Lesson 6 of 7
Objective: Students will evaluate a model of the cell process transcription to determine its accuracy.
In today's lesson, students evaluate a model of RNA transcription by using a portion of the hemoglobin gene to transcribed mRNA. Then they will compare transcription with DNA replication. By doing this activity, students will compare several cell processes that are important in maintaining homeostasis and cell replication. Here is what students will learn in this lesson.
Review the structure of DNA with students and explain to them that for this mini-model, they will be using Legos. Provide student groups with a double-stranded DNA molecule that contains 12 nucleotide pairs and is made of Legos and one set of Legos to construct the RNA strand. Ask students to make a key to help identify the parts of the DNA model. Ask students what is needed in an RNA strand that is different from the DNA strand. Using the student constructed key and a Vis-a-vis pen, have students label the leading and lagging strand and the components of the double-stranded DNA molecule. Next, students should label the unassembled components needed to transcribe the RNA strand. Have students unzip the DNA template and make a RNA strand from the leading strand of the DNA molecule.
Move about the room and evaluate the RNA strand construction. Review base-pairing rules with individual student groups, as necessary.
Have student consider the following questions and record their answers in their lab notebooks:
- How does the cell know where to start in RNA transcription?
- What problem(s) might occur to the mRNA if there is a mistake in transcription?
- Why is replacement of thymine with uracil necessary in RNA transcription?
(Note: Here is a sample student response to these questions. We will revisit these questions again at the end of the lesson.)
Using the Frayer method, define transcription.
Based on Fetters, Marcia. 2001. Building Block Transcription. The Science Teacher.
For this activity, students should work in pairs. Explain to students that one of them will play the role of the cytoplasm and one will play the role of the RNA polymerase. By working together, they will read the DNA template and construct a RNA strand that will be modified into the mRNA.
Using the student handout and the manipulatives, have students transcribe a portion of the gene for the hemoglobin protein. (Note: I laminate all of the manipulatives used in this activity so they can be used for more than one year.) First, the student playing the role of RNA polymerase will place the DNA strand marked "Beginning of Hemoglobin Gene" into the slit on the RNA polymerase sheet so that the first two nucleotides are visible. The student playing the role of cytoplasm will retrieve the RNA nucleotides that match the visible DNA nucleotides. Base-pairing rules should be followed. The RNA polymerase should attach the RNA nucleotides with tape (which will represent the covalent bonds between the nucleotides). Both students should repeat the steps until the entire RNA strand is synthesized.
When the strand is complete, students should summarize the activity and what they learned about RNA transcription in their lab notebook. They should explain how a gene is responsible for the construction of the RNA strand. (Note: I provide my students with keywords to help them in their answer. The keywords that they should include in their summary are: base-pairing rules, complementary nucleotides, DNA, gene, mRNA, nucleotide, nucleus, and RNA polymerase).
This student activity is based on Walden, Ingrid, et al. 2014. Serendip Studios. "From Gene to Protein."
As a class, revisit the hook activity from the beginning of the lesson. Combine all of the double stranded DNA molecules used in the hook activity to make a very long DNA strand (i.e. 48 nucleotides or longer). Be sure to include a start signal and stop signal to the long DNA strand. (Note: I assemble this model as the student pairs are working on the previous activity. Typically, I break one of the DNA template of twelve in half and insert the start signal. I attach the stop signal to the end of the DNA template and then place the remaining six nucleotides at the end of the long DNA template.)
Ask for four student volunteers and with the class' help create a mRNA strand. Remind student about the necessity for a start and stop signal. Encourage student volunteers to begin the RNA strand at the start signal. Once the student volunteers reach the stop signal, they have completed the RNA strand. Give students a chance to sketch the completed strand in their lab notebook (or they can use these templates). Use this time to explain to students that the completed strand is not actually mRNA. It is known as the primary RNA transcript and must be modified further. Using the Lego model and the attached attached powerpoint, modify the RNA strand to create mRNA. Guide students through the steps in which the primary RNA transcript is changed to make mRNA. Have student volunteers change the primary RNA transcript while explaining the process.
To help students understand how quickly all of this occurs show this video to summarize everything they have learned.
(Note: check out my teacher reflection in which I explain some helpful modifications for less artistic students.)
In their lab notebooks, students should make two columns. They should label one column similarities and the other column differences. Guide students in comparing the similarities and differences between DNA replication and RNA transcription.
Students should notice the following similarities:
- A polymerase enzyme is used in both transcription and replication. RNA polymerase is used in transcription and DNA polymerase is used in replication.
- Nucleotide monomers are added to the growing nucleic acid one at a time.
- Base-pairing rules are used to match each new nucleotide that is added to the RNA or DNA polymer.
- Both use a DNA template for instructions to make the new nucleic acid strand.
- Cytosine, guanine, and adenine are used in both cell processes.
Students should notice the following differences:
For DNA replication
- The entire strand of DNA is copied.
- The end product is a double stranded DNA molecule.
- A host of enzymes are used to copy DNA (DNA polymerase I and II, DNA ligase, DNA helicase, and DNA topoisomerase).
- Thymine is used in replication.
For RNA transcription
- A single gene from DNA is transcribed into a mRNA molecule
- The end product is a single-stranded RNA that is modified into mRNA.
- RNA polymerase is needed to transcribe RNA.
- Uracil is used in transcription.