In this lesson, students explore the preservation and expression of genetic information within the cell. Students make predictions about DNA replication based on its structure, and then assess their predictions after viewing a short video. They then view a video about mitosis and discuss how DNA replication relates to cell division. After using animations to learn about transcription and translation, students apply what they have learned to analyze a short DNA sequence and relate a sequence difference to one of Mendel's traits. Finally, students locate various regulatory and coding features of a human DNA sequence in an interactive activity and relate these features to what they have learned about gene expression.
- Describe the semi-conservative nature of DNA replication
- Describe how cells divide through the process of mitosis
- Use the terms DNA, RNA, protein, gene, nucleotide, and codon to accurately describe the role of the genetic code in protein synthesis
- Explain how transcription and translation serve the process of protein synthesis
- Use the genetic code table to translate an RNA sequence into an amino acid sequence
- Explain how a change in DNA sequence can alter the function of a protein
- Relate dominance and recessiveness to the functional state of a protein
Grade Level: 9-12
- Four to five 45-minute class periods
- How DNA Replicates QuickTime Video
- Mitosis QuickTime Video
- Cell Transcription and Translation Flash Interactive
- Some Genes Are Dominant Flash Interactive
- Explore a Stretch of Code Shockwave Interactive
- DNA Double Helix PDF Document
- Circular Genetic Code Table PDF Document
- Sequence Analysis Using the Genetic Code Table PDF Document
- Sequence Analysis Using the Genetic Code Table (Teacher Version) PDF Document
Before the Lesson
- If possible, arrange computer access so all students can work individually or in pairs.
- Make copies of all PDFs for each student or group of students.
- Prepare transparencies of the DNA double helix and the genetic code table or make them otherwise available to students.
Part I: DNA Replication
1. Draw on the board or display an overhead transparency of a diagram of a simple cell. Ask students, "Where in the cell is the genetic information that encodes the organism's traits?" The goal is to access students' understanding of DNA: that it exists, that it is located in the nucleus, and that it holds the genetic information that encodes an organism's traits. Once the discussion is underway, you may also want to display an overhead transparency of the DNA Double Helix PDF Document, or have students view a DNA diagram from their textbook.
2. Write on the board or display an overhead transparency with the following quote from the scientific paper published by James Watson and Francis Crick in Nature (April 25, 1953) in which they described the double helix structure of DNA:
- "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material."
Discuss with the class their ideas about what this statement means, drawing on their understanding of DNA processes from the earlier discussion.
3. Have students view the How DNA Replicates QuickTime Video. Then relate the video to the statement by Watson and Crick. Ask students to discuss why they think this type of replication is called "semi-conservative." Help them reach the understanding that it is because it produces DNA molecules that have one new strand and one old strand.
4. Have students view the Mitosis QuickTime Video. After they have watched it, conduct a guided class discussion to assess students' abilities to relate DNA replication to cell division. Discuss the following questions:
- At what point during the process of mitosis does DNA replication take place?
- What would happen if a cell divided without first replicating its DNA?
Part II: Transcription and Translation
5. Write on the board or display an overhead transparency of the so-called central dogma of biology: DNA _ RNA _ Protein. Briefly discuss with the class the flow of genetic information in the cell. Ask students, "Why do you think that an RNA copy of the gene is made, which then leaves the nucleus and goes to the cytoplasm to direct protein synthesis?" If necessary, bring out the fact that DNA remains in the nucleus where it is protected.
6. Go to the Cell Transcription and Translation Flash Interactive. Have students select the Overview tab and work through the activity screens. (Note: You may also want to have students read the background essay for additional information about this process.) Then ask students to answer the following questions in their notebooks:
- Why must transcription take place in the nucleus?
- What is the copy of the gene that moves to the cytoplasm called?
- During the process of translation, what is being translated into what?
7. Next, have students explore the process of transcription more deeply by selecting the Transcription tab and working through the related activity screens. (Note: You may also want to have students read the background essay for additional information about this process.) Students should use this information to revise their answers to the questions posed in Step 6. After they have completed the interactive activity, have some students report their findings to check for any misunderstandings. You may want to ask students the following questions to flesh out their understandings:
- Describe three regions of a typical gene that are associated with transcription.
- During transcription, which enzyme reads the DNA code and produces mRNA?
- How does the RNA code differ from the DNA code?
8. Now have students explore the process of translation more deeply by selecting the Translation tab and working through the activity screens. (Note: You may also want to have students read the background essay for additional information about this process.) After they have viewed the resource, have students report their findings. Discuss the following questions:
- What type of molecule in the cytoplasm reads the mRNA during protein synthesis?
