- Describe the structure of viruses.
- Outline the discovery and origins of viruses.
- Explain how viruses replicate.
- Explain how viruses cause human disease.
- Describe how viruses can be controlled.
- Identify how viruses are used in research and medicine.
protein coat that surrounds the DNA or RNA of a virus particle
period of dormancy of a virus inside a living body that may last for many years
substance containing modified pathogens that does not cause disease but provokes an immune response and results in immunity to the pathogen
individual virus particle that consists of nucleic acid within a protein capsid
At the end of the last lesson, you were asked which of the three domains of life do viruses belong to. Did you figure it out? None. Why? Viruses are usually considered to be nonliving. Viruses do not meet most of the criteria of life. They are not even cells.
An overview of viruses can be seen at http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/17/0h5Jd7sgQWY.
Characteristics of Viruses
An individual virus is called a virion. It is a tiny particle much smaller than a prokaryotic cell. Because viruses do not consist of cells, they also lack cell membranes, cytoplasm, ribosomes, and other cell organelles. Without these structures, they are unable to make proteins or even reproduce on their own. Instead, they must depend on a host cell to synthesize their proteins and to make copies of themselves. Viruses infect and live inside the cells of living organisms. When viruses infect the cells of their host, they may cause disease. For example, viruses cause AIDS, influenza (flu), chicken pox, and the common cold.
Although viruses are not classified as living things, they share two important traits with living things. They have genetic material, and they can evolve. This is why the classification of viruses has been controversial. It calls into question just what it means to be alive. What do you think? How would you classify viruses?
Structure and Classification of Viruses
Viruses vary in their structure. The structure of a virus is one trait that determines how it is classified.
Structure of Viruses
A virus particle consists of DNA or RNA within a protective protein coat called a capsid. The shape of the capsid may vary from one type of virus to another, as shown in Figure below.
Capsid Shapes in Viruses. Three shapes of viral capsids are shown here. They are helical (spiral), icosahedral (20-sided), and complex. Viruses with complex shapes may have extra structures such as protein tails.
Some viruses have an envelope of phospholipids and proteins. The envelope is made from portions of the host’s cell membrane. It surrounds the capsid and helps protect the virus from the host’s immune system. The envelope may also have receptor molecules that can bind with host cells. They make it easier for the virus to infect the cells.
Classification of Viruses
Viruses are classified on the basis of several traits. For example, they may be classified by capsid shape, presence or absence of an envelope, and type of nucleic acid. Most systems of classifying viruses identify at least 20 virus families. Table below shows four examples of virus families and their traits. Have any of these viruses made you sick?
Type of Nucleic Acid
Disease Caused by a Virus in this Family
acute respiratory disease
Discovery and Origin of Viruses
Viruses are so small that they can be seen only with an electron microscope. Before electron microscopes were invented, scientists knew viruses must exist. How did they know? They had demonstrated that particles smaller than bacteria cause disease.
Discovery of Viruses
Researchers used special filters to remove bacteria from tissues that were infected. If bacteria were causing the infection, the filtered tissues should no longer be able to make other organisms sick. However, the filtered tissues remained infective. This meant that something even smaller than bacteria was causing the infection.
Scientists did not actually see viruses for the first time until the 1930s. That’s when the electron microscope was invented. The virus shown in Figure below was the first one to be seen.
Tobacco Mosaic Virus. This tobacco mosaic virus was the first virus to be discovered. It was first seen with an electron microscope in 1935.
Origin of Viruses
Where did viruses come from? How did the first viruses arise? The answers to these questions are not known for certain. Several hypotheses have been proposed. The two main hypotheses are stated below. Both may be valid and explain the origin of different viruses.
- Small viruses started as runaway pieces of nucleic acid that originally came from living cells such as bacteria.
- Large viruses were once parasitic cells inside bigger host cells. Over time, genes needed to survive and reproduce outside host cells were lost.
Replication of Viruses
Populations of viruses do not grow through cell division because they are not cells. Instead, they use the machinery and metabolism of a host cell to produce new copies of themselves. After infecting a host cell, a virion uses the cell’s ribosomes, enzymes, ATP, and other components to replicate. Viruses vary in how they do this. For example:
- Some RNA viruses are translated directly into viral proteins in ribosomes of the host cell. The host ribosomes treat the viral RNA as though it were the host’s own mRNA.
- Some DNA viruses are first transcribed in the host cell into viral mRNA. Then the viral mRNA is translated by host cell ribosomes into viral proteins.
