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Lesson 2 Cell Theory

  Living or Not?

What about cell-like structures?
The cell theory only applies to living organisms and cells. It does not apply to cell-like structures, as these structures do not have all the characteristics of life. There are two such structures we will look at: viruses and prions.


A2.6 Images of different viruses

  Digging Deeper


© Oona Räisänen, via Wikimedia Commons
A2.8 T7 Bacteriophage

Bacteriophages are viruses that infect bacteria. They look similar to alien ships and can be quite deadly to bacteria. Scientists have studied these kinds of viruses in detail as they are quite interesting!

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A virus is a structure that is made up of a stretch of genetic material (like DNA) within a protein coat. This basically means that it is a piece of DNA inside a room. You probably know about common viruses such as the common cold and flu. Viruses work by getting inside the cells of the organism they infect and taking over the cells’ functions. The virus uses the cell to produce more viruses, which then burst out of the cell to infect other cells. When enough cells are infected, the organism becomes sick. Since viruses do not have the ability to produce new viruses without using cells, they are not considered to be alive.

Prions are another example of a non-cellular structure. A prion is a protein that changes from its normal form into a harmful particle. Just like viruses, prions use the cell to complete life functions and so are not cells.


© Lopez-Garcia, F., Zahn, R., Riek, R., Wuthrich, K. & RCSB, via Wikimedia Commons
A2.9 Structure of a cow prion protein

  Did You Know?


A2.7 Infection and disease control

There are some very interesting viruses out there! They are the stuff of nightmares, and they are often used as the villain in horror movies.

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  Did You Know?


A2.10 A cow

Mad cow disease is caused by a prion that attacks the brain of infected cows and humans, causing changes in personality and the loss of the ability to walk, as well as other symptoms.

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  Take Notes

Make sure you take notes on your readings to study from later. You should focus on what prions and viruses are and why they are considered non-living. Remember, if you have any questions or you do not understand something, ask your teacher!

  Practice Questions

Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

  1. What are the similarities and differences between viruses and prions?

    Viruses contain a piece of DNA in a protein coat, while prions are proteins that change into harmful substances. Both need to infect a cell to reproduce and carry out life functions.
  1. Write down one example of a virus and one example of a prion not included in your course content. Note: You will need to do research to find these examples.

    Answers will vary. Some common viruses are HIV, HPS, Rabies, Ebola, measles, polio, and zika. Some common prions are CJD, FFI, GSS, and CWD.

Interactive Activity


This activity shows you how the most common viruses look. Some are very odd looking! It will also give you lots of information on each virus, such as how it is spread and if there is a vaccine for it.

  1. Click on the play icon to open the activity. https://quick.adlc.ca/virus
  2. Click on any of the nine viruses pictured to see more information.
  3. You can move the image of the virus around to see all sides, or you can view the inside of the virus.
  4. When on the main page with all nine viruses, you can sort the viruses based on the characteristics listed across the top of the screen. If you click on hosts, you can choose to see the ones that infect humans specifically.
  5. Explore all the viruses and their characteristics.
  6. Please return to the top of this page and click on analysis to complete the analysis questions.
  1. Can you find the virus mentioned in the Digging Deeper above?

    This is the T7 virus shown on the bottom row in the interactive.
  1. Click on the “Show Relative Sizes” button found at the bottom of the screen. Which virus is the largest? Which is the smallest?

    The Ebola virus is the largest (it is huge in comparison!), and the Zika virus is among the smallest.
  1. Go to the HIV virus. Which continent has the largest amount of adult HIV infections?

    Africa has the largest amount of adult HIV infections. It is much higher than North America. In Africa, 4.5% of the adult population has HIV, while only 0.5% of the adult population in North America does.
  1. Go to the rabies virus. What are the symptoms that occur with this virus?

    This virus has neurological symptoms (the symptoms occur in the brain) that cause aggression. This often leads to biting, which is often how the virus is spread. Once symptoms appear, the infection is fatal. (This sounds like the zombie virus, doesn’t it?)
  1. Go to the influenza A virus. This is what the flu looks like. After reading the information, why do you think we need a new vaccine for the flu each year?

    The virus has its DNA split into many segments so that those segments can interact with other segments of different flu viruses. This allows the viruses to evolve and change very quickly. A new vaccine must be made for each flu season as a specific flu virus never lasts longer than a year. Every year, there is a new version of the flu virus we need to vaccinate against.


Virus Explorer ©2017 Howard Hughes Medical Institute