Introduction to Virology
Definition of a Virus:
Acellular infectious particle.
Composed of DNA or RNA.
Surrounded by a capsid (and sometimes an envelope).
Classified as an obligate intracellular parasite: can only replicate inside a host cell.
Do not have ribosomes; hence, they cannot translate proteins.
Structure of Viruses
Components:
Nucleic Acid Genome: Carries the genetic information.
Capsid: Made of protein subunits called capsomeres.
Envelope: Optional lipid membrane, often has viral spikes (glycoproteins) for host receptor binding.
Morphologies:
Can be icosahedral, helical, complex, or have odd shapes.
Envelope vs. Non-Enveloped Virus
Enveloped Viruses:
Generally more fragile.
Spread by close contact.
Exit cells via budding.
Non-Enveloped Viruses:
More stable in the environment.
Often cause cell lysis to release new virions.
Host Range and Tropism
Definition:
The range of hosts that a virus can infect is determined by viral receptor binding.
Examples of Viral Receptors:
HIV: Binds to CD4+ and CCR5/CXCR4 receptors.
SARS-CoV-2: Binds to ACE2 receptor.
Influenza: Binds to sialic acid.
Baltimore Classification System
Types of Viral Genomes:
I. dsDNA (double-stranded DNA)
II. ssDNA (single-stranded DNA)
III. dsRNA (double-stranded RNA)
IV. +ssRNA (positive single-stranded RNA)
V. -ssRNA (negative single-stranded RNA)
VI. ssRNA-RT (single-stranded RNA with reverse transcriptase)
VII. dsDNA-RT (double-stranded DNA with reverse transcriptase)
Size:
Smallest virus: Parvoviridae (approximately 20 nm).
Largest virus: Mimivirus (approximately 500 nm).
Subviral Agents
Viroids:
Composed of small circular ssRNA (affecting primarily plants) with no protein coat.
Satellites:
Require a helper virus to replicate.
Prions:
Misfolded proteins causing diseases like Kuru, Jakob-Creutzfeldt Disease (JD), and Bovine Spongiform Encephalopathy (BSE).
Viral Glycoproteins and Capsomeres
Glycoproteins (peplomers/spike proteins):
Facilitate the binding to host cell receptors.
Capsomeres:
Subunits that assemble into capsids.
Exam Review Questions
Causative agent of bubonic plague:
Caused by Yersinia pestis, not a giant virus like HIV.
Viral Capsid Morphologies:
Include icosahedral, helical, and complex shapes.
Viral Infection Cycle:
Steps include attachment, penetration, entry, uncoating, replication, transcription/translation, assembly, and release.
Lytic vs. Lysogenic Viruses:
Lytic: immediate hijacking and lysis of the cell.
Lysogenic: integration into host DNA, can remain dormant.
Innate vs. Adaptive Immunity:
Innate is fast and nonspecific, while adaptive is slower but highly specific.
Immunity Cells Prevent Pathogen Proliferation:
Examples include phagocytosis, antibody production, and cytotoxic killing.
Types of Vaccines:
Live-attenuated, inactivated, subunit, toxoid, viral vector, and mRNA vaccines.
Antivirals Targeting:
Block key steps in the viral life cycle without directly killing viruses.
COVID-19 Tests:
Antigen Tests: Detect viral proteins using antibodies on a test strip.
RNA World Hypothesis:
Proposes RNA was the first genetic material, possibly leading to virus-like particles.
Emergence of Viruses Hypotheses:
Regressive hypothesis, cellular origin hypothesis, and coevolution hypothesis.
Ecosystem Services Affected by Viruses:
Control populations, nutrient cycling, disease outbreaks affecting stability.
Biological Carbon Pump & Viral Shunt:
Important processes for carbon storage, nutrient availability, and energy flow in the biosphere.
Pandemic Contributing Factors:
Include high transmission rates, pathogen evolution, global travel, and public health challenges.
Giant Viruses Blurring Lines of Life:
Large genomes, complex functions, and replication inside hosts challenge definitions of life.
Medical Use of Viruses:
Gene therapy (using engineered viruses for gene delivery), oncolytic viruses (targeting cancer cells), and viral vectors in vaccines (such as in COVID-19).