Viruses Display Extraordinary Structural Diversit
- Viruses are acellular and lack a single evolutionary origin, unlike cells.
- They exhibit multiple structural solutions to parasitism, making them highly adaptable.
Shapes and Sizes of Viruses
- Viruses are categorized by their capsid shape:
- Icosahedral (spherical): Symmetrical, 20-sided structure (e.g., adenoviruses, coronaviruses).
- Helical (rod-shaped): Spiral capsid arrangement (e.g., tobacco mosaic virus).
- Complex: Icosahedral head + helical tail (e.g., bacteriophages).
- Viruses are extraordinarily small, ranging from 20 to 300 nanometers:
- Bacteriophage lambda: ~50 nm.
- Mimivirus: Up to 750 nm, one of the largest known viruses.
Note
- Viruses do not grow like living organisms.
- Their size is fixed, but they must remain small enough to enter their host cells and replicate efficiently.
Genetic Material: DNA or RNA?
- Unlike living organisms (DNA only), viruses use DNA or RNA in varying forms:
- Single-stranded (ss) or double-stranded (ds)
- Linear or circular
- RNA viruses are further classified:
- Positive-sense RNA: Acts directly as mRNA
- Negative-sense RNA: Requires transcription into mRNA first
Example
- Coronaviruses like SARS-CoV-2 have single-stranded positive-sense RNA genomes approximately 30,000 bases long.
- In contrast, HIV contains two single-stranded RNA molecules, which are reverse-transcribed into DNA within the host cell.
Enveloped vs. Non-Enveloped Viruses
- Non-enveloped (naked): Only a protein capsid; resistant to desiccation, detergents, heat, and pH changes (common in plants and bacteriophages).
- Enveloped: Host-derived lipid bilayer with viral glycoproteins; fragile outside hosts, easily destroyed by soap, alcohol, and drying (mostly animal viruses).
Hint
- Enveloped viruses are particularly common in those that infect animals.
- Their lipid envelope helps them evade the immune system by mimicking host cell membranes.
Examples of Viruses and Their Unique Features
1. Bacteriophage Lambda
- Infects E. coli.Structure: Icosahedral head (ds-DNA), protein tail (injects DNA), tail fibers (attach to host).
- DNA is injected through the bacterial cell wall.
Hint
In electron micrographs, look for a distinct head-tail structure to identify bacteriophages.
2. Coronaviruses
- Infect mammals and birds; cause respiratory diseases.
- Structure: Positive-sense ss-RNA, helical capsid, lipid envelope with spike glycoproteins forming a corona-like halo.
- RNA acts directly as mRNA, translated by host ribosomes.
Note
The COVID-19 pandemic was caused by SARS-CoV-2, a coronavirus that likely originated in bats and jumped to humans via an intermediate species.
3. HIV (Human Immunodeficiency Virus)
Definition
Retrovirus
An RNA virus that uses reverse transcriptase to produce DNA, which then integrates into the host genome.
- A retrovirus infecting human helper T-cells.
- Structure: Two copies of ss-RNA, cone-shaped capsid, enzymes (reverse transcriptase, integrase, protease), host-derived envelope with glycoproteins (gp120, gp41).
- Uses reverse transcription (RNA → DNA), which integrates into the host genome.
Analogy
Think of HIV’s strategy as planting a hidden blueprint in the host cell’s “factory,” ensuring the production of new viruses even when the immune system isn’t actively detecting the infection.
Theory of Knowledge
- How does the diversity of viruses challenge the traditional definitions of life?
- Do viruses represent a unique category of biological entities, or should they be considered non-living?
Self review
- Why are enveloped viruses less stable outside the host than non-enveloped viruses?
- Describe the structural adaptations of bacteriophage lambda that allow it to infect bacteria.
- What gives coronaviruses their crown-like appearance in electron micrographs?
