A2.3 Virus structure - student reading
The most well known virus today is the Coronavirus which caused the global covid-19 pandemic between 2019 and 2023.
This virus has a typical virus structure, made from genetic material (DNA or RNA) and a few proteins. The spike proteins are notorious. These surface proteins on the virus attach to receptors on human cell surfaces causing infection, but these are not the only proteins.
If we look at a more detailed diagram of the SARS cov-2 virus we can see a range of other structural proteins, and an envelope of phospholipid. In the centre there is some genetic material (ssRNA) which is covered in a type of capsid protein.
SARS Cov-2 is an 'enveloped virus' because it takes a covering of phospholipid bilayer from the membranes of host cells.
Some of its proteins are embedded in this phospholipid layer. These are visible from the outside.
However if we look inside we can see another protein, called a 'nucleocapsid', or 'capsid' protein. This molecule is soluble in water and binds to the generic material in the virus.
SARS Cov-2 virus particles contain 27 different proteins in total, most of them are non-structural proteins, often enzymes like proteinase, RNA polymerase and Helicase.
Diversity of virus structure
Viruses do not have a single cellular structure, not even cytoplasm. All viruses have genetic material, either DNA or RNA, and an outer protein coat, called a capsid. Some types of virus (like SARS cov-2) also have an outer envelope made of protein and phospholipid membranes which comes from the host cell. Viruses also contain a few additional proteins, such as enzymes.
There is a huge diversity of virus shapes, it is the most obvious difference between members of different viral families.
Each viral family has a consistent shape. In general, the shapes of viruses are classified into four groups:
- Enveloped viruses have membranes surrounding capsids. Influenza virus, Coronavirus and HIV, are enveloped.
- Head and tail viruses infect bacteria. They have a head that is similar to isometric viruses and a tail shape like filamentous viruses. Lambda bacteriophage is a good example of this sort of virus.
- Isometric viruses have shapes that are polyhedral (e.g. icosahedral), roughly spherical, such as Adenovirus or Herpes viruses.
- Filamentous viruses are long and cylindrical. Many plant viruses are filamentous, including TMV (Tobacco mosaic virus).
Types of genetic material, nucleic acid
Nearly all living organisms use DNA as their genetic material but viruses may use either DNA or RNA. The molecules of nucleic acid may be single- or double-stranded. It may also be linear or circular. Sometimes there is one molecule of nucleic acid, sometimes several. The virus core contains the whole genome.
The capsid protein molecules attached to the nucleic acid often forms a cylinder shape. This cylinder may be twisted inside a polyhedral capsid, or a spherical envelope. The envelope is rather fragile and damaged by disinfectants, alcohols and changes in temperature. Non-enveloped viruses are more resistant to changes in temperature, pH, and some disinfectants.
Fast evolution and mutation of viruses
Viral genomes tend to be small, containing only genes for essential proteins that the virus cannot obtain from the host cell.
In DNA viruses, the viral DNA directs the host cell’s replication proteins to synthesise new copies of the viral genome and to synthesis viral proteins.
RNA viruses contain only RNA as their genetic material. To replicate their genomes in the host cell, the RNA viruses must contain genes for enzymes that can (reverse) transcribe RNA into DNA. These RNA polymerase enzymes are more likely to make copying errors than DNA polymerases and, therefore, often make mistakes during transcription. For this reason, mutations in RNA viruses occur more frequently than in DNA viruses. This causes them to change and adapt more rapidly to their host. Human diseases caused by RNA viruses include COVID-19, HIV (AIDS), hepatitis C, and measles.
- Virus structure always contains nucleic acid and protein.
- There is a great diversity of virus shape: filamentous, isometric (or icosahedral), enveloped, and head and tail.
- Some viruses are covered by an envelope of phospholipid bilayer and protein.
- Viruses mutate and evolve quickly, especially RNA viruses.
- Non-enveloped viruses can be more resistant to changes in temperature, pH, and some disinfectants
- The virus core contains the small single-stranded or double-stranded genome or DNA or RNA that encodes the proteins that the virus cannot get from the host cell.