging from 1x10⁶ to 15x10⁶

Virus Size

• Virus sizes vary significantly, from 20-25 nm for the smallest (e.g., picornaviruses, parvoviridae, poliovirus) to 300 x 200 nm for the largest (e.g., poxviruses). The smallest viruses are slightly larger than ribosomes, whereas poxviruses have a size similar to certain bacteria.
• Viruses can be seen with a light microscope.
• Units of measurement include millimeters (mm), micrometers (µm), nanometers (nm), and Angstroms (Å).
• Methods for measuring virus size include ultrafiltration, ultracentrifugation, electron microscopy, X-ray crystallography, chromatography, and electrophoresis.

Virus Size Measurement Methods

• Ultrafiltration: Viruses are passed through a series of membranes with known pore sizes. The size of the virus can be determined by examining which membrane allows the virus to pass.
• Ultracentrifugation: High-speed centrifugation forces the viral particles to settle at the bottom of the tube based on their weight, size, and density.
• Electron Microscopy: This technique magnifies viruses to a high degree (10,000 to 100,000 times their original size) enabling detailed study of their morphology. The resolution is superior to traditional light microscopy (0.3 µm vs. 5 Å).
• X-ray Crystallography: This technique analyzes how X-rays are diffracted by a crystal structure. Scientists use mathematical methods to determine the structure of the pure virus preparation, and the data is used to understand morphology and structures of various viruses.

Electron Microscopy Examination Methods

• Shadow casting: Coating a virus sample with heavy metals (chromium, gold, platinum) to create a shadow effect that reveals the shape and structure.
• Negative staining: Using a contrasting heavy metal, like sodium phosphotungstate, to stain the background, while leaving the virus particles relatively transparent, enhancing clarity.
• Positive staining: Stains that bind directly to specific components of the virus particles, enhancing their visibility.
• Thin sectioning: Cutting extremely thin slices of specimens embedded in plastic material that are then studied under electron microscopy.
• Freeze-drying: Used to prevent structural distortion, the liquid sample is quickly frozen, and then the ice is sublimated.
• Carbon replicas: A technique similar to Plaster of Paris molds is used to capture and detail surface characteristics.

X-ray Crystallography

• X-rays passed through a virus crystal will diffract, forming cones of diffracted light that can be recorded.
• Special mathematical techniques analyze diffraction patterns to determine the structure.
• Requires highly pure virus particles for analysis.
• Provides information about the virus's components (proteins and nucleic acids) and structure, allowing comparisons between different viruses.