Today students are using two additional virus groups to better understand how scientists use many specific examples to explain the idea of evolution. Here is an overview of what students will learn today and why I use the methods I do to teach the content.
Yesterday, students constructed to models two of the common classification of viruses. Today they will construct two model models. Start the class by watching a part of the Jimmy Neutron episode Journey to the Center of Carl (I start at 4:02 and play until 5:21).
This video has several misconceptions in it. Have students write down what they think they are either in their lab notebook or give them more scaffolding to help in the identification by giving them this graphic organizer. (Note: Have students revisit this graphic organizer at end of the lesson to summarize the day's learning.)
Remind students that complex viruses are the catch-all drawer of the viruses.
(Answer: A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the organism model will provide insight into the workings of other living things.)
(Answer: The model organism most typically used for this virus group is the bacteriophage as much is known about it. There are three basic types of bacteriophages: one with an icosahedral head and a tail, another with just an icosahedral head, and a filamentous form.)
(Answer: Other complex viruses include the poxvirus.)
Have students return to the concept map that students started in yesterday's lesson. Have students add the structure of complex viruses and the theories regarding their evolution. Remind students that bacteriophages are polyphyletic, arose repeatedly in different hosts, and constitute 11 lines of descent. Tailed phages appear as monophyletic and as the oldest known virus group.
(Note: Bacteriophages were first described in 1915 by Twort and later independently by d'Herrelle in 1917.D’Hérelle coined the term bacteriophage, meaning “bacteria eater,” to describe the agent’s ability to kill bacteria. Despite the large amount of information accumulated about bacteriophages, the genomic sequence was not completed until 2003.)
Students should use the bacteriophage worksheet to construct a model of the T2 bacteriophage. Students will cut out the capsid and tail. Then fold them and tape the seams. The tail seams can be completely taped shut. To complete the structure of the tail, students should string pony beads of one color on a white fuzzy stick (also known as chenille sticks or pipe cleaners). Leave one seam open on the capsid so the DNA strand (yarn) can be placed inside. Choose a different color than the yarn used for the RNA strand. The fiber legs will be made from black fuzzy sticks (also known as chenille sticks or pipe cleaners). The bacteriophage has six legs which are made by wrapping 3 black fuzzy sticks around each other and inserting them in the end of the tail. The top part of the tail is taped to the capsid. Wind the pony bead fuzzy stick around the tail and wrap with clear packing to form the protein arrangement on the tail.
Have students determine how big to make the single stranded DNA genome by first calculating how long in nanometers the genome is. The bacteriophage is 168,903 base pairs. Each base pair is .34 nm.
168,903 bp *.34 nm/bp= ________ nm
A bacteriophage is 200 nm long. Determine the size ratio by dividing the length of the genome by the length of the bacteriophage capsid. Multiple the length of the paper model by the size ratio to determine how long to cut the yarn representing the DNA genome.
While students are cutting and assembling their models, I relate them the following scenario:
Enterobacteria phage T2 was first described in 1940s. The DNA genome is found in a icosahedral head or capsid. The tail of the T2 is hollow so its DNA can pass into the host cell after attachment of the phage to the outside of the cell. The tail attaches to the host cell with the help of tail fibers (which we do not show in the model we are making). The tail fibers are important not only in attachment, but also in recognizing the glycoproteins on the host cell surface. The capsid stays on the outside of the cell, only the nucleic acid enters the cell. Bacteriophages are specific for certain species of bacteria. Phages are found everywhere including soil, animal intestines, freshwater, and seawater.
Like all viruses, bacteriophages only have the genetic information need for replication of their nucleic acids and making of their capsid. The host cell makes everything else for the bacteriophage. Although the capsid in our model looks like one complete unit. The capsid actually consists of several different proteins as is the tail. While we will not show the different proteins for the capsid, we will show the different proteins for the tail by attaching a pipe cleaner that has been strung with pony beads. To hold the pony beads/pipe cleaner in place, tape the outside of the tail with clear packing tape. The bacteriophage has six legs that support it as it binds to the outside of a bacterium. These will be made with 3 black pipe cleaners folded in half and wrapped around each other. These legs are insert into the tail opening. (Note: The legs would be in the tail opening in real life as the nucleic acid would have no way to get into the bacteria if that was the case). Now we will determine the length of the single strand of DNA for this bacteriophage. Notice we use a different color of yarn since DNA is structurally different than RNA. The genome for the bacteriophage is 168,903 bp pairs. Each base pair is 0.34 nanometers. How long is the DNA genome? (168,903 bp *.34 nm/bp= ________ nm)
A bacteriophage is 200 nm long. Determine the size ratio by dividing the length of the genome by the length of the bacteriophage capsid. Multiple the length of the paper model by the size ratio to determine how long to cut the yarn representing the DNA genome. Cut the DNA to the correct size. Wrap the DNA in a figure 8 and then insert it in the opening in the capsid. Then the tape the seam shut.
(Note: I use this scenario because because there can be a lot of down time while they are constructing their models. I try to use this portion of the lesson to keep students on track and keep it pretty conversational by revealing the scenario to the students in a story-like manner.)
Remind students that enveloped viruses include HIV and the influenza virus.
(Answer: These are classified by the glycoprotein studded envelope that surrounds their capsid. They also contain a capsid and nucleic acid typically RNA. The envelope is made of lipids.)
Remind students that HIV are retroviruses.
(Answer: Some enveloped viruses like HIV also contain enzymes that convert the viral RNA to DNA and integrate it into the host genome.)
Students should create a model of the HIV capsid, which causes AIDS. It has the surface proteins on the outside of the virus. Print off enough virus templates for each student in the class. Instruct students to cut out the template around the solid black lines. Tape all the seams closed but one. Add the RNA genome to the virus by cutting two strands 3.3 meters long, wrapping them in a figure 8 pattern, and inserting them into the open seam. Use the templates for the viral enzymes found in the Structural Biology of HIV. Print off enough the viral enzyme images for each model being made. Then seal the final seam. Make an envelope out of a clear plastic fillable sphere (I found mine on Amazon.com, but they can be found at many craft stores) and place the paper capsid inside it. Glue jewels of two different colors (These can also be found at many craft stores.) to the outside of the sphere to represent the glycoproteins.
Read the following to the students:
The genome of HIV which is composed of two strands of RNA. The RNA strands are enclosed in a cone-shaped capsid which protects the RNA and delivers it to the cells that become infected with HIV.
Once the model is made, show students the interactive Structural Biology of HIV.
Have students return to the graphic organizer from the beginning of class. Students should pick two misconceptions and write a letter to the writers of Jimmy Neutron explaining to each misconception with the reason why that it is wrong. If students do not complete this assignment, they may work on it at home.