Describe the essential cell “machinery” that viruses use to make a new virus.

Page 1“Th e Ebola Wars: General Edition” by Addy, Iadarola, & Dube

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Part I –What Is a Virus? “I can’t believe I’m a college sophomore,” thought Terry in amazement while taking the train back home for the summer. “Th e school year went by so fast, but I made it through even though I had to change my major.”

A text popped up on Terry’s smartphone, “Terry, when r u going on ur trip? Let’s meet up before u go.” Terry recognized that the message was from Alex, a long-time friend from high school. “Leaving in a week,” Terry quickly texted back. “Never been on a trip outside of the US and can’t wait to work with doctors and nurses to help sick people. Alex, let’s meet up Fri.”

After a week of seeing family and friends, Terry was off to the west coast of Africa for ten weeks. Th is would be a life- changing experience. Th e agency placed Terry in a rural site where residents had limited access to medical care. Th e locals sometimes traveled miles to be seen at their clinic. Th e health care professionals on the team were unbelievably positive and devoted to educating and providing patients with the best care possible. Residents came in with many types of ailments. Terry’s responsibilities included greeting patients and their families, bandaging wounds, and providing additional assistance to doctors and nurses as needed.

Just like the school year, the summer experience seemed as though it was over in no time. Terry was about to return home when unexpected news came. Terry’s team was contacted by the Centers for Disease Control and Prevention (CDC) and notifi ed that they had likely been exposed to a patient infected with the Ebola virus as there was an outbreak near the medical relief site. Th e patient who had contracted the virus had died. Struck with fear, Terry quickly tried to determine which patient it could have been and her likelihood of being infected, but she found this nearly impossible to fi gure out. As a precaution, all individuals on the team returning to the US had to be screened and undergo a 21-day quarantine where they would be monitored for any Ebola-like symptoms. Not doing so could potentially place others at risk for contracting this deadly virus.

“I need to fi nd out more about viruses so that I can understand what’s going on,” thought Terry as she reached for her general biology textbook. Glancing through the section on virology, she was surprised to discover that viruses are tiny particles considered to be non-living since they cannot metabolize energy, do not create waste, do not grow, and require host cells to multiply. Indeed, in order to replicate, viruses hijack the machinery present within the cells that they infect. Th e additional viral particles produced inside host cells can exit and infect other cells. “Th ese viruses seem kind of creepy,” thought Terry. “Th ey’re like parasites to cells.”

Terry continued reading:

General EditionGeneral Edition

by Tracie M. Addy, Yale School of Medicine Teaching and Learning Center, Yale University Linda M. Iadarola , Department of Biological Sciences, Quinnipiac University Derek Dube, Department of Biology, University of Saint Joseph

The Ebola WarsThe Ebola Wars In every battle there comes a time when both sides consider themselves

beaten, then he who continues the attack

wins. –Ulysses S. Grant–

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Page 2“Th e Ebola Wars: General Edition” by Addy, Iadarola, & Dube

Viruses have either a DNA or RNA genome, which can be single- or double-stranded. Th ese genomes are housed in a capsid made of proteins. Viruses can be classifi ed by their specifi c genomes and the unique features of their capsids, including shape and protein constituents. Some viruses have lipid envelopes derived from host membranes that enclose the virus particle, while others do not. Surface glycoproteins on these membranes, or spike proteins protruding from the viral capsid in non-enveloped viruses, can play a role in viral attachment and entry into the host cell.

Terry spent the next few minutes summarizing the information she had just read.

Questions

1. Which structural features are in common to all viruses, and which are not? Complete the table below to answer this question based upon the information provided in the case.

Shared Attributes Diff erences

2. Why are viruses considered parasites?

3. Examine the diagram of the viral particle below. Label all of the important structures on this virus that you identifi ed in the table above.

4. Design an imaginary viral particle. Create a diagram of your virus and label its major features. Your virus should have a diff erent capsid shape (e.g., icosahedral, helical, complex) than the one above and be non-enveloped.

Figure 1. A general viral particle.

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Page 3“Th e Ebola Wars: General Edition” by Addy, Iadarola, & Dube

Part II – How Could a Virus Have Entered? Terry considered what she had learned so far. “So the Ebola virus could be inside my cells? But how? How exactly could a virus get inside? It seems to start at the plasma membrane… I’d better keep reading.”

In the fi rst step of infection, viral proteins interact specifi cally with host cells. Diff erent viruses like infl uenza, rabies and even Ebola each have unique glycoproteins on their surface that bind only to specifi c receptors on the particular host cells they infect. For example, the infl uenza virus uses the HA (hemagglutinin) protein for binding to the host cell receptors on respiratory epithelial cells. A rabies virus relies on the G protein protruding out of its viral envelope to attach to the host cell receptors of neurons. Ebola virus uses GP to bind to the host cell receptors on a wide variety of cells, beginning with the macrophages and dendritic cells of the lymphatic system. Once viruses attach to the host cell in this very specifi c manner, there are three major ways for them to complete the second stage of infection, entry into the host cell. Th e fi rst is by direct injection of its genome (DNA or RNA) into the host cell at the cell surface. Th e second is the binding of the virus to the plasma membrane, followed by the fusion of the viral envelope with the host membrane to transfer the viral genome (DNA or RNA) into the host cell. Th e third way is for the virus to bind to the host cell membrane and become internalized into the host cell via the endocytic pathway.