What is a Virus? What are Viruses made up of ?
2. How do viruses replicate?
Viruses recognise and bind to target cells by the interaction of molecules on their surface, with those on the target cell. These are called “surface receptors” – and work a little like a lock and key: a virus can only invade a cell is its surface molecule
(the key), matches the receptor (the lock) on the host cell. The SARS-CoV-2 key is the spike protein which matches the ACE2 receptor lock on target cells.
After binding to target cells, virus particles enter the cell either by fusion of the lipid envelope with the host cell membrane (e.g. HIV) or receptor-mediated endocytosis (e.g. influenza virus or coronavirus), in which receptor binding triggers the cell to engulf the virus particle. Once inside the cell, the viral capsid dissolves, uncoating the viral genome, which can then be replicated. New viruses are now made by the host replication and synthesis machinery. Finally mature virus progeny exit the host cell. Enveloped viruses do this by budding, picking up a coating of lipids from the host cell membrane on the way out. Box 1 shows the size of virus particles, but also some of the surface receptors on coronaviruses and influenza virus.