Coronaviruses, an orientation:
By Alfred Sibanda T.
What causes diseases?
Before we talk about coronavirus, what causes diseases in general? Well there are five classes of disease causing agents and these are: fungi, protozoa, bacteria, viruses and prions. Fungi and some protozoa can be seen by the naked eye, whereas the other three cannot, because they are very small. So how do we know these exist when we cannot see them? Scientists have come up with methods and tools to investigate natural phenomena, and one of these, the microscope, is the tool they use to view objects that are too small to be seen by the naked eye, and such objects include viruses, bacteria and protozoa.
The world is currently experiencing a pandemic, COVID-19, caused by one member of a family of viruses called the Coronaviridae and we shall be talking about this virus. From its structure, how it infects us, the symptoms of infection, the epidemiology, treatment and many more!
Just how small is coronavirus?
To put into perspective just how small coronavirus is, we should start from something we can all relate to. We have all seen a meter rule, it’s about half the height of a common door. Now, take a thousandth of that height, see how small it gets? We are now at the millimetre scale, yet that is not even close to the size of the virus. Now imagine shrinking that by a factor of a thousand again! We are now at the micrometer scale, and that is in total a millionth of the meter rule, is this how small coronavirus is? No! Shrink this by another factor of eight, and now we finally reach the size of the virus! This is of course unimaginably small, you still wouldn’t see a clump of, say a million coronaviruses.
|Unit of measurement||Fraction of 1 meter|
Size of coronavirus: 125nanometers diameter.
Discovery of viruses.
Many centuries ago, the cause of disease was not entirely known but in many communities, religious explanations were the prevailing ones, people saw sickness as punishment from the gods. In the 1800s, a scientist named Louis Pasteur proposed the germ theory of disease, here he postulated that diseases were caused by very small entities, indeed too small to be seen by the naked eye! Today we know just how right Louis Pasteur was. Pasteur’s explanation of disease however stumbled upon a rock, he couldn’t find the agent that was causing rabies, so he just speculated that rabies was caused by an organism so small that even the microscope could not sufficiently enlarge it into view. Today, with the electron microscope, viruses can be resolved and magnified sufficiently to be observed, proving Louis Pasteur right again. Later, in 1884, a microbiologist from France named Charles Chamberland invented a filter with pores smaller than bacteria, meaning that these pores could filter bacteria such that if a solution containing bacteria is passed through this filter, the bacteria are retained and the solution that comes out the other side (filtrate) is bacteria-free. What this means is that if we took an infected animal extract with bacteria and passed it through the filter, and then took the resulting solution and inoculated it into a healthy animal, it wouldn’t catch the disease from which the original animal suffered. In 1892, a Russian born scientist named Dmitry Ivanovsky used one of the filters invented by Chamberland to filter a crushed sample of a tobacco plant that had been infected by the tobacco mosaic virus (of course this exact phrase wasn’t used then). The resulting solution was seen to still be infectious! This meant that the organism causing the disease was very small, smaller than bacteria! This organism is today known as the virus. The details of many more viruses have since been elucidated, including coronaviruses. The first coronavirus to infect humans was discovered in the 1960s.
Structure of the virus
Viruses take many different shapes and forms, most are basically made up of a central core that contains nucleic acids and a covering. The nucleic acid in a virus is either RNA (Ribonucleic acid) or DNA (Deoxyribonucleic acid) but never both. Normally, RNA is known to exist as a single strand and DNA is known to exist mostly as a double stranded helix, but in viruses, RNA can exist as a double strand and DNA as a single strand, a feature that has not been observed in living organisms. This nucleic acid is the one carrying all the information necessary for the virus to be able to perform all its functions, including binding and gaining entry into living organisms, and multiplying to produce even more viruses. Living cells have information that only allows copying DNA into RNA in order to make proteins, so DNA viruses just make use of this machinery to copy their DNA and reproduce, but what of RNA viruses? RNA in viruses comes in two forms, it can be positive sense RNA or negative sense RNA. Positive sense RNA is RNA that has the same polarity as normal cellular RNA and hence can be directly translated into viral proteins. Negative sense RNA however is of opposite polarity to normal cellular RNA and thus cannot be directly translated into viral proteins, as such, viruses with this kind of RNA come with their own enzymes that are able to copy this negative sense RNA and convert it into positive sense RNA which can be translated by the cell into viral proteins.
Surrounding the nucleic acid is a protein coat called a capsid. Capsids take two general forms, they can either be helical capsids or icosahedral capsids. These capsids protect the nucleic acid, as it is the material that carries all the information required for reproduction of the virus. Some viruses go on to have another covering, the envelope. The envelope is a lipoprotein complex and if present, it carries the glycoproteins that aid the virus to gain entry into the cells the virus infects.
This was a brief and general description of the structure of viruses, which of these components are actually specific to coronaviruses? Well, coronaviruses are RNA viruses with single stranded positive sense RNA, and this means that once they get into the cell, their RNA can be translated to give early viral proteins that are later used to aid in making even more viral proteins, eventually leading to replication of the virus.
Next the coronavirus then has a coiled helical capsid protecting the RNA. Enclosing everything is the viral envelope with the characteristic crown shape that gave rise to the name of the virus (corona- crown). The spike proteins project as the crown appendages. These, as we shall later see, are quite essential for the virus to gain entry into the host cells. The envelope of the virus is actually derived from the host itself! After production of viral components inside the cell, the virus buds off the endoplasmic reticulum, carrying with it the membrane of the endoplasmic reticulum and it is this membrane that makes the envelope. The guts!
The dryer balls have an appearance pretty much similar to that of coronaviruses.
The table below summarises the key components of the coronaviruses and their roles.
|Viral envelope||– Helps virus bind to the host cell in order to infect
– Helps the virus avoid immune system
– Protects the virus contents
|Viral capsid||– Protects the viral genome
– Pathogenicity determination
– Viral self-assembly
|RNA||– Contains information required for synthesis of the entire virus in the host.|
Correlating structure with function in the viral capsid Agbandje-McKenna, MAVIS Chapman, MICHAEL S ,2006 Hodder Arnold, UK.
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