Viruses are structurally diverse, acellular parasites whose host-dependent replication cycles, multiple evolutionary origins, and rapid genetic change shape infection and immune escape.
Viruses contain genetic material enclosed in a protein coat, and many also have a lipid envelope with attachment proteins.
The genome stores instructions for making viral components; the capsid protects it and helps deliver it into a host cell. Envelopes are host-derived membranes carrying viral proteins that recognize receptors.
Describe a virus structurally:
An enveloped virus may bind a host receptor with a spike protein, fuse membranes, and release its genome into the cytoplasm.
A virus particle has structure but lacks the complete machinery for independent metabolism and protein synthesis.
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 geometry, envelope and attachment structures because each lineage solves protection and host entry in a different way.
Helical, icosahedral and complex capsids package genomes efficiently; envelopes can aid membrane fusion but are fragile outside hosts. Shape alone does not determine host range; receptor matching and replication machinery matter too.
Compare the features that affect infection:
A non-enveloped capsid may survive drying better than an enveloped virus, while an enveloped virus may fuse efficiently with a host membrane.
A distinctive shape is not enough to identify a virus species or predict its symptoms.
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 a lytic cycle, a virus enters a host cell, copies its genome, makes viral proteins, assembles new particles and lyses the cell to release them.
Viral genes redirect host resources toward genome replication and capsid production. Assembly followed by membrane or cell-wall disruption produces many infectious particles but kills the host cell.
Trace the sequence:
A bacteriophage can inject its genome into a bacterium, assemble many phage particles, and rupture the bacterium so the particles infect neighbouring cells.
The lytic cycle is not just ‘virus reproduction’; the timing of genome replication, assembly and release matters.
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 a lysogenic cycle, viral genetic material becomes associated with the host genome or persists in a stable form and is copied when the host cell divides.
The integrated viral sequence can remain inactive while host genes are expressed. Stress or another trigger can induce the viral genome to leave this state and enter a lytic cycle.
Distinguish the states:
A bacterium carrying a silent prophage can divide without bursting; DNA damage may trigger excision and production of new phage particles.
Lysogeny is not permanent immunity: induction can switch the infection to a destructive phase.
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.
Viruses can evolve rapidly when short generation times, large population sizes and high mutation or recombination rates create variation for selection.
Selection favours variants that transmit or replicate more successfully in a particular host and environment. Drug or immune pressure can change which variants increase, but mutation alone does not guarantee a beneficial change.
Explain rapid evolution with the full chain:
If a treatment suppresses susceptible viruses, a resistant variant may become common because it leaves more descendants under that treatment.
An individual virus does not evolve during treatment; the population’s variant frequencies change.
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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Marking guidance:
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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.