To prepare for the lesson, make a class set of the Binary Alphabet (S-B-8-2_Binary Alphabet.doc) and individual copies of the Break the Code activity (S-B-8-2_Break the Code Worksheet and KEY.doc). If you prefer, write your own phrase in binary for your students to solve, e.g., “Go Wildcats.”
From the Protein Synthesis Props (S-B-8-2_Protein Synthesis Worksheet and KEY.doc), copy and cut out nucleotide cards. You will need to make: 22 A, 11 T, 11 U, 8 C, and 8 G cards for the role-play activity. Tape or staple the DNA nucleotide cards, in order, to yarn or string (see illustration below).
On the board or white board, affix one side of the double-stranded DNA nucleotides. If you have a white board, use magnetic tape; they will slide when unzipping the DNA. Cut out the amino acid “puzzle” according to the instructions on page 7 in the Protein Synthesis Props document (S-B-8-2_Protein Synthesis Props-PDF.pdf). Move desks to the sides of room to leave an open space in the middle.
Copy and cut out enough True–False cards on page 11 of the Protein Synthesis Props document for each student pair (S-B-8-2_Protein Synthesis Props-PDF.pdf).
Introducing the Lesson: The Genetic Code
Give students the Break the Code activity (S-B-8-2_Break the Code Worksheet and KEY.doc). Explain that the phrase is written in the same language used by computers, Binary Code. Binary means that it is made up of two parts, in this case, two numbers. Computers run on electricity so it recognizes “on” (voltage 1) or “off” (voltage 0). Ask, “What information would you need to break the code?” (Students may suggest: how many letters are in the words, how many digits—0 and 1’s—make up a letter, and what is the code.)
Give students the binary alphabet. Explain that each letter is formed from a sequence of 8 bits (binary digit), i.e., every 8 bits represents a new letter (8 bits = 1 byte). Ask, “How many characters can be represented with this system?” Help students solve this mathematically using 28, which equals 256 possible characters, but do not use 8 • 2. In the code, there are no spaces between bits, and a space between words is given as 8 zeros, 00000000. Have students solve the phrase and write their names.
Tell students that the genetic code is similar to binary code. Ask, “What are the nitrogen bases that make up DNA?”
- adenine (A)
- thymine (T)
- guanine (G)
- cytosine (C)
The sequence of nucleotides in DNA codes for the sequence of amino acids of a protein. Ask students:
- “How many amino acids are found in living things?” (20 different amino acids make up living things)
- “How many nucleotides will be needed to code for all the different amino acids?”
Help students do the math if you observe that they are guessing: 41 = 4 (one nucleotide codes for one amino acid; there could only be 4 amino acids), 42 = 16 (still not enough), 43 = 64 (more than enough). This means that it takes three nucleotides in a specific order to code for a specific amino acid.
Give students the Codon Chart on pages 1 and 2 in the Protein Synthesis Props document (S-B-8-2_Protein Synthesis Props-PDF.pdf). Show students how to read the chart to determine the amino acid. Randomly call out three nitrogen bases to check students’ understanding.
Role-Play Activity: Transcription and Translation
Students who are not participating in the role-play activity can write a summary of each step in the activity. You will need:
- DNA/RNA Nucleotide cards (S-B-8-2_Protein Synthesis Props-PDF.pdf)
- 15 students to act as mRNA nucleotides
- 4 students to act as tRNA with the amino acids
- 2 students to act as the ribosome; one to translate codons and the other to hold the growing polypeptide chain
- 1 student to act as a rough endoplasmic reticulum (ER)
- 1 student to act as a Golgi apparatus
Step 1: Genetic Code
Hand out DNA Nucleotide cards to students for review of bonding of nitrogen bases of DNA (S-B-8-2_Protein Synthesis Props-PDF.pdf
). Affix the “original strand” to the board. Ask students to bring up the nucleotides that make the complimentary strand of DNA and hold them in place in the right positions. Check their understanding by asking why the “T” was put with the “A,” or why the “G” goes with the “C,” and so on. Lead them to say that “A” is the complement of “T,” and remind them that straight letters go together and curved letters go together in a DNA molecule.
Ask the class:
- “What holds the two strands of DNA together?” (hydrogen bonds between the base pairs)
- “What makes up the ‘backbone’ of the DNA molecule?” (phosphate and deoxyribose sugar)
Step 2: Transcription
Hand out RNA cards to students (S-B-8-2_Protein Synthesis Props-PDF.pdf). Ask:
- “What kind of nucleotides are these?” (RNA)
- “How do you know?” (Uracil, rather than thymine, is present.)
