Virology Quiz

Virology

• Virology is a branch of science that deals with viruses and viral diseases.
• Viruses are the smallest infectious agents and are obligate intracellular parasites.

General Properties of Viruses

• Viruses are smaller than bacteria, ranging in size between 20-300 nanometers.
• Viruses contain only one type of nucleic acid, either DNA or RNA, but never both.
• Viruses consist of nucleic acid surrounded by a protein coat, and some have additional lipoprotein envelope.
• Viruses lack cellular organelles, such as mitochondria and ribosomes.
• Viruses are obligate cellular parasites, replicating only inside living cells.
• Viruses replicate through replication of their nucleic acid and synthesis of viral protein.
• Viruses do not multiply in media.
• Viruses do not undergo binary fission.

Comparison between Viruses and Bacteria

• Viruses have a size range of 20-300 nm, while bacteria have a size of 1000 nm.
• Viruses have DNA or RNA, but not both, while bacteria have both DNA and RNA.
• Viruses have an envelope present in some, while bacteria have a cell wall.
• Viruses do not multiply by binary fission, while bacteria do.
• Viruses are sensitive to antibiotics, while bacteria are not.
• Viruses grow only in the living host cell, while bacteria grow in culture media.

Viral Structure

• Viruses are composed of nucleic acid (DNA or RNA) surrounded by a protein coat called the capsid.
• The capsid is composed of small structural units called capsomeres.
• The capsid protects nucleic acid from inactivation by outer physical conditions.
• The complete infectious virus particle is called a virion.
• The basic virus structure is DNA or RNA + capsid protein = nucleocapsid = naked virus.

Types of Symmetry of Virus Particles

• Viruses are divided into three groups based on the morphology of the nucleocapsid and arrangement of capsomeres:
  • Icosahedral (Cubic) symmetry
  • Helical symmetry
  • Complex structures

Classification of Viruses

• Classification of viruses is based on the following characteristics:
  • Virion morphology, including size, shape, type of symmetry, presence or absence of envelope
  • Virus genome properties, including type of nucleic acid, size of genome strand, linear or circular, positive or negative
  • Virus protein properties, including number, size, functional activities, amino acid sequences, and reverse transcriptase
  • Biological properties, including natural host range, mode of transmission, vector relationships, pathogenicity, tissue tropisms

Viral Replication

• As obligate intracellular parasites, viruses must enter and replicate in living cells to "reproduce" themselves.
• The "growth cycle" involves specific attachment of virus, penetration and uncoating, nucleic acid transcription, protein synthesis, maturation and assembly of virions, and release from the cell by lysis.
• The typical infectious cycle consists of:
  • Attachment: specific binding between viral capsid proteins and specific receptors on the host cellular surface
  • Penetration: follows attachment, involving receptor-mediated endocytosis in non-enveloped viruses or membrane fusion in enveloped viruses
  • Uncoating: release of viral genome from the capsid
  • Transcription and Translation: synthesis of viral protein and genome replication
  • Assembly: formation of new virions
  • Release: release of new virions from the cell by lysis