In either case, the newly made viral proteins assemble to form new virions. The virions may then direct the production of an enzyme that breaks down the host cell wall. This allows the virions to burst out of the cell. The host cell is destroyed in the process. The newly released virus particles are free to infect other cells of the host.
Viruses and Human Disease
Viruses cause many human diseases. In addition to the diseases mentioned above, viruses cause rabies, measles, diarrheal diseases, hepatitis, polio, and cold sores (see Figure below). Viral diseases range from mild to fatal. One way viruses cause disease is by causing host cells to burst open and die. Viruses may also cause disease without killing host cells. They may cause illness by disrupting homeostasis in host cells.
Cold Sore. Cold sores are caused by a herpes virus.
Some viruses live in a dormant state inside the body. This is called latency. For example, the virus that causes chicken pox may infect a young child and cause the short-term disease chicken pox. Then the virus may remain latent in nerve cells within the body for decades. The virus may re-emerge later in life as the disease called shingles. In shingles, the virus causes painful skin rashes with blisters (see Figure below).
Shingles. Shingles is a disease caused by the same virus that causes chicken pox.
Some viruses can cause cancer. For example, human papillomavirus (HPV) causes cancer of the cervix in females. Hepatitis B virus causes cancer of the liver. A viral cancer is likely to develop only after a person has been infected with a virus for many years.
Control of Viruses
Viral diseases can be difficult to treat. They live inside the cells of their host, so it is hard to destroy them without killing host cells. Antibiotics also have no effect on viruses. Antiviral drugs are available, but only for a limited number of viruses.
Many viral diseases can be prevented by giving people vaccines (see Figure below). A vaccine is a substance that contains pathogens such as viruses. The pathogens have been changed in some way so they no longer cause disease. However, they can still provoke a response from the host’s immune system. This results in immunity, or the ability to resist the pathogen. Vaccines have been produced for the viruses that cause measles, chicken pox, mumps, polio, and several other diseases.
Vaccination. A child receives a vaccine to prevent a viral disease. How does the vaccine prevent the disease?
Viruses in Research and Medicine
Viruses are important tools in scientific research and medicine. Viral research has increased our understanding of fundamental biological processes involving DNA, RNA, and proteins. Viruses that infect cancer cells are being studied for their use in cancer treatment. Viruses are also being used in gene therapy to treat genetic disorders, as explained in Figure below.
Using a Virus in Gene Therapy. A normal human gene is inserted into a virus. The virus carries the gene into a human host cell. The gene enters the nucleus and becomes part of the DNA. The normal gene can then be used to make normal proteins. It can also be copied and passed to daughter cells in the host.
- Viruses are tiny particles, smaller than prokaryotic cells. They are not cells and cannot replicate without help, but they have nucleic acids and can evolve.
- Viruses can be classified on the basis of capsid shape, presence or absence of an envelope, and type of nucleic acid.
- Viruses were assumed to exist before they were first seen with an electron microscope in the 1930s. Multiple hypotheses for viral origins have been proposed.
- After infecting a host cell, a virus uses the cell’s machinery and metabolism to produce new copies of itself.
- Viruses cause many human diseases by killing host cells or disturbing their homeostasis. Viruses are not affected by antibiotics. Several viral diseases can be treated with antiviral drugs or prevented with vaccines.
- Viruses are useful tools in scientific research and medicine. Viruses help us understand molecular biology. They are also used in gene therapy.
Lesson Review Questions
1. How do viruses differ from living things? How are they similar to living things?
2. Describe variation in capsid shape in viruses.
3. State two hypotheses for the origin of viruses.
4. Describe how viruses replicate.
5. How do viruses cause human disease?
6. Apply lesson concepts to decide how strep throat and flu can be treated or prevented. Create a chart to summarize your ideas.
7. Viruses often infect bacteria. Some of them destroy the bacterial cells they infect. How could this information be applied to finding a cure for bacterial infections?
8. Why did scientists think viruses must exist even before they ever saw them with an electron microscope?
9. Why are viruses especially useful tools for understanding molecular biology? What might scientists learn by studying how viruses invade and use host cells?
Points to Consider
In this chapter, you read about two of the three domains of life: Bacteria and Archaea. The next chapter introduces the simplest, smallest members of the third domain, the Eukarya.
- Some Eukarya are single-celled organisms. What do you think they are?
- How might single-celled eukaryotes differ from single-celled prokaryotes? How might they be the same?
Opening image copyright Michael Taylor, 2010. Used under license from Shutterstock.com.