Biology · Viruses & Acellular Biological Agents
Virions, genomes, capsids and envelopes
A drawing of a spherical virus and a drawing of a small bacterium can both look like circles. Their contents change the comparison: the bacterium has cellular machinery, while the virus particle carries material that participates in a host-dependent life cycle.
A membrane separates a cell from its surroundings. DNA and RNA carry information, while ribosomes translate messenger RNA into protein. Those ideas are developed in Cells, Membranes & Transport, DNA, Genomes, Replication & Repair and Gene Expression & Protein Synthesis; they are also introduced briefly where needed here. Evolution concerns inherited population change, rather than a deliberate response by one particle.
Back to the Biology mapBy the end, you can
Distinguish a virus lineage from a virion, and identify hereditary material, a capsid and an optional envelope without mistaking a particle for a cell.
A virion is one physical part of the history
A virion is a virus particle, considered as a physical package containing a viral genome and associated structures. Virus can refer more broadly to the biological entity and its lineage across infection and transmission. Counting particles answers a different question from identifying the lineage or following activity inside a cell.
The genome is hereditary nucleic acid. Different viruses use DNA or RNA, in different organisations. A protein capsid encloses the genome in a typical virion. An enveloped particle additionally has a lipid-containing membrane with associated proteins. An envelope is an acquired and modified part of the particle; it does not supply the full organisation of a cellular plasma membrane.
Virions do not carry their own ribosomes. Viral proteins are made through translation in host cells. Some particles contain enzymes or other proteins as well as structural proteins, so a genome-and-coat diagram is a selected comparison rather than a complete molecular inventory.
A capsid and an optional envelope are distinct boundaries
Read the diagram in words
Both panels place a seven-point schematic genome path inside the inner capsid radius 2.7. The protein coat spans 2.7–3. The enveloped panel adds a distinct outer boundary at 3.8–4. Dimensions are assigned to separate the regions.
| Selected layout | Boundary material | Inner radius | Outer radius |
|---|---|---|---|
| Selected non-enveloped particle | Protein capsid | 2.7 | 3 |
| Selected enveloped particle | Protein capsid | 2.7 | 3 |
| Selected enveloped particle | Lipid-containing envelope | 3.8 | 4 |
| Selected layout | Schematic x | Schematic y |
|---|---|---|
| Selected non-enveloped particle | 0 | 0 |
| Selected enveloped particle | 0 | 0 |
| Selected layout | Nucleic-acid path position | Schematic x | Schematic y |
|---|---|---|---|
| Selected non-enveloped particle | 1 | -1.8 | 0 |
| Selected non-enveloped particle | 2 | -1.2 | 0.8 |
| Selected non-enveloped particle | 3 | -0.6 | -0.8 |
| Selected non-enveloped particle | 4 | 0 | 0.8 |
| Selected non-enveloped particle | 5 | 0.6 | -0.8 |
| Selected non-enveloped particle | 6 | 1.2 | 0.8 |
| Selected non-enveloped particle | 7 | 1.8 | 0 |
| Selected enveloped particle | 1 | -1.8 | 0 |
| Selected enveloped particle | 2 | -1.2 | 0.8 |
| Selected enveloped particle | 3 | -0.6 | -0.8 |
| Selected enveloped particle | 4 | 0 | 0.8 |
| Selected enveloped particle | 5 | 0.6 | -0.8 |
| Selected enveloped particle | 6 | 1.2 | 0.8 |
| Selected enveloped particle | 7 | 1.8 | 0 |
Selected non-enveloped particle · Hereditary nucleic acid · Selected section; arbitrary dimensions · No envelope assigned in this selected section
Selected enveloped particle · Hereditary nucleic acid · Selected section; arbitrary dimensions
Radius (drawing units)
Drawing units
Additional lipid-containing boundary
No measured sizes or universal shape
Original non-enveloped and enveloped schematic comparisons. Not a species-specific particle, molecular stoichiometry or measured dimension. No nucleus, ribosomes or cell cytoplasm is assigned.
