VIRUS

VIRUS

  • Viruses are the simplest form of life known and lack cellular organization.

  • They are obligate intracellular parasites, requiring host cells for replication.

  • Contain only one type of nucleic acid (DNA or RNA) but not both.

  • They lack the necessary enzymes for protein and nucleic acid synthesis.

  • Viruses are unaffected by antibacterial antibiotics.

MORPHOLOGY OF VIRUSES

Size

  • Viruses are much smaller than bacteria; measured in nanometers (nm).

  • Extracellular infectious virus particle is called the virion.

  • Known as ‘filterable viruses’ due to passing through filters that block bacteria.

  • Some larger viruses, like poxviruses (300 nm), can be seen under a light microscope.

  • Smallest viruses (parvovirus) are nearly as small as the largest proteins.

Measuring the Size of Viruses

  1. Passing through Collodion Membrane: Early method using filters of graded porosity.

  2. Electron Microscopy: Most widely used method for estimating virus size.

  3. Sedimentation in the Ultracentrifuge: Calculating size based on sedimentation rates.

  4. Comparative Measurements: Reference measurements with bacteria and bacteriophages.

SHAPE OF VIRUS

  • Various shapes among different virus groups:

    • Most animal viruses: Roughly spherical.

    • Poxviruses: Brick-shaped.

    • Rabies virus: Bullet-shaped.

    • Tobacco mosaic virus: Rod-shaped.

    • Bacteriophages: Complex morphology.

STRUCTURE OF A VIRUS

A. Viral Capsid

  • Viruses consist of a nucleic acid core surrounded by a protein coat called capsid.

  • The capsid plus the nucleic acid is termed the nucleocapsid.

  • Comprised of multiple capsomers, forming a protective shell.

Functions of Capsid

i. Protection: Safeguards viral genome from destruction. ii. Binding sites: Provides attachment sites for host cell receptors. iii. Facilitates assembly: Aids in packaging viral genetic info. iv. Vehicle of transmission: Assists in virus transmission to hosts. v. Defense: Vital in host defense against infection. vi. Symmetry: Provides structural symmetry to the virus.

B. Virus Symmetry

  • Viral architecture is categorized into three types:

  1. Icosahedral Symmetry: 12 vertices, 20 triangular facets; consists of pentagonal (pentons) and hexagonal (hexons) capsomers.

  2. Helical Symmetry: Nucleic acid and capsomers form a helix; found in certain RNA viruses (e.g., influenza).

  3. Complex Symmetry: Viruses (e.g., poxviruses) with structures that do not conform to icosahedral or helical symmetry.

C. Viral Envelope

  • Virions may be enveloped or naked (nonenveloped).

  • Enveloped Viruses: Outer lipid covering derived from the host cell’s plasma membrane; glycoprotein in nature.

  • Envelopes are susceptible to lipid solvents; naked viruses are more resistant.

D. Peplomers

  • Glycoproteins projecting from the envelope surface; vary in type (e.g., hemagglutinin and neuraminidase in influenza).

Functions of Peplomers

i. Mediate attachment: Initiate virus entry into host cells. ii. Receptor attachment: Some glycoproteins cause agglutination in red blood cells. iii. Enzymatic activity: Enzymes like neuraminidase cleave host glycoproteins. iv. Major antigens: Important for developing protective immunity.

E. Viral Nucleic Acids

  • Contain either DNA or RNA for replication.

  • Genuinely classified by type (single or double-stranded, circular or linear, segmented or non-segmented).

  • The characteristics of the nucleic acid are critical for virus classification.