What Is A Virus?
Viruses are non-cellular, infectious particles that are not alive. Why?
- Viruses are much smaller and less complex than cells. Viruses have no nucleus, cytoplasm or membranes.
- Viruses do not grow.
- Viruses do not carry out cellular functions or maintain homeostasis because they do not generate metabolic energy.
- In order to reproduce, viruses require a host cell. Therefore viruses are obligate intracellular parasites.
However, viruses are still important microbes to discuss; they have significant impact upon and close ties to living things. They also share some of the characteristics of living things, such as genetic material.
Navigate to the sidebar and download the Virus Video Notes Guide to preview important ideas about Viruses that we will learn about in this lesson. Use this information to study for your upcoming quiz.
Examples of viruses and/or some diseases that they cause include:
- AIDS (HIV virus)
- Flu (influenza virus)
- Common cold
- Avian Flu
- Chicken and cow pox
Viral Structure and Function
Now that you know a little about viruses, take a moment to think about their size. Viruses are tiny compared to living things. In fact, the largest virus is only as big as the smallest bacterium! Bacteria average about 1000nm in size, whereas a virus is about 20-400nm. To view a virus, you would need an electron microscope!
A variety of viruses exist, but each share the same basic anatomy:
- a nucleic acid (DNA or RNA but not both)
- that is surrounded by a capsid (protein coat).
Viruses with RNA as the genetic code are called retroviruses.
Viruses vary in the amount of other machinery that they may have; some may have an extra envelope outside the capsid (examples: Influenza or HIV). The envelope comes from lipids within the virus’s host cell. Other viruses, like the bacteriophage, have special proteins that help to inject its DNA into its host cell: bacteria.
Viruses can also vary in their shape. (Make sure to use the link to see them!)
Most viruses are very selective about the type of cell they invade. There are specific sites called receptor sites that the viruses attach to on the host cell’s surface.
Viral Reproduction: Life Cycles of Viruses
All viruses must invade a host cell and reproduce through the lytic or lysogenic cycles. Watch the presentation to learn more about each cycle.
Other Non-Living Infectious Particles: Viroids and Prions
Viroids and prions are infectious, non-living particles that are even smaller than viruses! These tiny particles challenge our traditional view of infectious agents. Examine details about both below and compare to how a virus operates:
- Viroids – consist of a single, tiny strand of RNA without a protein coat. Most are plant pathogens.
- Prions – prions are small sequences of misfolded proteins. They are the only type of pathogen that does not use DNA or RNA to cause disease. Prions attack animals, causing holes in brain tissue; Mad cow disease, or Bovine spongiform encephalopathy (BSE) is a prion-caused disease.
Use the interactive to review the structure and reproductive cycles of viruses.