The mechanism of viral replication

Viruses multiply within host cells – they require the host’s genetic replication machinery to express their own genetic code. Outside the host, viruses can’t multiply – but they can remain dangerous even on their own. Viruses can live for hours or even days, in water droplets in the air or on hard surfaces in the home – that’s why maintaining good ventilation and washing your hands regularly can help keep an infection at bay.

In order to complete their replication cycle, viruses must enter a host cell, hijack its genetic replication machinery, and reassemble – exiting the host as new viral particles (Figure 1).

Figure 1 – Schematic viral replication.

SARS-CoV-2 reaches human cells through the upper airway tract – entering the body through the mouth or nose. Spike proteins on the surface of SARS-CoV-2 help it attach to the human host cells. It can then inject its own genetic material. SARS-CoV-2 hijacks the hosts genetic replication machinery, ensuring that its own structural proteins are transcribed. Once the virus has replicated everything it needs – it re-assembles into new and complete virus particles. The new, complete viruses then prepare for ejection from the host cell – a process called ‘budding’.

Finally, the virus undergoes one last modification – a cleavage of its Spike protein by a host cell enzyme. This cleavage primes the virus to go on and infect more host cells. Some variants have a higher proportion of the snipped Spike protein – this is why it’s so important to understand variant profiles alongside the mechanism of viral infection.

If you would like further scientific details on virus mechanisms, you can read this article by Ward et al (2020)