1. Virus
INTRODUCTION
Viruses are tiny organisms that may lead to mild to severe illnesses in humans, animals
and plants. Ex- flu or a cold to something more life threatening like HIV/AIDS.
OBJECTIVES
How big are viruses?
Are viruses alive?
Structure of a virus
Receptors
How do viruses infect?
Life cycle of a basic virus
2. Size of viruses
The virus particles are 100 times smaller than a single bacteria cell. The bacterial cell
alone is more than 10 times smaller than a human cell and a human cell is 10 times
smaller than the diameter of a single human hair.
Are viruses alive?
Viruses by themselves are not alive. They cannot grow or multiply on their own and need
to enter a human or animal cell and take over the cell to help them multiply. These
viruses may also infect bacterial cells.
The virus particle or the virions attack the cell and take over its machinery to carry out
their own life processes of multiplication and growth. An infected cell will produce viral
particles instead of its usual products.
Structure of a virus
A virion (virus particle) has three main parts:
Nucleic acid – this is the core of the virus with the DNA or RNA
(deoxyribonucleic acid and ribonucleic acid respectively). The DNA or RNA
holds all of the information for the virus and that makes it unique and helps it
multiply.
Protein Coat (capsid) – This is covering over the nucleic acid that protects it.
Lipid membrane (envelope) – this covers the capsid. Many viruses do not have this
envelope and are called naked viruses.
Receptors
Viruses are not simply taken into cells. They must first attach to a receptor on the cell
surface. Each virus has its specific receptor, usually a vital component of the cell surface.
It is the distribution of these receptor molecules on host cells that determines the cell-
3. preference of viruses. For example, the cold and flu virus prefers the mucus lining cells
of the lungs and the airways.
How do viruses infect?
Viruses do not have the chemical machinery needed to survive on their own. They, thus
seek out host cells in which they can multiply. These viruses enter the body from the
environment or other individuals from soil to water to air via nose, mouth, or any breaks
in the skin and seek a cell to infect.
A cold or flu virus for example will target cells that line the respiratory (i.e. the lungs) or
digestive (i.e. the stomach) tracts. The HIV (human immunodeficiency virus) that causes
AIDS attacks the T-cells (a type of white blood cell that fights infection and disease) of
the immune system.
Video
Life cycle of a basic virus
There are a few basic steps that all infecting viruses follow and these are
called the lytic cycle. These include:
1.A virus particle attaches to a host cell. This is called the process of
adsorption
2.The particle injects its DNA or RNA into the host cell called entry.
3.The invading DNA or RNA takes over the cell and recruits the host’s
enzymes
4.The cellular enzymes start making new virus particles called
replication
5.The particles of the virus created by the cell come together to form
new viruses. This is called assembly
6.The newly formed viruses kill the cell so that they may break free and
search for a new host cell. This is called release.
Viral life cycle
Viruses are similar to living organisms, however there are differences. One of the ways a
virus can be seen as living is that a virus needs to replicate and create progeny. However,
unlike organisms, a virus cannot survive on its own. It is only active when replicating
within a host, using a hosts' resources and food. Once inside a host, a virus's sole purpose
is to make as many copies of itself, and infect other host cells; everything it does is to
benefit its fitness and increase the number of its offspring.
4. Objectives
1 Overview
2 Exposure of host
3 Viral Entry
4 Viral replication
5 Viral shedding
6 Viral latency
Overview
A virus is totally dependent on a host cell.[1] Most viruses are species specific, and they
only infect a narrow range of plants, animals, bacteria or fungi.
Exposure of host
Usually viral infection occurs when a virus enters the host, either:
through a physical breach (a cut in the skin)
direct inoculation (e.g.mosquito bite[2])
direct infection of the surface itself (inhalation of the virus into trachea[3])
It is usually only after a virus enters a host that it can gain access to possible susceptible
cells.
Viral Entry
Main article: Viral entry
For the virus to reproduce and thereby establish infection, it must enter cells of the host
organism and use those cells' materials. To enter the cells, proteins on the surface of the
virus interact with proteins of the cell. Attachment, or adsorption, occurs between the
viral particle and the host cell membrane. A hole forms in the cell membrane, then the
virus particle or its genetic contents are released into the host cell, where viral
reproduction may commence.
Viral replication
Main article: Viral replication
Next, a virus must take control of the host cell's replication mechanisms. It is at this stage
a distinction between susceptibility and permissibility of a host cell is made.
Permissibility determines the outcome of the infection. After control is established and
the environment is set for the virus to begin making copies of itself, replication occurs
quickly.
5. Viral shedding
Main article: Viral shedding
After a virus has made many copies of itself, it usually has exhausted the cell of its
resources. The host cell is now no longer useful to the virus, therefore the cell often dies
and the newly produced viruses must find a new host. The process by which virus
progeny are released to find new hosts, is called shedding. This is the final stage in the
viral life cycle.
Viral latency
Main article: Viral latency
Some viruses can "hide" within a cell, either to evade the host cell defenses or immune
system, or simply because it is not in the best interest of the virus to continually replicate.
This hiding is deemed latency. During this time, the virus does not produce any progeny,
it remains inactive until external stimuli—such as light or stress—prompts it to activate.
Two Life Cycles of a Virus
Lytic versus lysogenic life cycles:
In the lytic stage, many viral particles are made and copies are sent back into the
environment.
A virus is found in this phase when conditions are favorable, i.e. when bacteria is
"growing like crazy"
6. The virus attaches to bacteria (host)
The virus inserts its DNA into the
bacteria
The virus takes over the cell's machinery
The virus reproduces itself and self-
assembles.
The host cell is destroyed
In the lysogenic phase there is no pathology.
Under certain conditions the lysogenic lifestyle can switch to a lytic lifestyle.
A virus is found at this stage under harsh conditions.
7. The virus is a prophage at this stage.
The virus binds to bacteria (host)
The virus inserts its DNA into the
bacteria
The viral DNA gets incorporated into the
cell's chromosome
Viral DNA is replicated
along with chromosomal material
