VIRUS
Definition
A virus is defined as an infectious agent of small size and simple composition that can multiply only in living cells of animals, plants or bacteria. Accordingly, the term ‘virus’ originated from a Latin word that means ‘poison’.
Characteristics
In terms of size, viruses are known as the smallest in the world of microbes. In comparison with a virus, bacteria are considered giants. For example, bacterial cells typically range between 200 -1000 nm (nanometre) while viruses typically range between 20-250 nm in diameter. Bacteria can be seen under light microscope easily but viruses are a thousand times smaller than bacteria that they need a special microscope called an electron microscope to be visible to the human eyes.
Figure 1: The Human Immunodeficiency Virus (HIV)
The structure of a virus varies from one to another but viruses usually contain either an RNA or DNA genome surrounded by a protective, virus-coded protein coat. The viral genome that is associated with basic proteins is packaged inside a symmetric protein capsid. The nucleic acid-associated protein, called nucleoprotein, together with the genome forms the nucleocapsid. In enveloped viruses, the nucleocapsid is surrounded by a lipid bilayer derived from the modified host cell membrane and studded with an outer layer of virus envelope glycoproteins. A complete virus particle is called a virion which functions to deliver its genetic material into the host cell to be expressed. Below are examples of commonly known viruses.
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Other than that, some viruses are ‘enveloped’ in the cell membrane of their host. The membrane may be the outer membrane or the inner membrane of host cell such as the nuclear membrane or endoplasmic reticulum. This makes the virus gain an outer lipid bilayer known as ‘viral envelope’. The lipid bilayer is studded with proteins of both the viral genome and the host genome. The envelope gives the virus some advantages over the unenveloped virus. For example, they have better protection from host’s immune system, enzymes and some chemicals. Other than that, the envelope with protein receptors also helps viruses binds to host cell easily. However, since viruses are made up of lipids, they are susceptible to inactivation by environmental factors such as detergent.
Classification
Viruses were initially grouped according to their shared morphology. For example, viruses are grouped based on the shape of their capsids which are either icosahedral, helical or complex. Figure 1 above shows the HIV as an icosahedral virus, the TMV as helical and a common complex shaped bacteriophage that is neither icosahedral nor helical in shape. However, with the advancement in technology, scientists were able to later group viruses based on the type of nucleic acid they contain (DNA or RNA) and whether their nucleic acid was single or double stranded. More recently, viruses are further classified based on the molecular analysis of their viral replicative cycles. Below is a flow chart on the classification of viruses.
Origin of Virus
The origin of virus is still unclear. However, scientists have come up with three theories on the origin of virus. The first theory, the regressive hypothesis, suggests that viruses may had come from cells that parasitized larger cells. The cellular origin hypothesis on the other hand suggests that viruses may have evolved from bits of DNA or RNA that escaped from the genes from a larger organism. Last but not least, the co-evolution hypothesis suggests that viruses may have evolved from complex molecules of protein and nucleic acid at the same time that cells first appeared on Earth and has been dependent on living cells since and remain unknown due to their submicroscopic size.
Virus in Human History
In known human history, several pandemics has been recorded. Some of these pandemics were caused by viruses. At the beginning of the 20th century, the deadly Spanish flu caused by H1N1 influenza A virus resulted in the death of 500 million people. The years that follows shows the emergence of other lethal viral pathogens such as the ebolavirus and Marburgvirus that results in outbreaks in certain parts of the world. Currently, the whole world is facing a pandemic caused by a novel coronavirus known as COVID-19.