Wendy has taught high school Biology and has a master's degree in education.
A Telltale Rash
When we get sick, it can sometimes be hard to tell exactly what we have. Is it caused by bacteria or a virus? Is it a common cold or an infection? We often head to the doctor for answers. However, for generations there was one childhood illness that was easily identifiable by parents everywhere. Its calling card was the rash of red spots that suddenly appeared, quickly spreading all over the body as well as to the rest of the kids in the house. The miserable itching, blistering and scabbing of the skin could only mean one thing: Chickenpox had arrived.
Chickenpox, is caused by the varicella zoster virus, or VZV for short. For decades, this illness was a very normal part of childhood. However, with the arrival of its vaccine in 1995, it has become much more rare. In this lesson, we'll zoom in on this minuscule but powerful organism and learn more about its structure and function.
Varicella Zoster Virus
Let's first look at characteristics of viruses in general. These tiny, alien-like creatures are composed of a protein shell with genetic information inside. A virus is an extremely small infection agent that hijacks cells of living organisms in order to replicate themselves and spread.
Viruses are classified according to their chemical, physical and morphological properties. Based on this classification, the varicella zoster virus is part of the herpesvirus family. You may have experienced cold sores, which are also caused by the herpesvirus, but a different type. Specifically, VZV is known as human herpesvirus 3. VZV is very specific in the organisms that it targets, as it only infects human cells.
Let's take a closer look at the structure of the VZV. It is composed of four main parts: the envelope, tegument, capsid, and core. The envelope is the outer covering of the virus. It is made of a lipid bilayer, studded with spiky glycoproteins. These glycoproteins are there to bind with the cell receptors of the host cell, allowing the virus a gateway into its host.
The tegument is the next layer inside the envelope, and it is composed of another layer of viral proteins. Next we find the protein shell mentioned earlier, called the nucleocapsid. Like the nucleus of a living cell, the nucleocapsid contains the very important DNA. Think of a capsule in which important items are stored. And the shape of this shell is most extraordinary. As if straight out of a geometry textbook, it is a twenty-faced icosahedron.
Tucked within the nucleocapsid is the core, which is the lifeline for the virus. Genetic information in the form of a linear double strand of DNA is safely encapsulated, only to be released when infecting a host cell. Of all the types of herpesvirus, VZV contains the smallest genome.
Infecting a Host Cell
You may be wondering exactly how the varicella zoster virus infects its host. VZV is transmitted by droplets, meaning that it can be transmitted if an infected person shares their food or drink. Coughing and sneezing into someone's face also helps to pass the virus along.
Once in, the virus goes to work in the upper respiratory tract of its host. The process goes like this:
- First, it uses those spiky proteins to bind to cell receptors on the host cells.
- The envelope fuses to the host cell membrane.
- The capsid with the genetic information and the tegument proteins are now released into the doomed host cell.
- The capsid takes its precious cargo and heads for the nucleus of the host cell.
- The virus releases its own DNA and commandeers the cell machinery into replicating viral DNA instead of its own.
- Fresh new viruses are assembled and are released to infect more host cells.
When all is finished, the host cell is dead, and the virus has successfully spread. The infected individual must now endure the symptoms of chickenpox, which include headache, fever and the infamous rash.
Once the bout of the chickenpox has ended, a person will typically never get it again. However, this virus is not necessarily finished. It may not be causing infection anymore, but the virus essentially goes into hibernation within the body. In a latent state, the VZV continues to reside in the nerve ganglia of the body. Like the chilling epilogue of a suspense novel, it can once again come to life in later years as herpes zoster, commonly known as shingles, a painful skin condition. Fortunately, a vaccine now exists for this as well.
Chickenpox is a condition characterized by a rash of red spots and an itching, blistering and scabbing of the skin. It is caused by the varicella zoster virus, or VZV. Also called human herpesvirus 3, it only infects humans. VZV has four main parts: the envelope, tegument, nucleocapsid and core. The envelope is a lipid bilayer with proteins that allow it to bind to host cells. The tegument contains proteins as well. The nucleocapsid is a protein shell and contains within it the DNA, which is the core. VZV travels to new hosts via droplets. It binds to the host cell and releases its DNA, using cell machinery to replicate itself. VZV goes dormant in the body and can emerge in the form of shingles, a painful skin condition, later in life.
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