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Author: Jason Lavender, Duke University
Editor: Wei-Chung Chen
©SCICOM MIT
Viruses: One of Human s Strongest Enemies
What are viruses? Viruses are responsible for causing the person to become sick
due to a cold or flu. On a larger scale, viruses are also responsible for causing many
deadly diseases and this include AIDS (acquired immunodeficiency syndrome), Ebola
hemorrhagic fever, infectious hepatitis and herpes.
Compared to bacteria and normal cells, virus particles are nothing alike. Virus
particles are about one-millionth of an inch (17 to 300 nanometers) long. Viruses are
about a thousand times smaller than bacteria, and bacteria are much smaller than most
human cells. Viruses are so small that most cannot be seen with a light microscope, but
must be observed with an electron microscope. Viruses are made of three major
components. First components are nucleic acids, which include DNA and RNA and they
provide a set of genetic instructions for future viral reproduction. Secondly, there are
protein coatings that protect the nucleic acids. Lastly, there are lipid coatings that
surround the protein coatings, but this is not present in all viruses. Viruses with lipid
coatings are called enveloped viruses as opposed to naked viruses. Shape of the virus
varies greatly ranging from round popcorn balls to a spider or the Apollo lunar lander.
One thing that greatly distinguishes viruses from bacteria and cells is that they do
not carry enzymes needed to carry out the chemical reactions for life. Instead, they carry
only one or two enzymes that decode their genetic instructions. Thus, viruses depend on
host cells for viral production and nutrient. In this case, host cells include bacteria, plant
cells, and animal cells. Outside of a host cell, viruses cannot function. For this reason,
viruses are sandwiched in between living things from nonliving things. Most scientists
agree that viruses are alive because of what happens when they infect a host cell.
Viruses are everywhere around people, and waiting for the opportunity to infect
host cells that would provide the necessary nutrients for survival. Since they possess
small size, they can enter into our body through the nose, mouth or breaks in the skin.
Once they enter inside the body, they would find a particular host cell to infect.
Specifically, cold viruses will attack cells that line the respiratory tract or digestive tract.
In addition, HIV (human immunodeficiency virus), which causes AIDS, targets white
blood cells such as T-cells that protect the body from foreign particles.
After they enter into the body, viruses follow a particular cycle for the
reproduction of new viruses. This cycle is called the lytic cycle. First, the virus particle
attaches itself to the host cell. Second, the particle releases its DNA or RNA into the host
cell. The injected genetic instruction recruits the necessary enzymes inside host cell.
These enzymes will be responsible for making different components of a complete virus.
After these parts are completed, they are assembled together to form new virus particles.
Finally, these new viral particles would break out of the host cells and invade other host
cells. There are two ways that the viruses break out of the host cell. First, they simply kill
the host cell by breaking open the host cell. The second way is by pinching out from the
cell membrane and break away (budding) with a piece of the cell membrane surrounding
them. This is how enveloped viruses leave the cell. In this way, the host cell is not
destroyed.
During the viral attack, the human body responds to the infection by producing
chemicals called pyrogens that cause the body temperature to rise. This rise in
temperature helps to fight the infection by slowing down the rate of viral reproduction.
The rationale is that chemical reactions in the human body operate at an optimal
temperature of 98.6 degrees Fahrenheit (37 degrees Celsius). If your temperature rises
slightly above this, the reactions slow down. This immune response continues until the
viruses are eliminated from your body.
While some viruses go through the lytic cycle for generating new viruses, there
are viruses that undergo a different type of cycle. This cycle is called the lysogenic cycle.
In this cycle, some viruses, such as herpes and HIV, do not reproduce right away. Instead,
they mix their genetic instructions into the host cell's genetic instructions. When the host
cell reproduces, the viral genetic instructions get copied into the host cell's offspring. The
host cells may undergo many rounds of reproduction, and then some environmental or
predetermined genetic signal will stir the "sleeping" viral instructions. The viral genetic
instructions will then take over the host's machinery and make new viruses.
To reduce the spread of viruses, there are several precautions that people can take.
First, people can cover their nose or mouth when they sneeze. Another methods include
washing your hands after you leave the restroom. Lastly, it is important to avoid contact
with the bodily fluids of others.