In the search to discover the agents that cause mad cow disease, scrapie in sheep, and CJD and kuru in humans, diseased brain tissues were passed through a fine filter to remove bacteria. The filtrate was still infectious, indicating that something smaller than bacteria must be the causative agent. If a virus was responsible for these brain diseases, then the infectious agent would contain either RNA or DNA. Other possibilities were that the agent was a carbohydrate, fat, or protein. Tissue filtrates were treated with agents that destroyed just one of these chemicals and then injected into a healthy animal, with the results as follows. What is the infectious agent? •Amylase digests carbohydrates; tissue filtrate still infects healthy test animal.
•Lipase digests fats; tissue filtrate still infects healthy test animal.
•Formaldehyde and/or heat denatures DNA and RNA; tissue filtrate still infects healthy test animal.
•Trypsin digests protein; tissue filtrate does not infect healthy test animal.

Question

Quizplus · Accepted Answer

The fact that trypsin digestion destroys the infectious agent indicates that it is a protein, ruling out options A, B, and D. The fact that formaldehyde and/or heat treatment does not destroy the infectious agent rules out DNA and RNA. Therefore, the infectious agent must be a protein, making option C the best choice. Option E is incorrect because trypsin digestion rules out the possibility of carbohydrates or fats being the infectious agent.

In the Search to Discover the Agents That Cause Mad