A cross-section and a three-dimensional shape answer different questions
A circular cross-section can make enclosing boundaries visible without claiming that every particle is a perfect sphere. Capsids can have helical, roughly icosahedral or more complex organisations. A tailed phage combines regions with different roles; its outline is not the outline of every virus that infects bacteria.
The term non-enveloped describes the absence of that outer lipid envelope, not the absence of a protein covering. Conversely, an envelope does not imply that a particle has a nucleus, cytoplasm or an energy-generating cellular system. A shared lipid component is only one part of a larger structural comparison.
There are also virus-derived and related genetic elements whose present forms do not fit the familiar particle diagram. The ICTV operational definition recognises descent from major virion-protein-encoding entities. A simple picture should therefore explain its selected particle, rather than become a universal checklist for every classified entity.
Keep the counted object explicit
A genome copy, a protein subunit and an assembled particle are different units. Detecting nucleic acid does not by itself show that an intact particle is present, or that the detected material can establish a productive infection. Structure, activity and hereditary identity require different evidence.
In the constructed inventory, rows identify the counted entity before the number. The example supplies no biological assay or conversion factor from genome copies to infective particles. Its purpose is to prevent different counts from being silently treated as equivalent.
Genomes, protein pieces and particles have different denominators
Read the diagram in words
The genome row is 8 + 4 = 12; the protein-piece row is 16 + 8 = 24; complete packages number 8. These rows have different object units and cannot be added to count particles.
| Counted material | Assigned to assembled packages | Outside complete packages | Total objects of this kind |
|---|---|---|---|
| Genome objects | 8 | 4 | 12 |
| Abstract protein pieces | 16 | 8 | 24 |
| Complete schematic particles | 8 | 0 | 8 |
Eight of twelve genome objects are packaged
Original finite account with one genome object and two abstract coat pieces assigned to each of eight particles. Two coat pieces are accounting symbols, not real capsid stoichiometry.
Worked example
Three counts can describe the same fictional record differently
The assigned record contains 12 genome objects, eight complete schematic particles and four unassembled genome objects. Each of the eight particles contains exactly one assigned genome. The accounting is therefore 8 packaged + 4 unassembled = 12 genome objects.
Separately, the record contains 24 individual protein pieces, 16 assigned to assembled coats and eight unassembled. This invented two-piece coat is an accounting symbol, not a real capsid stoichiometry. The 24 protein pieces cannot be added to 12 genomes to claim 36 particles.
The ratio 8 ÷ 12 = two-thirds describes the fraction of assigned genome objects in packages. It is not infectivity, viability, transmission probability or a real genome-to-particle conversion.
Check your understanding
Does the presence of an envelope make a virion a cell?
Show reasoning
No. A lipid-containing envelope is one structure. It does not give the particle ribosomes or a complete cellular metabolism, and the lineage remains dependent on host cells.
Why are 12 genome objects not necessarily 12 complete particles in the constructed inventory?
Show reasoning
Four genome objects are assigned outside the eight complete packages. The counted material is hereditary nucleic acid, whereas the particle count concerns assembled packages.
Can the two-thirds packaging fraction be reported as a measured infectivity fraction?
Show reasoning
No. The model assigns packaging only and contains no evidence about productive infection. Changing the counted outcome would require additional evidence.
What to carry forward
Name the particle, boundary and counted material before deciding what a picture or number establishes.
References
Resources supporting this lesson’s explanations and examples.
- Biology 2e: selected virus, host and acellular-agent sections (2018; selected sections 21.1, 21.2 and 21.4)Mary Ann Clark, Matthew Douglas and Jung ChoiTextbook sections
- International Code of Virus Classification and Nomenclature (Current visible code checked 13 October 2026)International Committee on Taxonomy of VirusesOfficial classification rules