- “What is another structural difference between DNA and RNA?” (DNA has D or deoxyribose sugar and RNA has ribose sugar. DNA is double-stranded and RNA is a single-strand molecule.)
Unzip the DNA molecule by moving the complimentary strand away. Tell students the part of the DNA that is transcribed is the gene. A gene may consist of thousands of nucleotides. You act as RNA polymerase, linking students in 3' to 5' order of the complimentary strands. When mRNA is transcribed, students leave the “nucleus.”
Check for understanding by asking questions such as:
- “What is transcription?” (copy the genetic code to mRNA)
- “How is RNA different from DNA?” (RNA is single-stranded; it can move out of the nucleus; it has uracil instead of thymine.)
- “What is the role of RNA polymerase?” (It is an enzyme that synthesizes mRNA from the instructions on the DNA molecule.)
- “What is the relationship between mRNA and DNA?” (mRNA resembles the genetic code except it has uracil instead of thymine.)
- “Do you think that this step requires energy input or release?” (requires energy input since it is a synthesis reaction)
Step 3: Translation
Two students act as the ribosome. One student will read and look for the anticodon, while the other holds the amino acid chain. The tRNA students will be carrying the amino acids required by the anticodon. Note: There are no tRNA cut-outs; the students themselves will act as the tRNA.
When outside the nucleus, the ribosome directs the mRNA to the rough ER. Explain to students that a ribosome bound to ER will pass the newly formed amino acid through the ER membrane. Ask:
- “How does the ribosome know where to begin?” (the start codon on the mRNA)
- “What nucleotides make up the start codon?” (AUG)
- “What is the anticodon found on the tRNA?” (UAC)
Direct the activity so that before one tRNA leaves the mRNA-ribosome complex, the next tRNA has taken its position on the mRNA strand. Explain that this represents the two binding sites on the ribosome. The binding sites hold only two tRNAs. One tRNA holds the growing amino acid chain, while the other tRNA brings in a new amino acid. The Ribosome binds the amino acids together, moving the mRNA through like the links on a bicycle chain, releasing the first tRNA. Continue this sequence until the stop codon is reached. Ask:
- “How many nucleotides does the ribosome have to ‘read’ at one time?” (3)
- “What do we call that sequence of 3 nucleotides?” (a codon)
- “What do we call the sequence of 3 nucleotides on the tRNA?” (an anticodon)
- “Why would it be important for the ribosome to always have a tRNA acting as a place holder?” (This allows the ribosome to read the adjacent codon and not “shifting” the codon by one or more nucleotides.)
Part 2: Reviewing Protein Synthesis
Give students the Protein Synthesis worksheet (S-B-8-2_Protein Synthesis Worksheet and KEY.doc). Remind them how the nitrogen bases pair in the DNA and in RNA. They each need a copy of the tRNA anticodon chart (they can use the mRNA codon chart, but remind them to look at the mRNA codes, not the tRNA anticodons). Note that on Part 2 of the worksheet, although the coding strand is reversed with the template from Part 1, different amino acids will be synthesized. After they have completed the worksheet, discuss their results.
Give True–False cards to students found on page 11 of Protein Synthesis Props (S-B-8-2_Protein Synthesis Props-PDF.pdf). Have them write two lists: true statements and false statements. When students sort the cards correctly, ask them if they can change a false statement to make it a true statement.
- Students who may be going beyond the standards can research the human genome project and write a summary of what the project accomplished. Information can be found at:http://web.ornl.gov/sci/techresources/Human_Genome/index.shtml.
- Students who may be going beyond the standards can complete the following activity after Step 3 of the Role-Play Activity:
o Step 4: Simplified Posttranslational Modification (Folding and Packaging)
§ Remind students that proteins are three-dimensional molecules, not one-dimensional chains. Within the lumen of Rough ER, the protein starts folding: Have the rough ER fold up the puzzle to form a box. Next, a membrane bound vesicle transports the protein to the Golgi apparatus for further modification: The rough ER places the amino acid sequence in a “trash bag” to represent the membrane bound vesicle. Recall the organelles found in the eukaryote cell.
§ In the Golgi apparatus, altering the protein may continue by adding a long carbohydrate chain or phosphates. This may signal the protein’s destination: The Golgi apparatus places a label and a stamp on the box. The label may send the protein to the cell membrane where it is immediately released outside the cell. The label might act as a chemical signal regulating when the protein will be released outside the cell membrane.