A2.3 Viruses

Question 1

A2.3.1—Structural features common to viruses

Answer 1

Relatively few features are shared by all viruses: small, fixed size; nucleic acid (DNA or RNA) as genetic
material; a capsid made of protein; no cytoplasm; and few or no enzymes

Question 2

A2.3.2—Diversity of structure in viruses

Answer 2

Students should understand that viruses are highly diverse in their shape and structure. Genetic material
may be RNA or DNA, which can be either single- or double-stranded. Some viruses are enveloped in host cell membrane and others are not enveloped. Virus examples include bacteriophage lambda,
coronaviruses and HIV.

Question 3

A2.3.3—Lytic cycle of a virus

Answer 3

Students should appreciate that viruses rely on a host cell for energy supply, nutrition, protein synthesis
and other life functions. Use bacteriophage lambda as an example of the phases in a lytic cycle.

Question 4

A.2.3.4—Lysogenic cycle of a virus

Answer 4

Use bacteriophage lambda as an example.

Question 5

A2.3.5—Evidence for several origins of viruses from other organisms

Answer 5

The diversity of viruses suggests several possible origins. Viruses share an extreme form of obligate
parasitism as a mode of existence, so the structural features that they have in common could be regarded
as convergent evolution. The genetic code is shared between viruses and living organisms.

Question 6

A2.3.6—Rapid evolution in viruses

Answer 6

Include reasons for very rapid rates of evolution in some viruses. Use two examples of rapid evolution:
evolution of influenza viruses and of HIV. Consider the consequences for treating diseases caused by
rapidly evolving viruses.