Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 19.1 A Virus Consists of a Nucleic Acid Surrounded by a Protein Coat
Viruses are infectious particles composed of nucleic acids (DNA or RNA) enclosed in a protective protein coat. They exist in a unique area between living organisms and nonliving chemicals, dependent entirely on host cells to replicate.
Discovery of Viruses
- Adolf Mayer (1883): Discovered tobacco mosaic disease was contagious via sap, suspecting bacteria.
- Dmitri Ivanowsky: Showed infectious agent passed through bacterial filters.
- Martinus Beijerinck: Proposed agent replicated only in host cells; coined the term "virus."
- Wendell Stanley (1935): Crystallized tobacco mosaic virus, confirming its non-cellular nature.
Structure of Viruses
- Extremely small (20 nm to 1,500 nm); much simpler than cells.
- Composed of:
- Nucleic acid: DNA or RNA, single- or double-stranded.
- Capsid: Protein shell made of capsomeres.
- Types of virus structures:
- Helical viruses: rod-shaped (e.g., TMV).
- Icosahedral viruses: 20-sided (e.g., adenovirus).
- Enveloped viruses: have membranes from host (e.g., influenza).
- Complex viruses: such as bacteriophages with head-tail structures.
Viral Genomes
- Genome can be:
- Single or multiple segments.
- DNA or RNA, single- or double-stranded.
- Range from a few genes to thousands.
General Viral Replicative Cycle
- Entry: Virus binds and enters host cell, injecting genome.
- Genome replication: Host enzymes replicate viral genome.
- Protein synthesis: Host ribosomes synthesize viral proteins.
- Assembly and exit: New viruses assemble and exit the host cell.
Host Range
- Each virus targets specific species or cell types (host range).
- Examples:
- Broad: West Nile virus (mosquitoes, birds, humans).
- Narrow: Measles virus (humans only).
Viruses: Living or Nonliving?
- Cannot metabolize or reproduce outside host cells.
- Exist in a gray zone between life and non-life—"borrowed life."
In a Nutshell
Viruses are infectious particles made of nucleic acid enclosed by protein capsids, lacking metabolic machinery. They depend on host cells for replication, display diverse genome types and shapes, and exist at the boundary between living organisms and nonliving chemicals.