Viruses are structurally diverse, acellular parasites whose host-dependent replication cycles, multiple evolutionary origins, and rapid genetic change shape infection and immune escape.
All viruses are small particles of fixed size with a DNA or RNA genome enclosed by a protein capsid. They are non-cellular and can reproduce only inside host cells.
Viruses have no cytoplasm or ribosomes and few or no metabolic enzymes, so they depend on host energy, nutrients, protein synthesis and other life functions. Some, but not all, viruses also have a host-derived lipid envelope.
Features shared by all viruses: nucleic-acid genome, protein capsid, small fixed particle size and absence of cytoplasm. Envelope, spikes and many enzymes are variable rather than universal.
A virus particle can carry genes and a protective capsid yet cannot translate its own proteins because it has no ribosomes.
Do not define every virus as enveloped or as containing both DNA and RNA. A viral genome is DNA or RNA, and an envelope is optional.
This objective is assessed through structured response, commonly using State / Label / Describe.
State / Label / Describe / Justify
Build the answer around this relationship: Every virus has genetic material enclosed by a protein capsid.
Describing viruses as cells that merely lack a nucleus instead of recognizing that they are acellular.
Representative question
Describe one reason that viruses are not considered to be living.
a. (viruses) do not have a metabolism of their own (rely on a host cell for chemical processes) / cannot carry out transcription/protein synthesis / cannot perform ATP synthesis;
b. they are not made of cells (but consist only of a nucleic acid core and protein coat/capsid);
c. they do not carry out all reproduction but only multiply inside other living cells;
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Viruses vary in genome type, capsid shape and whether they carry a host-derived membrane envelope.
| Example | Genome | Outer structure | Distinctive form |
|---|---|---|---|
| Bacteriophage lambda | Double-stranded DNA | Non-enveloped protein capsid with tail | Complex head-tail particle that infects E. coli |
| Coronavirus | Single-stranded RNA | Enveloped with projecting spike proteins | Roughly spherical enveloped particle |
| HIV | Two copies of single-stranded RNA | Enveloped | Conical capsid inside the envelope |
Across viruses, genetic material may be DNA or RNA and single- or double-stranded. Capsids can adopt different geometries, and envelopes are present in some lineages but absent in others.
An envelope can aid fusion with a host membrane but is more easily disrupted by detergents or drying than a protein capsid.
Shape alone does not determine host range or identify a species; receptor matching and replication compatibility also matter.
This objective is assessed through structured response, commonly using Outline / State / Describe.
Outline / State / Describe
Build the answer around this relationship: Viral genomes may be DNA or RNA and may be single- or double-stranded.
Claiming that a virus contains both DNA and RNA instead of one type of nucleic acid genome.
Representative question
Describe, with examples, the diversity of structure in viruses.
a. variation in size/shape
OR
helical versus spherical / spikes versus no spikes / other contrasting shapes;
b. head and tail in bacteriophage (lambda) and polyhedral/spherical in HIV/COVID-19;
c. DNA or RNA as genetic material;
d. retroviruses/HIV has reverse transcriptase (but other viruses do not);
e. DNA in bacteriophage lambda and RNA in HIV/COVID-19;
f. single-stranded or double stranded DNA/RNA/nucleic acid;
g. bacteriophage lambda with double stranded and HIV/COVID-19 with single stranded;
h. circular or linear DNA / linear DNA in bacteriophage lambda;
i. variation in number of DNA/RNA/nucleic acid molecules
OR
HIV/other virus example with more than one molecule (of DNA/RNA);
j. some viruses are enveloped (in plasma membrane from the host cell);
k. bacteriophage lambda is a naked virus and HIV/COVID-19 is enveloped;
I. rapid evolution due to short generation time/rapid mutation rate/no proofreading/intense natural selection;
m. changes in influenza coat proteins/coronavirus spike proteins;
Marking guidance:
Accept other verified and valid examples in marking points b, e, g, i, k and m.
Accept coronavirus instead of COVID-19.
Accept diagrams to show examples of structural differences for marking points b, e, g, i, k and m. Reject statements that some viruses have plasma membranes.
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In lambda's lytic cycle, the phage attaches to E. coli, injects its double-stranded DNA, redirects host resources, assembles new phages and lyses the bacterium.
The viral DNA remains separate from the bacterial chromosome. Viral genes use host energy, nutrients, ribosomes and enzymes to copy phage DNA and synthesize capsid and tail proteins; host DNA may be degraded.
Sequence: attachment -> DNA injection -> viral genome replication and protein synthesis -> assembly of heads, tails and genomes -> bacterial lysis and release.
One infected E. coli cell can assemble many lambda particles before the cell wall ruptures and releases them to infect neighbouring bacteria.
