5.1: Viruses

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What causes the common cold?

That miserable cough and runny nose is caused by one villain: a virus. Viruses come in many different shapes, including the prickly balls you see here. They are so tiny that they can only be seen with a very powerful microscope.

What is a Virus?

We have all heard of viruses. The flu, the common cold, and many other diseases are caused by viruses. But what is a virus? Do you think viruses are living? Which domain do they belong to? Bacteria? Archaea? Eukarya?

Are Viruses Alive?

The answer is actually “no.” A virus is essentially DNA or RNA surrounded by a coat of protein (Figure below). It is not made of a cell, and cannot maintain a stable internal environment (homeostasis). Recall that a cell is the basic unit of living organisms. So if a virus is not made of at least one cell, can it be living? Viruses also cannot reproduce on their own—they need to infect a host cell to reproduce. So a virus is very different from any of the organisms that fall into the three domains of life.

Though viruses are not considered living, they share two important traits with living organisms. They have genetic material like all cells do (though they are not made of cells), and they can evolve. The genetic material of a virus can change (mutate), altering the traits of the virus. As the process of evolution has resulted in all life on the planet today, the classification of viruses has been controversial. It calls into question the very definition of life.

Replication

Viruses infect a variety of organisms, including plants, animals, and bacteria, injecting its genetic material into a cell of the host organism. Once inside the host cell, they use the cell’s own ATP (energy), ribosomes, enzymes, and other cellular parts to make copies of themselves. The host cell makes a copy of the viral DNA and produces viral proteins. These are then packaged into new viruses. So viruses cannot replicate or reproduce on their own; they rely on a host cell to make additional viruses.

Viruses and Human Disease

Viruses cause many human diseases. In addition to the flu and the common cold, viruses cause rabies, diarrheal diseases, AIDS, cold sores, and many other diseases (Figure below). Viral diseases range from mild to fatal.

Summary

A virus is composed of DNA or RNA surrounded by a coat of protein.
Viruses are not considered living things because they cannot reproduce on their own, and they are not comprised of cells.

Explore More

Use the resources below to answer the questions that follow.

Explore More I

Viruses at http://www.youtube.com/watch?v=L8oHs7G_syI (8:06)

How do viruses reproduce? How does this differ from other organisms?
What kinds of nucleic acids can viruses have?
Explain one of the theories as to how viruses came to be.
What is the importance of the "envelope" to a virus? What is the envelope made of?
What is a difference between the lytic cycle of a virus and the lysogenic cycle?