Not quite alive — but undeniably consequential. Tiny protein-coated genomes that outsource everything to a host.
Every sub-topic below feeds at least one of these questions.
How can viruses exist with so few genes?
In what ways do viruses vary?
An extra 6 sub-topics for HL — same syllabus, deeper mechanism.
Structural features common to viruses
Diversity of structure in viruses
Lytic cycle of a virus
Lysogenic cycle of a virus
Evidence for several origins of viruses from other organisms
Consider the consequences for treating diseases caused by rapidly evolving viruses.
Viruses are wildly diverse — but five features are shared across all of them. None of those features is enough to be alive.
Bacteria are 10–100× larger (~2,000–3,000 nm).
Single- or double-stranded.
Repeating protein subunits forming a defined shape.
And few or no enzymes either.
No cytoplasm, no enzymes, no metabolism. A virus is an instruction set in a box. It can do nothing on its own; everything happens through hijacked host machinery.
Within the basic plan (genetic material + capsid), viruses vary in almost every parameter. The IB names three exemplars: bacteriophage lambda, coronaviruses, HIV.
Icosahedral — a 20-faced polyhedron (common). Helical — a rod or spiral. Complex — bacteriophages have an icosahedral head plus a helical tail plus fibre legs.
Single-stranded DNA · double-stranded DNA · single-stranded RNA · double-stranded RNA. Lambda is dsDNA; HIV is ssRNA; SARS-CoV-2 is ssRNA.
Some viruses (HIV, coronaviruses, influenza) acquire an outer envelope of host-cell membrane during release. Others (like lambda) are non-enveloped.
Lambda's fast option: enter the cell, take it over, make hundreds of copies, blow it up. Five clear stages.
Note: in the lytic cycle, phage DNA stays separate from the host chromosome the entire time. It hijacks; it does not integrate.
Lambda's slow option: integrate into the bacterial chromosome and ride along through host generations, then break out when conditions are right.
No single hypothesis explains all viruses. The diversity may reflect multiple independent origins — and the structural similarities may be convergent evolution from being obligate parasites.
Claim: viruses are older than cells — pre-cellular self-replicators that predated the first cell.
For: some viral genes have no cellular equivalent. Against: modern viruses require cells to replicate, so they can't have existed before cells.
Claim: viruses are bits of DNA or RNA that escaped from cells and became independent infectious agents.
For: bacterial cells exchange genetic material — a known escape mechanism. Multiple escapes would explain diversity. Against: most viral genes aren't found in cells.
Claim: viruses were once cellular parasites that lost everything but the bare essentials of parasitism.
For: giant viruses have genomes resembling those of parasitic bacteria. Against: the smallest known cellular parasites don't look like viruses.
Common features across virus families (capsid, small genome, obligate parasitism) may be convergent evolution — the same lifestyle selects for the same general design, even from different origins.
Viruses (especially RNA viruses) evolve at extraordinary rates. Three drivers — and two named examples.
Gradual accumulation of point mutations in HA and NA surface proteins (antigens). New strains emerge each season — slight enough that previous immunity is partly lost. This is why the flu vaccine is updated every year.
Two influenza strains co-infect a single host (e.g. a pig). When new virions assemble, gene segments from the two strains recombine. The result: a virus with HA + NA antigens nobody has seen before. Can cause pandemics.
HIV is a retrovirus — its RNA is reverse-transcribed into DNA before integrating into the host genome. Reverse transcriptase is exceptionally error-prone. The result: within a single patient, HIV evolves into a swarm of related strains, and drug resistance can develop in months.
If you can't define one of these in a sentence, that's where to revise next.
“What mechanisms contribute to convergent evolution?”
“To what extent is the natural history of life characterized by increasing complexity or simplicity?”