Viruses, Viroids, and Prions: Quick Notes
Viruses
Definition: Small, non-cellular, non-living pathogenic particles, lacking organelles.
Structure: Composed of a capsid (protein coat) and nucleic acid (DNA or RNA). Some have an envelope from the host cell membrane.
Properties: Do not make/use energy, do not move/grow independently, only reproduce inside a host cell, do not produce waste, extremely small ().
Characteristics: Infect specific host species, some (bacteriophages) infect bacteria by injecting DNA, leaving the capsid outside.
Importance: Cause diseases in humans, animals, plants, and bacteria; some are linked to cancer (oncoviruses); important in ecosystem population control.
Classification: Based on nucleic acid type (RNA/DNA), envelope presence, capsid shape, and host infected.
Replication: Requires a host cell. Follows two cycles:
Lytic cycle: Virus is active, viral reproduction occurs, host cell bursts (e.g., influenza).
Lysogenic cycle: Virus is dormant, viral DNA integrates into host genome (prophage) and reproduces with host cell; can become lytic later (e.g., herpes, HIV).
Vaccines: Introduce weakened virus to trigger an immune response and antibody memory; effectiveness varies due to changing viruses.
Applications: Used to control harmful bacteria and in gene therapy as vectors to deliver drugs or genes to target cells.
Viroids
Definition: Small, circular RNA molecules without a protein coat.
Role: Pathogens primarily affecting higher plants, causing diseases like potato spindle tuber.
Prions
Definition: Abnormally folded infectious proteins, primarily affecting the brain and nervous system.
Mechanism: Convert normal prion proteins into abnormal ones.
Transmission: Can spread by consuming infected tissues, leading to brain damage (e.g., Mad Cow Disease (BSE) in cattle, Creutzfeldt-Jakob Disease (CJD) in humans).