Viruses: General Properties and Structure

Viruses: General Properties

• Smallest infectious agents, 20-300 nm in diameter
• Contain either DNA or RNA
• Lack active protein synthesizing apparatus, nucleus, cytoplasm, mitochondria or ribosomes
• No metabolic activity outside a host cell, obligate intracellular parasites
• Can only replicate inside living cells, not on inanimate media

Basic Virus Structure

• Genome: DNA or RNA, single-stranded (ss) or double-stranded (ds), circular or linear
• Capsid: Protein coat surrounding the genome
  • Provides structural symmetry
  • Facilitates attachment to host cells
  • Protects the viral genome from nucleases in the bloodstream
• Capsomeres: Structural units making up the capsid, composed of one or several proteins

Virus Structure: Envelope

• Lipoprotein surrounding the capsid
• May contain host cell material as well as viral origin
• Virus-encoded glycoproteins are exposed on the surface of the envelope
• Most human helical viruses are enveloped, while icosahedral can be either enveloped or non-enveloped
• Nucleocapsid: The protein-nucleic acid complex
• Virion: The complete infective virus particle

Additional Viral Components:

• Enzymes, such as RNA and DNA polymerase, neuraminidase, reverse transcriptase
• Host tissue or cell tropism

Properties of Naked Capsid Viruses

• More stable, resistant to heat, drying, detergents, and alcohols
• Transmitted by feco-oral route (HAV, poliovirus, rotavirus)

Properties of Enveloped Viruses

• More unstable, sensitive to heat, drying, detergents, and alcohols
• Often transmitted by direct contact (blood, sexual contact), such as HIV, HBV, HCV, rabies virus, measles, mumps, rubella viruses

Viruses: Symmetry

• Cubic Symmetry (Icosahedral): Have 60 subunits on the surface of an icosahedron, with fivefold, threefold, and twofold rotational symmetry
• Helical Symmetry: Elongated nucleocapsid, capsomeres arranged around a spiral of nucleic acid, most helical viruses are enveloped
• Complex Symmetry: Does not conform to cubic or helical symmetry

Atypical Virus-like Agents

• Defective Viruses: Viral nucleic acid and proteins but cannot replicate without a helper virus
• Pseudovirions: Contain host cell DNA instead of viral DNA within the capsid, can infect cells but do not replicate
• Viroids: Solely composed of a single molecule of circular RNA without a protein coat or envelope, cause plant diseases.
• Prions: Infectious particles composed solely of protein without detectable nucleic acid, cause slow diseases like Creutzfeldt-Jacob Disease (CJD) and scrapie.

Major Diseases Caused by Human Viruses

• Many viral diseases exist, causing a wide range of symptoms and severity.

Viral Life Cycle

• Attachment: Reactive sites on the virus surface interact with specific receptors on susceptible host cells.
• Penetration: The coat of enveloped viruses fuses with the host cell membrane and releases the nucleocapsid into the host cytoplasm. Other viruses enter via endocytosis.
• Uncoating: Viral capsid is broken down by viral or cellular enzymes, releasing the viral nucleic acid into the host cell.
• Biosynthesis: Viral nucleic acid and protein synthesis occurs, utilizing the host cell's machinery.
  • Capsid/Envelope: Structural proteins that build the virion.
  • Enzymes: DNA/RNA polymerase, other replication enzymes.
• Assembly: Viral nucleocapsids are assembled, in either the nucleus (e.g., Herpes virus, Adenovirus), cytoplasm (e.g., Poliovirus), or at the cell surface (e.g., Influenza virus).
  • Inclusion bodies: Accumulation of virions at sites of assembly, visible under a light microscope.
• Release: New virions are released from the infected cell.
  • Budding: Enveloped viruses.
  • Lysis: Non-enveloped viruses, causing cell death.

This quiz covers the fundamental characteristics and structure of viruses, detailing their size, genetic material, and components such as the capsid and envelope. Test your understanding of how viruses function as obligate intracellular parasites and their replication process within host cells.