Unit 3 Notes
The Viruses
Discovery and Structure of Viruses
At the beginning of the 1900’s, the word “virus” was being used in science to refer to any unseen or unknown
entity involved with infectious disease, and came from the word for “poison”. But viruses had not been
discovered yet, because no microscope was powerful enough to see them yet. Viruses are obligate
intracellular parasites, meaning that they cannot reproduce on their own; rather, they require a host cell to
multiply.
One of the first scientists to study viruses in depth was Dmitri Iwanowski, who was interested in tobacco
mosaic virus (TMV), which was a disease affecting tobacco plants in 1892. He figured out that the organism
that causes TMV was so small that it went right through a filter he used to trap the “bacteria”. In the 1930’s, a
scientist named Wendell Stanley found that you could form crystals of TMV, which meant that it was some sort
of chemical molecule causing the disease. But in 1933, the electron microscope was invented and by 1941,
scientists were finally able to see viruses.
The Structure of Viruses
Viruses are among the smallest microbes that cause disease in humans. In fact, 500 or more viruses could fit
into a single bacterial cell! Scientists have discovered that viruses come in many shapes and sizes. Look at the
figure below showing sizes:
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Now look at the figure below with just a few representative shapes of viruses:
The Components of Viruses
Viruses are very different than plant and animal cells. All viruses consist of a core of nucleic acid with a
protein coat. The nucleic acid core can consist of either DNA or RNA, but not both. The nucleic acid can exist
as a single molecule (see measles virus above), or in segments (see influenza virus above). Here are some parts
of the virus worth mentioning:
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Capsid – the outer protein coat
Capsomere – smaller protein units that are bound together chemically to form the capsid
Genome – the nucleic acid core of the virus
Nucleocapsid – the combination of genome and capsid
Envelope – some viruses have an enclosing structure similar to the cell membrane
Spikes – projections on the envelope that help the virus contact its host cell
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Viral Classification
Viruses are overall classified on the basis of whether they contain DNA or RNA in their core. They can be
further classified by either plant, animal, or bacterial according to which type of organism they infect. In the
laboratory, they can also be classified according to the organ or organ system they infect. See chart below:
Category
Dermotropic
Tissue Affected
Skin and subcutaneous tissue
Neurotropic
Brain and central nervous
system tissue
Internal organs
Viscerotropic
Pneumotropic
Lungs and other respiratory
structures
Example Diseases
Chicken pox, shingles, measles, mumps,
smallpox, rubella, herpes simplex
Rabies, West Nile virus, polio
Yellow fever, AIDS, Hepatitis A and B, Mono,
Dengue fever
Influenza, common cold
Viral Replication
Viruses are not generally considered to be alive. They do not take in nutrients, they do not produce waste
products. They do not grow in size, and they have no metabolism. But there is one thing viruses do well – they
replicate! There are two distinct ways that viruses
replicate:
I. Lytic Phase – this is when viruses replicate in a
host cell, and cause the host cell to “lyse” or burst.
When it bursts, it releases the “baby viruses”. Here
are the stages of the lytic cycle:
1. Attachment stage – viruses will attach to and
replicate within specific cells only. For
example, hepatitis viruses replicate only in
liver cells. This is because there are proteins
in the capsid surface of the virus, which only
“recognizes” certain host cells. These are
called receptor sites, kind of like a lock and
key mechanism. When the two sites meet, the
cell membrane opens to the virus and it enters.
2. Penetration stage – When the virus enters the
host cell.
3. Uncoating stage – Once inside a host cell, the
enzymes of the cell cytoplasm remove the
capsid from the viral genome. The nucleic
acid is released.
4. Synthesis stage – This occurs in one of two
ways, depending on whether the virus
contains DNA or RNA. Just like protein
synthesis in our cells.
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5. Assembly stage – this is when all of the necessary building blocks are produced through protein
synthesis, they are combined to form new viral particles.
6. Release stage – as the virus kills the host cell, after the cell dies, new viruses are released into the body.
II. Lysogenic Phase – sometimes, a virus
enters a host cell and does not replicate
immediately. The name for this viral cycle is the
lysogenic cycle. During this cycle, the virus
incorporates its genes into the host cell’s genome
and becomes a part of the cell. It then remains
inside the host cell and multiplies when the
host cell multiplies. AIDS is an example of this
type. It can live in the host cell for many years
before the person actually contracts the disease.
Host Cell Damage
In many cases, when a virus infect a cell, it causes cell death or at least cell damage. The specific imprint that
the virus leaves is called the cytopathic effect (CPE). By observing the effects under the microscope, scientists
can sometimes identify the type of viral infection. Many viruses have a characteristic CPE.
Defense Against Viruses
As you know, some viral disease can kill us, such as AIDS, while others do not kill us. Some examples of
viruses that do not kill us are measles, chickenpox, herpes simplex, mononucleosis, and many others. So our
bodies must be defending itself against them, right?
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It is our immune system that defends our bodies against viruses. Specifically our antibodies. Antibodies are
protein molecules synthesized by the immune system in response to the presence of viruses. When the
antibodies are produced, they “attack and kill” the virus particles. Beyond this, there are some drugs that can be
used against viruses, which inhibit replication. However, these drugs cannot actually “kill” the virus, just slow
it down. There are 3 types of viral vaccines:
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Inactivated vaccines – those, such as the polio vaccine, that are made from real inactivated virus
particles. This is where the viral genome is destroyed, but the capsid is intact, so it stimulates our
bodies to produce antibodies against the virus.
Attenuated vaccines - these are vaccines made from “live” viruses (complete viruses), but in such
low doses that they do not cause disease. These are more effective than the above; however, they
also carry the slight risk of contracting the disease. Examples of these are chickenpox vaccine, and
measles, mumps and rubella vaccines.
Genetically engineered vaccine – in this case, viral proteins are produced by yeast cells that have
been altered to express the genes from a specific virus. The proteins are used in the vaccine. An
example of this is the hepatitis B vaccine. Very safe because no actual virus particles are used.
How Are Viruses Grown in the Lab?
Because viruses must have a live host cell in order to replicate, they are sometimes a challenge to grow in the
lab. Viruses can be cultured in the lab in the following ways:
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Inside a live animal
Inside a fertilized chicken egg
Viral cell culture, where scientists can grow them in a test tube with special nutrients
Viroids and Prions
A viroid is an ultramicroscopic, single-stranded molecule of RNA without any protein coat. They infect plants,
and cause stunted growth and abnormal development. A prion is a proteinaceous infectious particle (a single
viral piece of protein with no capsid). This is the cause of mad cow disease. These can survive heat, radiation,
and chemical treatment that normally inactivates viruses.
Viruses and Cancer
Scientists know that cancer is when normal cells begin to multiply without control. Viruses are considered to
be carcinogens (cancer-causing agents). Examples of such viruses are:
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Herpesviruses – believed to cause psychosis as you age, as well as Kaposi’s sarcoma, a form of cancer
mostly found in AIDS patients.
Epstein-Barr viruses – believed to increase the risk of getting multiple sclerosis and Burkitt’s
lymphoma, a form of cancer
Papillomavirus - which causes genital warts, is believed to be a leading cause of cervical cancer in
women.
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