Preview
IB DP Biology — A — Unity and Diversity | SL | A2.3 Viruses
📋 What You Will Learn
- Identify the structural components of viruses — capsid, nucleic acid core, and envelope — and explain how each contributes to function
- Explain the lytic and lysogenic replication cycles of bacteriophages, including all key stages in correct sequence
- Describe how animal viruses enter host cells and hijack cellular machinery to produce new viral particles
- Classify viruses by type of nucleic acid (DNA or RNA), strandedness, and presence or absence of a lipid envelope
- Analyse the relationship between viral structure and host specificity, applying this to unfamiliar viral examples
- Evaluate the argument for whether viruses should be considered living organisms, using biological criteria
🔍 Guiding Question Are viruses living organisms, and what does the answer reveal about the nature of life itself?
🌍 Why This Matters In 2019, a novel coronavirus with a genome of just 30,000 nucleotides — smaller than many individual human genes — brought global society to a halt. SARS-CoV-2 is not alive by most biological definitions, yet it redirected the machinery of trillions of human cells to copy itself. Understanding how viruses are built and how they replicate is not academic curiosity: it is the foundation of every antiviral drug, every vaccine, and every pandemic response strategy ever developed.
What Exactly Is a Virus?
Here is the problem biology had to solve: how do you classify an entity that carries genetic information, evolves by natural selection, and causes disease — but cannot grow, cannot metabolise, and cannot reproduce without hijacking another organism's cellular machinery? Viruses sit at the boundary of the living and non-living, and that boundary turns out to be one of the most philosophically and scientifically interesting places in all of biology.
Before exploring how viruses work, we need precise language. The terms below are not just vocabulary — they are the conceptual tools that make the rest of this topic coherent.
📖 Key Definition — Virus A virus is an acellular, non-living infectious agent consisting of a nucleic acid core (DNA or RNA) enclosed within a protein coat called a capsid, and in some cases surrounded by a lipid envelope derived from the host cell membrane. Viruses are obligate intracellular parasites — they can only replicate inside a living host cell.
📖 Key Definition — Capsid The capsid is the protein coat that surrounds and protects the viral nucleic acid. It is composed of repeating protein subunits called capsomeres, which self-assemble into characteristic geometric shapes — most commonly icosahedral (20-faced) or helical arrangements.
📖 Key Definition — Virion A virion is the complete, mature, extracellular form of a virus — the infectious particle as it exists outside a host cell. It is structurally intact and capable of infecting a new host.
Viral Structure
Every virus faces the same engineering challenge: protect the genetic material during transit between hosts, then deliver it precisely into the right type of cell. The solution is elegant in its simplicity — a genome wrapped in protein, sometimes wrapped again in lipid. But within that simplicity lies extraordinary diversity.
The Nucleic Acid Core
The viral genome is the heart of the virion. Unlike all cellular life, which uses double-stranded DNA as its primary genetic material, viruses use an extraordinary variety of nucleic acid types. The genome may be DNA or RNA, single-stranded or double-stranded, linear or circular, and in some viruses it is segmented into multiple separate pieces.
This diversity is not random — it reflects different evolutionary strategies for replication. RNA viruses mutate far more rapidly than DNA viruses because RNA polymerases lack the proofreading mechanisms of DNA polymerases. This high mutation rate is why influenza and HIV evolve so quickly, making vaccine development so challenging.
📖 Key Definition — Nucleic Acid Types in Viruses Viruses may contain: double-stranded DNA (dsDNA, e.g.