31st July 2023

Molecular classification

Viruses can also be classified based on their genomic structure and the resultant method of viral replication. This is known as the Baltimore Classification System (See image below).

For the MRCP exam, being able to determine where a virus may fit within the 7 categories of the Baltimore system is unlikely to be very important. However, it can help us understand how different viruses replicate and hence cause disease, which is crucial to designing antiviral drugs and vaccines. Hence it is worthwhile being aware of. Basic sciences are on the MRCP curriculum.

Some important viruses, as per their molecular classification, include:

• Group I – herpes simplex virus (HSV)
• Group IV – coronaviruses, dengue virus, and poliovirus
• Group V – Ebola virus
• Group VI – human immunodeficiency virus (HIV)

The Baltimore Classification System

Image Source: Wikimedia Commons (2009). Baltimore Classification, Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:Baltimore_Classification.png (Accessed: 17 September 2022).

The 7 groups are depicted here, categorised by how they produce the mRNA used to replicate. ds = double stranded, ss = single stranded, (+) = positive sense, (-) = negative sense.