Author: Holly M.
Third Year Medical Student
What is a virus?
Viruses are the smallest type of microbe. They are much smaller than the cells found in our bodies. Unlike cells in our body, and bacterial cells, viruses are only able to replicate inside other organisms’ cells, called host cells. This means that they are not considered to be living organisms. Scientists estimate that there are over 1031 viruses in existence at any given moment, and that all types of organisms (including bacteria) are home to at least 1 type of virus. Like other types of organisms, viruses are able to evolve and have genetic variation.
When outside of a host cell, the virus is covered by a capsid.
A capsid is a protein cover for a virus. It is made up of capsomers which are proteins grouped together. The capsid itself contains genetic information, either in the form of DNA or RNA. Capsids can come in a variety of different shapes, including filamentous, tail-head and icosahedral. Icosahedral capsids are 20 faced. Filamentous capsids are long and thin, and may also be called helical or rod-shaped. Tail-head capsids are capsids made up of an icosahedral part and a rod shaped part.
The capsid can also be covered by an additional external membrane envelope, although this is not a universal feature. Sometimes there are glycoproteins attached to this external membrane envelope and these can help the virus to bind to host cells. The envelope is made up of lipids. The virus RNA / DNA does not contain the instructions to make this envelope, so it is taken from the host cell when the virus leaves it.
As previously covered, the genetic material found in a virus can be in the form of RNA or DNA. This will vary from virus type to virus type, and can be found in different formats, such as double stranded DNA, double stranded RNA, single stranded RNA and single stranded DNA. If the virus genome is in a different format to that of it’s host cell, then enzymes (such as reverse transcriptase) found within the capsid are used to transform the genetic material so it matches.
When a virus comes into contact with a host cell, it will recognise it and bind to it using a receptor on the host cell, this is called attachment. The virus inserts its genetic material into the host’s genetic material and uses the host cell’s machinery to create more copies of itself. Eventually, the copies of the virus will burst out of the host cell, causing it to die. The copies of the virus can then go on to infect more host cells. This state of virus reproduction is called the ‘lytic phase’. Sometimes, a virus can lie dormant (inactive) within a host cell for a long period of time without replicating, this is known as the ‘lysogenic phase’.
What are examples of common viruses?
There are lots of examples of viruses that you will be familiar with. In humans, viruses are the cause of influenza, smallpox, chicken pox, herpes, shingles, ebola and HIV (which can develop into AIDS).
HIV infects a specific type of cell in the body called CD4 cells. This is a type of T cell. T cells are a type of cell that are involved in the immune system, so are involved in protecting the body and helping it to fight infections. As more and more CD4 cells are destroyed, the body’s immune system stops being able to work as effectively, this means that the person is more likely to become unwell. Eventually, opportunistic pathogens or cancers that people with a normal immune system would be able to fight off can infect the person. This is then when the person is said to have AIDS.
What treatments are there for viruses?
Drugs used to treat viruses are called anti-viral medications. Antibiotics don’t work against viruses, as they are designed to target bacterial cell features that viruses don’t have. Antibiotics can normally be used to treat a range of different bacteria, however anti-viral medications are normally only used to treat one specific type of virus. Similarly to bacteria, viruses can also mutate over time and so can become resistant to anti-viral medications.
There are anti-virals for common viruses such as hepatitis B and C, influenza, herpes and HIV. Anti-viral drugs work by inhibiting viral enzymes, so that the virus cannot replicate itself using the host cell machinery. If the virus is unable to replicate, it means that it cannot infect the host.
What is COVID-19?
COVID-19 is a strain of virus that has been the cause of the current pandemic (2020). It belongs to a group of viruses called Betacoronaviruses. Other viruses in this group include the SARSr-CoV-2 (severe acute respiratory syndrome related coronavirus), which caused the SARS outbreak.
In terms of structure, COVID-19 is spherical. Its envelope contains spike glycoproteins and an enzyme called hemagglutinin esterase. Hemagglutinin esterase helps COVID-19 to enter host cells by destroying receptors (sialic acid receptors) found on host cells. Influenza C viruses also have hemagglutinin esterase enzymes on their surfaces. COVID-19 enters host cells via the ACE2 receptor found on host cells. ACE2 (angiotensin converting enzyme 2) is found on the cell membrane of lungs, arteries, heart, kidneys and intestines in humans, it helps to control our blood pressure.
The genetic information in the virus is stored as single stranded RNA. Coronaviruses have the largest genomes out of all the RNA viruses we have ever discovered. This RNA is associated with a nucleoprotein inside a capsid. Also found inside the capsid are several enzymes, including RNA polymerase and RNA helicase, which are commonly found in RNA viruses. However, COVID-19 also contains some enzymes that are much rarer, including putative sequence-specific endoribonuclease, 3′-to-5′ exoribonuclease, 2′-O-ribose methyltransferase and ADP ribose 1′-phosphatase.
It’s important for scientists to have a good understanding of the structure of COVID-19 so that they can develop treatments that will specifically target it.
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These are some of the resources I used to write this article. If you have enjoyed learning more about viruses, these might be a good place to start with some further reading. Remember to write any reflections you have in a journal, so you can use them in your personal statement or interviews.
Genotype and Phenotype of COVID19: their roles in pathogenesis Leila Mousavizadeh Sorayya Ghasemi