Lytic replication makes components before release; a phage does not grow larger and split like a bacterial cell.
This objective is assessed through multiple choice, commonly using Identify / Compare / Contrast.
Identify / Compare / Contrast
Build the answer around this relationship: Lambda phage attaches to E. coli and injects its DNA into the host.
Reversing the cycles by placing viral DNA integration in the lytic cycle.
Representative question
What is a difference between the lytic and the lysogenic cycle of the bacteriophage lambda?
Only in the lytic cycle is lambda DNA inserted into the host DNA.
Only in the lysogenic cycle is the host metabolism used to replicate.
Only in the lytic cycle is the host metabolism used to replicate.
Only in the lysogenic cycle is lambda DNA inserted into the host DNA.
D
In lambda's lysogenic cycle, phage DNA integrates into the E. coli chromosome as a prophage instead of immediately producing new virions.
The prophage is copied whenever the bacterial chromosome replicates and is passed to daughter cells during binary fission. Stress such as DNA damage can induce excision and entry into the lytic cycle.
Sequence: attachment and injection -> integration as prophage -> copying with the host genome -> induction -> excision and lytic replication.
A lysogenic bacterium can divide repeatedly without bursting; after induction, the same viral DNA directs phage assembly and eventual lysis.
A prophage is not an inactive virus particle hidden inside the cell; it is viral DNA integrated into the host chromosome.
This objective is assessed through structured response, commonly using Explain.
Explain
Build the answer around this relationship: Lambda DNA integrates into the bacterial chromosome during lysogeny.
Representative question
Explain how the bacteriophage lambda reproduces in the lysogenic cycle.
a. the virus attaches to maltoporin/protein in the host/bacterium membrane;
b. the viral DNA/genome is integrated into a host/bacterium;
c. when the host DNA replicates/does mitosis, viral DNA is also replicated;
d. all daughter cells contain viral DNA/genetic material;
e. the inserted DNA is known as a prophage;
f. bacteriophage/prophage remains in dormant state in host/bacterium OR
later prophage may exit to initiate lytic cycle;
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The diversity of viruses is consistent with more than one evolutionary origin, rather than one simple family tree from a single first virus.
Some viruses resemble escaped genetic elements, some may descend from reduced cellular organisms, and others may have assembled from ancient replicators. Similar functions can evolve independently, so shared genes must be interpreted with context.
When comparing origins, ask:
A capsid gene shared by distant viruses may indicate transfer or convergent function; it does not alone prove one direct ancestor.
‘All viruses evolved from one virus’ is stronger than the evidence supports.
Some viruses evolve rapidly because short generation times, very large populations, mutation and recombination continually create heritable variation.
RNA-copying enzymes often lack effective proofreading. Influenza changes by antigenic drift through accumulated mutations and can change abruptly by antigenic shift through genome-segment reassortment. HIV reverse transcriptase is error-prone, producing many variants.
| Virus | Rapid-change mechanism | Consequence |
|---|---|---|
| Influenza | Antigenic drift; occasional antigenic shift | Immunity may recognize new strains poorly, so vaccine composition requires review and updating |
| HIV | Frequent reverse-transcriptase errors plus selection | Drug-resistant variants can rise, supporting combination therapy and careful adherence |
If one HIV variant resists a drug, treatment suppresses susceptible variants and the resistant lineage can leave more descendants; using multiple drugs makes simultaneous resistance less likely.
Mutation is not directed by treatment, and an individual virion does not adapt. Selection changes variant frequencies in the viral population.
This objective is assessed through structured response, commonly using Suggest / Outline.
Suggest / Outline
Build the answer around this relationship: Error-prone replication creates mutations especially rapidly in many RNA viruses.
Attributing rapid viral evolution only to mutation while omitting replication rate, recombination, or selection.
Representative question
Outline two reasons for the very rapid rates of evolution in some viruses.
a. rapid rate of mutation;
b. in RNA viruses / retroviruses enzymes (RNA replicase/polymerases/reverse transcriptase) do not proofread or correct errors (unlike DNA polymerase);
c. exchange of genetic material with each other (in the same generation)/ recombination and horizontal gene transfer;
d. viruses have short generation times / high reproduction rates / reproduce/replicate fast;
e. transmission of viruses between species can lead to new strains;
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A strong HL virus answer usually combines two moves: define the boundary, then explain the mechanism or consequence. Viruses have DNA or RNA plus a capsid but lack the machinery for independent metabolism. Their diversity is described by genome, capsid, and envelope. Lambda phage lets you contrast lytic takeover with lysogenic integration. Origin questions require competing hypotheses and polyphyly. Evolution questions require mutation, recombination, short cycles, large populations, and examples such as influenza or HIV.