- What is an RNA codon?
- How does the ribosome know where to begin reading the mRNA?
- What is the function of the tRNA?
- What type of chemical bond links the amino acids together?
- How does the ribosome know when to stop reading the mRNA?
9. Conclude this activity with a brief class discussion that summarizes the processes of DNA replication, transcription, and translation. Students sometimes confuse these processes, especially transcription and translation. Bring out in the discussion that to transcribe means to make a copy of something, while to translate means to go from one form of expression (language) to another—specifically, from nucleic acids to amino acids.
Part III: The Genetic Code
10. Discuss with the class what they think is meant by the genetic code. Be sure that students understand that the genetic code refers not to DNA itself, but to the relationship and processes that allow a specific DNA sequence to determine a specific amino acid sequence. Remind them also that these amino acid sequences form proteins and that proteins influence the collection of traits an organism has.
11. Explain to students that a monk named Gregor Mendel was one of the first to systematically observe traits that resulted from genetic variations. Although Mendel knew nothing about DNA, his studies of inheritance patterns in pea plants led him to conclude that traits were determined by factors (now known to be the genes) that are passed from one generation to the next. Ask students to think about what kind of differences in these factors (genetic variations) would affect certain pea plant traits, such as color or height. Have students offer some predictions about how many genes are involved in such variations.
12. Have students view the Some Genes Are Dominant Flash Interactive. Then discuss the following questions:
- What is the relationship between genes and alleles?
- How do the gene variations Mendel studied influence the traits of pea plants?
- How might protein synthesis play out differently as a result of genetic variations?
Students should understand that each of the traits Mendel studied was controlled by one gene that occurred in two contrasting forms, called "alleles." For example, the gene for seed color occurs in one form that produces green seeds and one form that produces yellow seeds. The gene for plant height occurs in one form that produces tall plants and one that produces short plants. Scientists have since discovered that the allele for the tall peas produces an enzyme (a type of protein) that allows the plant to respond to a growth hormone. The allele for the short peas produces an enzyme that is just 5 percent as active as that for tall plants. The exercise that follows allows students to trace the steps from DNA to protein to see how these differences are possible.
13. Provide each student with a copy of the Sequence Analysis Using the Genetic Code Table PDF Document. Explain that these short sequences of code represent alleles of the gene that controls the height of pea plants. The codes listed are only a partial segment of a full gene sequence, and do not contain the start codon or stop codon. Then display an overhead transparency of the Circular Genetic Code Table PDF Document, or distribute a hard copy of the table to students. Remind students that during transcription, the nucleotide uracil (U) is substituted for thymine (T) in the mRNA sequence, and that during translation, the cell uses triplet codons in the mRNA molecule to specify the amino acid sequences for particular proteins. Also remind students that the ribosome begins reading the mRNA at the position of the first AUG codon. Ask students to use the genetic code table to determine which amino acid AUG corresponds to. (methionine)
14. Instruct students to supply the missing mRNA sequence for Allele 1 in the space provided, and then use the genetic code table to supply the missing amino acid sequence for Allele 1 based on the mRNA codons. As students work, verify that they have correctly identified the mRNA sequence (GGU AAA GCU CCU), and have correctly used the genetic code table to obtain the amino acid sequence: Gly, Lys, Ala, Pro.
Next, instruct students to supply the missing mRNA sequence for Allele 2 in the space provided, and then use the genetic code table to supply the missing amino acid sequence for Allele 2 based on the mRNA codons. Verify that students have correctly identified the mRNA sequence (GGU AAA ACU CCU), and have correctly used the genetic code table to obtain the amino acid sequence: Gly, Lys, Thr, Pro.
15. When students are finished, have a class discussion about what they found as they performed the sequence analysis on the two alleles. Lead students toward the observation that Allele 1, which results in tall peas, produces a protein that contains the amino acid alanine, while Allele 2, which results in short peas, produces a protein that instead contains the amino acid threonine in that position.
Finally, discuss the following questions:
- What is the phenotype of a pea plant that is heterozygous for the stem length trait?
- Why is the tall allele dominant over the short allele?
Check for Understanding
The Explore a Stretch of Code Shockwave Interactive displays 12,587 bases of DNA sequence from human chromosome 1. Instruct students to use the interactive activity to obtain and record in their notebooks information about:
- On and off switches
- Start and stop codons
- Introns and exons
- Hitchhiking and ancient codes
- Sites of variation
Have students describe in their notebooks how each of the above terms relates to the expression of genetic information, especially with regard to transcription and translation. When students are finished, discuss these relationships as a class.