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Teaching A2.3 Viruses to IB Diploma students

Guiding questions

In what ways do viruses vary?

Viruses vary in size and shape, structure and shape of their capsid, chemical composition and structure of the genome, and method of replication. The genome of a virus may consist of DNA or RNA, which may be single stranded or double stranded, linear or circular. The entire genome may occupy either one nucleic acid molecule (single segment genome) or several nucleic acid segments (multi-segment genome). The different types of genome have different types of replication.

How can viruses exist with so few genes?

The purpose of a virus is to replicate itself by infecting new host cells. Viruses generally have small genomes with a small number of genes and hence express a small number of proteins (enzymes and capsid proteins). Viruses have small genomes for several reasons:

  • Efficiency: Viruses need to replicate quickly within a host cell. The presence of a small genome means viruses are able to achieve rapid replication without the need for complex regulatory mechanisms.
  • Host dependency: Unlike cells, viruses do not have the ability to synthesize their own proteins and therefore rely on the host cell's ribosomes and enzymes to replicate. By having a small genome, viruses are able to use the host cell's protein synthesis system more easily, which allows for efficient replication. This means that viruses are true parasites.
  • Mutation rate: Mutant viruses appear very quickly which allows them to adapt quickly to changing environments and host defences. The rapid appearance of mutants is due to two factors. If the virus has a DNA genome and uses host cell enzymes to replicate its genome mutations will occur at the same frequency that is found within the host cell and the ‘proof-reading’ function of the enzymes will correct most, but not all errors in the newly synthesised virus DNA. However, the small virus genome means that very large numbers of new genomes are produced quickly, and the large number means that mutants are more likely to arise much faster than is seen in the larger and more slowly replicated host cell genome. For viruses that encode their own replication enzymes such as those with RNA genomes, the RNA replicating enzymes cannot correct bases that have been introduced in error leading to a high level of mutations in the newly synthesised genome.
  • Genome packaging: viruses need to package their genome within a capsid for transmission to other host cells. Having a small genome makes it easier to package the genetic material within the capsid.

Overall, the small genome size of viruses underpins several aspects of their evolution and gives rise to their reliance on host cells for replication.

The third topic in the new 2025 IB Biology syllabus is A2.3 Viruses (Additional Higher level). HIV was featured as a sub-topic in the previous syllabus in the context of infection and the immune system, but viruses (Figure 1) including retroviruses, are now present in their own expanded and unified topic that explores viruses at a broader and deeper level with more explicit links to other parts of the syllabus, especially evolution.

The coronavirus disease 2019 (COVID-19) pandemic, caused by the SARS-CoV-2 virus, has forced the science of virology into the spotlight and has illustrated the complex interplay between science and society (especially politics) providing many NOS and TOK opportunities in the teaching of A2.3. 

Students will enter the IB Biology programme with varying levels of understanding of viruses.

An introductory question as a starting activity could be for students to: write down and summarise what they already know about viruses. A second question could be are viruses ‘alive’? A third question might be why study viruses?

Students of course will be familiar with a range of viral diseases, but may not know that viral infections in plants (e.g., dwarf mosaic virus in corn) have a major economic impact on crop production. Planting of virus-resistant varieties, developed by traditional breeding methods and more recently by genetic engineering techniques, can reduce crop losses significantly.

Diseases caused by defective genes, e.g., cystic fibrosis, may be treated by using viral vectors to introduce a normal copy of a defective gene into patients (gene therapy). Several viruses are associated with human cancers and female students may be familiar from vaccination that cervical cancer is associated with the human papillomavirus. The Epstein-Barr virus is associated with lymphoma and hepatitis B and C viruses with liver cancer.

Viruses can differ in many aspects such as host range (number of species and cell types that they can infect), types of genome and structural features. The wide diversity of viral shape and structure can be rationalised in terms of the classification of a virus based on whether it is a DNA or RNA virus, or single-stranded or double-stranded. Another classification of viruses involves the presence or absence of an envelope (derived from the host cell membrane) (A2.3.2).

An extension activity related to viral diversity and classification is for students to research or be introduced to the Baltimore classification of viruses (Figure 2) via ATL A2.3A 2 on page 90 of the textbook based on the nature of their genes and their modes of synthesis of mRNA. Students could be encouraged to summarise and evaluate the approach.

The key element of the Baltimore system is that all viruses must make mRNA as they all use the host ribosome to generate protein and ribosomes can only recognise mRNA. For this reason mRNA is represented as the central molecule. Once that is understood by students it is clear that you can describe how the production of mRNA can be achieved in the least number of steps. For example, if the genome RNA is positive-sense to make more it is necessary to synthesise a negative sense strand which will be the template for production of the positive-sense mRNA. If the genome is negative-sense it can be used as a template to directly produce mRNA without the need to produce an intermediate (though to replicate a negative or positive sense molecule you obviously have to make an opposite sense-strand, hence you can also infer something about the mode of replication).

David Baltimore (originator of the Baltimore classification system) elaborated his scheme because he was working on retroviruses and discovered that they were different (for which he was a joint recipient of a Nobel prize). He proposed that they have a separate classification within his scheme as a result – class VI(6).

One teaching approach to the topic may be to introduce a simple generalised structure of a virus (genome and capsid) and its generalised replication inside a specific host cell. The structural features common to viruses could also be introduced (A2.3.1).

Students can then be introduced to a group of viruses known as bacteriophages which infect bacteria. Lambda phage can be used to illustrate the lytic and lysogenic reproductive cycles (A2.3.4). The use of ‘phages’ in the Hershey-Chase experiment (A1.2.14) and the importance of phages in the early days of molecular biology should be mentioned. To continue reading this blog, please click here.

Chris Talbot has taught IB Chemistry, IB Biology, and TOK at schools in Singapore for over 20 years. He is the author of numerous science textbooks, including Biology for the IB Diploma Third edition and Chemistry for the MYP 4&5: By Concept, developed in cooperation with the International Baccalaureate®.

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