Virology: Virus Structure and Classification

Virus Structure and Function

• A supercapsid has the same functions as a capsid: adhesive and invasive, protective, and principal antigens against which the host mounts its immune response.
• The presence of an envelope makes a virus more sensitive to heat, detergents, and lipid solvents.

Classification of Viruses

• Based on the type of nucleic acid: DNA or RNA (but not both), number of strands, and polarity.
• According to nucleocapsid symmetry: icosahedral or helical.
• According to structure: presence or absence of a second membrane/supercapsid.
• Viruses can be classified into three size groups: small (30-40 nm), middle (80-150 nm), and large (200-400 nm).
• According to shape: spherical, rod-shaped, spermatozoid, bullet-shaped, star-shaped, or crown-shaped.

Microbial Genetics

• The science of genetics defines and analyzes heredity, or constancy and change in physiologic functions.
• The unit of heredity is the gene, a segment of DNA that carries information for a specific biochemical or physiologic property.
• Genetic information is stored as a sequence of bases in DNA.
• Prokaryotic genes are carried on the bacterial chromosome, a single circular DNA molecule.
• Bacteria are haploid, having a single chromosome, in contrast to human cells, which are diploid.
• Plasmids carry genes associated with specialized functions.

History of Microbiology

• Ivanovsky showed that organisms passed through filters of small pore size and lacked a cellular structure.
• This discovery led to the development of virology and immunology.
• The stages of development of microbiology are:
  • Morphological stage (XVII-XIX century)
  • Physiological stage (XIX-XX century)
  • Immunological and development of virology (XX century)
  • Modern – molecular-genetic stage (XX-XXI century)

Morphology and Bacterial Taxonomy

• Bacterial taxonomy comprises three components: classification, identification, and nomenclature.
• Classification is based on phenotypic properties: morphological, physiological, and biochemical.
• Morphology is characterized by shape and structure of microbial cell.
• Physiological properties are characterized by growth on artificial nutrient media and character of growth on liquid media.
• Biochemical properties are based on type of metabolism, fermentation of carbohydrates, and proteolytic activity.

Learn about the functions of supercapsid, the differences between enveloped and non-enveloped viruses, and the properties used to classify viruses.