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Covid-19 / Coronavirus/ Sars-CoV-2 – the basics
Coronavirus (Sars-CoV-2) structure. Credit | NIH NIAID
Structure of a virus: notice the nucleic acid center, surrounded by a protein capsid +/- a fatty acid envelope. Notice the spike-like envelope proteins. Credit | Ben Taylor (public domain)
Cryo-EM structure of coronavirus cell covered with spike proteins so it can bind to the ACE2 receptor on the host cell. The red areas in the spike protein are cleaved by the cell’s protease so the virus can enter the cell. Credit | David Veesler, University of Washington
Virus BasicS
What is a virus? A virus is literally a package of nucleic acid (either DNA or RNA) enclosed in a protein shell. Sometimes that shell has a fatty membrane around it. Viruses constitute their own kingdom. One of the interesting things about them is that they do not have the ability to independently reproduce or make copies of themselves. They accomplish this by hijacking other cells that do have the machinery to reproduce. That’s why they attach to other kinds of cells (including bacterial cells and human cells).
The viral life cycle involves a protein on the surface of the virus binding to a receptor on the surface of the cell that is being hijacked. For this coronavirus, the spike protein binds to a receptor on the human cell (often a lung cell) called ACE2. The virus doesn’t just get access; the spike protein needs to be cleaved (broken) by an enzyme present on our own cells called a protease.
Once the virus enters, it breaks down to release its nucleic acid (Coronavirus) or injects its nucleic acid (bacteriophage) into the cell’s cytoplasm. It then uses the invaded cell’s machinery to make more copies of its DNA, turn it into mRNA (messenger RNA), and finally into proteins using the cell’s endoplasmic reticulum. Those proteins reassemble into virus particles, and those particles are released from the hijacked cell into the host.