Image from: ACS Sens. 2020, 5, 12, 3739–3769 Publication Date:November 23, 2020 https://doi.org/10.1021/acssensors.0c01961
iGEM Project 2023 | Part 3
Before continuing to discuss the history of these viruses, we cannot avoid mentioning their structure. In fact, phages represent an extraordinary example “perfection” in nature. Their structure is extraordinarily ordered, geometric, almost as if it had been designed, yet it is nothing more than the result of millions of years of evolution.
A phage is nothing more than a small set of genetic information enclosed in an ordered box of proteins. The viral genome is essential: no frills or useless information, only genes essentials to survive, all wrapped up in a few proteins that repeat themselves in constant units to form a rigid structure called capsid.
The typical anatomy of these organisms can be divided into two categories: the so-called head-tail phages and the cylindrical ones. The former are perhaps those that are most present in the common imagination and that come to mind when we think of a “virus”. They consist of an icosahedral head given by the repetition of one or two protein units, and a tail that ends with fibers that closely resemble “little paws”. This structure also reminds us, perhaps, of one of the most iconic ways in which the infection of the bacterial cell can take place: the phage lands on the surface of the cell and, like a syringe, injects its own genome inside. Keep in mind that this structure and this mechanism are valid only for some bacteriophages.
We mentioned the cylindrical phages a little while ago, these are perhaps less fascinating in their structure than the previous ones, but they remain important. Moreover, in this case they are formed by the repetition of a fundamental unit which is flanked by some other proteins positioned at the two opposite ends which are responsible for two events: they mediate the interaction and recognition with the host cell, and they complete the assembly of the capsid itself.