Unit 3 NotesThe 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:

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 Tissue Affected Example DiseasesDermotropic Skin and subcutaneous tissue Chicken pox, shingles, measles, mumps,

smallpox, rubella, herpes simplexNeurotropic Brain and central nervous

system tissueRabies, West Nile virus, polio

Viscerotropic Internal organs Yellow fever, AIDS, Hepatitis A and B, Mono, Dengue fever

Pneumotropic Lungs and other respiratory structures

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:

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:

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:

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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