What did we know about the structure of the
SARS-CoV-2 virus and how it replicates?
(What was understood by March 2021)
You may wish to read the following pages first in order to better understand this page:
What we knew
What we knew about the origin of Covid-19
Click on dark blue words or terms to see their meaning in the glossary.
Like other coronaviruses, SARS-CoV-2 has a spherical structure with protruding spikes that resemble a crown. The name “coronavirus” comes from corona, the Latin word for “crown.”
The virus is enclosed within a lipid envelope. Inside this envelope is a long, single strand of RNA — a genetic molecule similar in structure to DNA, the well-known “blueprint of life” in living organisms (but only to half of its double helix). However, while DNA serves as the genetic blueprint for most life forms, some viruses, including SARS-CoV-2, use RNA instead. Unlike living organisms, viruses cannot reproduce on their own, which is why they are often considered to exist on the boundary between living and non-living entities.
Within the viral envelope, the RNA is protected by a protein shell called a capsid. This is a long, tube-like structure that is coiled and packed tightly inside the envelope. The RNA it contains holds all the genetic instructions necessary for assembling new copies of the virus.
The virus enters human cells using its spike proteins, which bind to a specific enzyme receptor called ACE2 on the surface of mammalian cells. The ACE2 receptor normally works in tandem with the ACE receptor to regulate molecules that influence blood pressure and other physiological functions. The binding of the viral spike to ACE2 is analogous to a key fitting into a lock, initiating a process that fuses the virus with the host cell membrane.
To facilitate this fusion, another enzyme on the cell membrane, TMPRSS2, possibly in combination with furin, cleaves the spike protein. This splitting releases what are essentially molecular “pliers” that pinch the viral envelope and the cell membrane together, creating an opening through which the viral RNA enters the cell. Once inside, the virus hijacks the host cell’s machinery to replicate itself.
The RNA from the virus contains information “written” in its combination of nucleotides (the molecular units displayed in colors in the diagram above) along the strand. This information is basically an instruction manual for creating a copy of the virus.
Normally, human cells create proteins using these instructions from their own messenger RNA (mRNA), which is transcribed (copied) from genes in the cell’s DNA. The viral RNA, however, bypasses this process by directly inserting its own genetic instructions into the cell’s ribosomes — organelles that are the molecular assembly lines that produce proteins from mRNA. These instructions guide the production of viral proteins, including the building blocks of new viruses and the enzymes necessary for their assembly. (In fact, each set of three nucleotides on the DNA or RNA corresponds to a single unit of the protein being created. The assembly is linear but as they then fold at specific positions they become magnificent structures.)
One key enzyme produced, polymerase, is the tool responsible for the replication of the entire viral RNA genome for each new copy of the virus. This animation shows how RNA is copied by the polymerase enzyme. (In the animation the blue RNA strand is an original and the red RNA strand is a new copy.)
As replication proceeds, newly formed viral particles bud off from the host cell. The envelope of SARS-CoV-2, like the membrane of an animal cell, is composed of a lipid bilayer,. This envelope is not synthesized from scratch but is instead taken from the membrane of a host cell organelle, which wraps around the newly created viral RNA and proteins. (This structural feature makes SARS-CoV-2 particularly vulnerable to soap, as the lipid bilayer can be easily disrupted. Washing hands with soap for at least twenty seconds effectively destroys the virus by breaking apart its membrane, rendering it incapable of infecting new cells.)
Continue with What we knew about Covid-19 spreading in a population.
What we knew
What we knew about the origin of COVID-19
What we knew about the structure of the SARS-CoV-2 virus
What we knew about the virus spreading in a population
DW: Researchers succeed in taking first 3D photo of coronavirus
(January 20, 2021)
©2020, 2021 Dr. Michael Herrera