outline how viruses are classified, limited to the type of nucleic acid (RNA or DNA) and whether this is single stranded or double stranded

Classification of Viruses – Cambridge AS & A Level Biology (9700)

Learning Objective

Students will be able to recognise viruses as non‑cellular infectious agents, state the basic components of a virus, and outline how viruses are classified according to the type of nucleic acid they contain (RNA or DNA) and whether that nucleic acid is single‑stranded or double‑stranded.

Key Definition (syllabus wording)

  • All viruses are non‑cellular structures consisting of a nucleic‑acid core (DNA or RNA) surrounded by a protein capsid.

  • Some viruses also possess a lipid envelope derived from the host cell membrane.

  • Viruses are obligate intracellular parasites – they can replicate only inside a suitable host cell.

Why the classification matters

  • The nature of the viral genome (DNA vs RNA, single‑ vs double‑stranded) determines the replication pathway a virus must use inside the host.

  • Knowing the genome type helps predict:

    • How the viral genome is transcribed into messenger RNA, and

    • Which antiviral strategies are likely to be effective.

  • This classification underpins the study of viral pathogens in Topic 10 (Infectious diseases) and the host response in Topic 11 (Immunity).

Classification by nucleic‑acid type and strand configuration

Genome typeStrand configurationKey replication featureTypical example families
DNADouble‑stranded (dsDNA)Transcribed to mRNA by the host DNA‑dependent RNA polymerase.Herpesviridae, Adenoviridae, Poxviridae
DNASingle‑stranded (ssDNA)Converted to dsDNA by host DNA polymerase before transcription.Parvoviridae, Circoviridae
RNADouble‑stranded (dsRNA)Each strand serves as a template for synthesis of (+)‑sense mRNA.Reoviridae, Totiviridae
RNASingle‑stranded, positive‑sense (ssRNA⁺)Genome can act directly as mRNA for protein synthesis.Picornaviridae, Flaviviridae, Coronaviridae
RNASingle‑stranded, negative‑sense (ssRNA⁻)Requires a viral RNA‑dependent RNA polymerase to make (+)‑sense mRNA.Orthomyxoviridae, Rhabdoviridae, Filoviridae

Application to Infectious Diseases (Topic 10)

Understanding the genome classification allows students to predict the replication strategy of important human pathogens:

  • Human Immunodeficiency Virus (HIV) – a ssRNA⁺ retrovirus (family Retroviridae). Its RNA genome is first reverse‑transcribed into dsDNA, which is then integrated into the host genome.

  • Influenza virus – a ssRNA⁻ virus (family Orthomyxoviridae). It carries its own RNA‑dependent RNA polymerase to synthesise (+)‑sense mRNA from the negative‑sense genome.

By linking the classification to these examples, students can see how genome type influences disease‑causing mechanisms and informs vaccine or drug design.

Link to Immunity (Topic 11) – brief note

Because the viral genome determines the proteins expressed on the virion surface, the classification also helps explain which viral antigens are recognised by the immune system and why certain vaccines target specific genome types (e.g., inactivated dsDNA viruses vs. live‑attenuated ssRNA⁺ viruses).

Suggested visual aid

Flow‑chart idea: Start with “DNA or RNA?” → “Double‑stranded or single‑stranded?” → “If single‑stranded RNA, is it +‑sense or –‑sense?” → final box with the corresponding virus families. This diagram reinforces the step‑by‑step decision process required by the syllabus.