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Defines what viruses are and how they are different from living organisms.

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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, and they can evolve. 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.

These little "alien" looking creatures are viruses, and these specific viruses infect Escherichia coli bacteria. Shown is a representation of viruses infecting a cell. The virus lands on the outside of the cell and injects its genetic material into the cell.

Viral classification

Viruses are identified by four characteristics:

  1. The type of nucleic acid they posses
  2. Whether they are enveloped
  3. The pattern of reproduction they follow
  4. The type of organism and cells they infect

Nucleic Acid:

  • a virus will only contain DNA or RNA, never both
  • a virus will have either  single strand of DNA or RNA or it will have double strand DNA or RNA
  • RNA retro viruses have RNA as their nucleic acid, but also contain an enzyme called reverse transcriptase that will help the infected host cell convert the viral RNA to viral DNA


  • some viruses have only an outer protein coat ( a capsid
  • other viruses have the capsid, but it is surrounded by an outer layer of lipid (the envelope)
  • so viruses are called enveloped or noneveloped

Pattern of Reproduction:  Lytic or Lysogenic ??

        Lytic Viruses

License: CC BY-NC 3.0

http://apbiologydodd.wikispaces.com/Viruses+and+DNA+Technology  Creative Commons

  1.  The lytic virus attaches to are receptor on the surface to the target cell and inserts its nucleic acid into the host cell.
  2. The virus takes over the metabolism of the host cell, forcing the host cell to make copies of the viral nucleic acid and segments of the viral protein coat
  3. The virus causes the host cell to assemble new viral particles which accumulate inside to host cell and are released either by exocytosis (in the case of enveloped viruses) or by lysis (rupture of the host cell) which is pictured above
  4. The host cell stops its normal function shortly after becoming infected and dies when it ruptures or in the case of exocytosis, it runs out of raw materials

Link to a Youtube that has great graphics showing how a lytic virus works:

http://www.youtube.com/watch?v=cE0qdqoBFa8  (time 3:57min)

    Lysogenic (Latent) Viruses

License: CC BY-NC 3.0

http://en.wikipedia.org/wiki/File:Phage2.JPG Creative Commons

 In the diagram above you should recognize the lytic cycle, but a lysogenic virus has options.  It can enter a host cell, insert its DNA into the host genome and remain dormant.  If the host cell goes through cell division the viral DNA will be replicated along with the host DNA and passed on to the daughter cells. Under conditions that vary with different viruses the viral DNA can take itself out of the host genome and become lytic, forcing the host to make copies and releasing them as seen in the lytic cycle.

Only DNA viruses and RNA retroviruses can be lysogenic. 

Organisms and Cells Infected

In order for a virus to infect a cell the virus must have a structure on its protein coat that is compatible with a recepton on the cell membrane of the cell to be infected.  Once again this requires a "fit" between the structure on the virus and the receptor of the cell to be infected (the host cell).  This is similar to the "lock and key" that is used to explain the specificity of enzymes and their substrate.

Viruses that only infect bacteria are known as bacteriophages. Scientists did a lot of research using this type of virus because it was safer to use; it never infects eukaryotic cells like plants and animals.

Other viruses are exclusive to plants like the 1st virus every seen with an electron microscope, the Tobacco Mossaic Virus. 

Viruses that infect animals may only infect certain cells within the animal.  HIV (huma immunodeficiency virus) primarily infects cells of the immune system called helper T cells.  The rabies virus infects cells of the nervous system.

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.

Cold sores are caused by a herpes virus.


Virus Review

Use the resources below to answer the questions that follow.

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


  1. Is a virus a living thing? Why or why not?

  2. Name some examples of human diseases caused by a virus.


  • ATP: Adenosine triphosphate; a usable form of energy inside the cell.
  • host cell: Cell infected by a virus.
  • replicate: To make a copy of, as in viral replication or DNA replication.
  • ribosome: Cell structure on which proteins are made; not surrounded by a membrane; found in both prokaryotic and eukaryotic cells.
  • virus: Non-living particle consisting of RNA or DNA surrounded by a protein coat.


  • 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.

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Image Attributions

  1. [1]^ License: CC BY-NC 3.0
  2. [2]^ License: CC BY-NC 3.0

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