Written by Kelly A. Hogan and James Oh, University of North Carolina at Chapel Hill
– To see how a DNA template codes for a protein
– To practice determining what amino acid a tRNA carries based on its anticodon
– To see how a single DNA change affects the protein’s structure
Activity Description: Student volunteers will be used to demonstrate protein synthesis starting with a DNA template. The rest of the class will help by thinking through the play with prompted questions.
Time Needed: 30 minutes as a whole class demonstration or longer if beginning with student group discussion
– Large cardstock pieces of four different colors to write sequences on (one color for DNA, one color for mRNA, a third color for tRNA, and a fourth color for amino acids)
– String to hang cardstock signs around the necks of students
– A baseball cap for the 5’ cap of mRNA and a scarf or boa to represent the 3’ poly(A) tail
– Student handouts of the mRNA codon chart or a projection of the chart via PowerPoint
– Large binder clips to link amino acids together
- Explain to the students that they will be putting on a play called “From DNA to Protein.” Choose students to perform through a random selection, such as calling out a row and seat number. The worksheet below gives the titles for 7 acts of this play. For each act, the class (maybe in partners) will come up with some of the details of that act. You may choose to have them think through the whole process first, but I prefer having them think through one act at a time and then performing that act. You can perform the entire play a second time to show them the process continuously. I have fun randomly choosing volunteers with an easy web application that randomizes my class roster: http://classtools.net/education-games-php/fruit_machine/
- Act 1: Transcription: Set aside an area of your room that you designate as the nucleus and another area that is the cytoplasm. Have students “wear” a DNA sequence. Have six students wear DNA triplet codes in row 1: TAC GGA CTC CTC TTC ACT, and have six students next to the other students wearing the complementary sequence of the other DNA strand: ATG CCT GAG GAG AAG TGA. Engage the students by asking them to name the complementary codons before handing them their sequences.
- Bring another student up who will be RNA polymerase. As RNA polymerase walks over to the DNA, have the two rows of DNA move apart. Ask them what kind of bond is being disrupted when DNA is pulled apart (Hydrogen bonds). Choose row 1 to be the template strand. RNA polymerase will then have a pile of RNA codons that are complementary to hang around their necks (have a different colored cardstock form DNA) to make mRNA. Once again, have students participate to call out the codons. Discuss how uracil replaces thymine in mRNA. Students can link arms to show the covalent bonds formed between the RNA nucleotides.
- Act 2: mRNA Matures: The students wearing mRNA will now be processed. Call up other students to represent enzymes that perform the processing steps. Have a student add a 5` cap by having the first mRNA student stick their arm out. The enzyme can put a baseball cap on the arm. Have another student remove one of the two GAGs that will arbitrarily represent an intron. Have the remaining students splice their codons together. Lastly, have the last of the mRNA codon students hold a long scarf or boa to represent the poly(A) tail.
- The mRNA students should move as a group to the area of the room labeled cytoplasm. Stress that only after processing does the mRNA exit the nucleus. You may decide now is a good time to discuss the differences between prokaryotes and eukaryotes (with no nucleus and splicing, prokaryotes can begin translation as transcription is still finishing). You may also discuss the advantages of a nucleus and the stability of DNA vs. the short half-life of mRNA and how mRNA breakdown affects gene expression.
- Act 3: Translation Initiation: I like to use two chairs in the front of the room for the ribosome grooves in which tRNA students will temporarily sit. Translation is the most difficult aspect of protein synthesis because students often get confused about what the tRNA anticodon is complementary to. I have the chain of mRNA students stand behind the chairs with the first two codons aligned with the two chairs. I then have several students floating around the cytoplasm that are wearing tRNA anticodons around their neck. They are also holding amino acid names, but the audience cannot see which ones each tRNA is holding yet. It is useful to ask the students what the anticodon of the initiator tRNA should be that will recognize the first codon (UAC). The student wearing UAC will sit in the first chair. Ask them, “What amino acid will this tRNA be carrying?” (Met). This is often the first place students have trouble because they look for the anticodon sequence on the mRNA codon chart instead of the codon sequence. Ask them, “Which of our tRNAs should sit down in the second chair?” (The tRNA with GGA around its neck). Then ask them, “What amino acid is this second tRNA holding?” (Pro).
- Act 4: My Growing Polypeptide: Have the tRNA students in the chairs use binder clips to attach the first two amino acids together. The initiator tRNA can move out (point out that this tRNA can be recycled and can be charged with another Met). The tRNA codon now holding the first two amino acids will shift over as will the mRNA chain of students standing above. The next tRNA will sit down in the empty chair. Continue forming peptide bonds (with binder clips) and continue having students name the tRNA anticodon and amino acid that will enter. Repeat until the stop codon is reached.
- Act 5: When Will It End?: Students will reach the codon that they will recognize as a stop codon. Discuss how the ribosome, mRNA, and tRNAs are released. Stress that the mRNA still has some more time and has not been degraded yet as other ribosomes can attach to it. (In fact, multiple ribosomes can be attached to the same mRNA once the start codon emerges from the first ribosome.)
- Act 6: Just the Beginning for a Young Protein: Have one student hold the four amino acids that are clipped with binder clips and folded, and clip them in a new way. Discuss primary, secondary, and tertiary structure. Have the student with the protein then move to another area of the classroom and discuss the secretory route for a protein (through ER, Golgi, vesicles, and plasma membrane) or how some proteins will remain in the cell.
- Act 7: Am I Normal?: You can choose to re-act the play changing one of the codons in the DNA to illustrate a base substitution. Show students how one amino acid would be different and discuss implications of a mutation. Alternatively, you can have students use the sequence on their worksheet to see this easily (right side of sheet).
Worksheet: From DNA to Protein Worksheet