While teaching biology and their applications to my undergraduate students, I often make use of analogies that bring home the concepts in a simple yet effective manner. I find analogies very useful, especially during online teaching sessions, in helping students visualize the concepts being discussed.

For example, the concept of prokaryotic and eukaryotic cells. In prokaryotic cells, all metabolic activities happen within a single cell membrane without any compartments whereas in eukaryotes, each organelle has its own membrane and a different function. This concept can be explained by comparing a prokaryotic cell to a studio apartment where everything is accommodated within a single room, whereas a eukaryotic cell can be compared to a 4 or 5 BHK apartment with dedicated rooms for specific work such as kitchen, bedroom, study, drawing room etc. This comparison gives the students a start towards a clear and complete understanding of the features of these two types of cells.

Analogies can be used to bring home various concepts in immunology, bioprocess technology, or molecular biology, such as self and non-self recognition, innate and acquired immunity, replication, transcription, translation, etc. Following are the examples of analogies that I have found useful in my classroom.

1. The innate immunity can be compared with the police force in a city, which is always present and looks after the law and order in the city. The police van keeps going around in all the lanes and by-lanes of the city to counter any untoward incident that may happen during their rounds. Similarly, the phagocytic cells of our body also circulate throughout the body and any non-self substances or foreign organisms encountered are phagocytosed and eliminated. 

2. Just as the police use a photograph of the culprit to search an area, the immune cells such as macrophages and neutrophils can recognize specific epitopes on the pathogen cell surface with the help of TOLL-like receptors. The antigenic determinants can be compared to the fingerprints of an individual that are unique and can be used for identifying and eliminating the pathogens.

The police use hand-cuffs to prevent the thief from running away. This can be compared to the antibodies that opsonize the antigen and help to anchor the pathogen on the phagocytic cell surface with the Fc receptors and facilitate phagocytosis.

3. Just like a security guard can catch hold of a thief before he can break into a house, when a pathogen tries to enter the body through any of the mucosal linings of respiratory tract, digestive tract or urogenital tract then the IgA antibodies and the mucosal macrophages catch hold and engulf these pathogens and eliminate them.

4. If a gang of terrorists or foreign enemy (highly virulent pathogens) enters the country in huge numbers, with advance arms and ammunitions (capsule or toxin secretion), then the normal police cannot overpower them. In such cases, special forces, such as the NSG or the Military (Acquired Immunity) need to be called, strictly under orders from the Chief of Army or the President of the country.. Similarly, cells of the acquired immune system such as the T helper and T cytotoxic cells can only be activated when presented with the antigen by the Antigen Presenting Cells (Macrophages, Dendritic cells, Neutrophils etc.)

5. When terrorists are hidden in a deserted house (intracellular pathogens) and a local informer helps to identify that house (Antibodies), then the military may decide to destroy the entire house by bombing it instead of ambushing the terrorists individually. Similarly, the T cytotoxic cell also recognizes the infected self-cell and destroys it completely along with the pathogens.

6. Compared to a new and unknown enemy, the military can elimiate a known enemy faster and with enhanced capacity, as it is already aware of its strengths and weaknesses. Similarly, when the same pathogen infects the second time the immunological memory of the acquired immunity elicits a secondary response which is enhanced and faster than the primary response. 

7. During gene expression, the processes of transcription and translation can be compared to an interior designing company, in which the chief designer (DNA) makes the designs and hands over the responsibility of executing designs of individual rooms to assistant managers (mRNA). These managers then instruct the artisans and carpenters (ribosomes and tRNA), to build different furniture pieces such as chairs, tables, cupboards (proteins), using pieces of wood (amino acids).

8. Plasmid vectors in bacterial cells can be explained by using the analogy of a person staying in our house as a guest. An unknown person is not allowed to enter or stay, similarly only when the plasmid has an origin of replication recognizable to the enzymes of the host cell, it can replicate in the host. If the guest is a paying guest, he/​she will be allowed to stay for a longer period of time, likewise, only if the plasmid is providing any survival advantage to the host cell (Antibiotic resistance marker) will it be retained in the host.

9. In Bioprocess technology, the process of primary and secondary screening of organisms for production of biological products, such as antibiotics, growth factors, organic acids, etc., can be compared to the audition and elimination rounds of reality talent hunt shows, such as Indian Idol, Dance India Dance, etc. Here the candidates (wild type organisms) are screened and the most talented ones (potential strains) are selected for the final round. Before the final round, the candidates are groomed (strain improvement by mutagenesis) before the winner (Industrial strain) is selected.

Teachers in the field of biological sciences can use these as well as other such analogies in their teaching pedagogies and improve the attention and understanding of their students. However, it should be kept in mind that the analogies should not confuse students or infuse misconceptions about the topic. Teachers have to choose the analogy that is closest to the concept being explained. The analogies are not a substitute for the explanation, hence the concepts have to be introduced first and theoretically explained, and only then the analogy should be given.