Professional Development AP® Biology From Gene to Protein— A Historical Perspective Curriculum Module The College Board The College Board is a not-for-profit membership association whose mission is to connect students to college success and opportunity. Founded in 1900, the association is composed of more than 5,700 schools, colleges, universities and other educational organizations. Each year, the College Board serves seven million students and their parents, 23,000 high schools, and 3,800 colleges through major programs and services in college readiness, college admission, guidance, assessment, financial aid and enrollment. Among its widely recognized programs are the SAT®, the PSAT/NMSQT®, the Advanced Placement Program® (AP®), SpringBoard and ACCUPLACER.
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Visit the College Board on the Web: www. 3 Lesson 1: Protein vs. 5 Lesson 2: The Watson and Crick Model of DNA. 15 Lesson 3: Replication of DNA.
19 Lesson 4: Transcription—DNA→RNA. 29 Lesson 5: Translation—DNA→RNA→Protein. 33 Lesson 6: Gene Regulation—the Operon Model. 41 Alignment with AP® Exam Questions.47 About the Contributors.
Zedalis The Bishop’s School La Jolla, California Jamie A. Kelso The Bishop’s School La Jolla, California A “Big Idea” in biology is that living systems store, retrieve, transmit, and respond to information critical to life processes. Heritable information provides for continuity of life, and the storage and transfer of this information are necessary for life to continue. In most cases, this information is passed from parent to offspring via deoxyribonucleic acid, or DNA.
This double-stranded molecule provides a simple and elegant solution for the transmission of heritable information: By using each strand as a template, existing information can be preserved and duplicated with high fidelity. For information in DNA to direct cellular processes, it must be transcribed (DNA→RNA) and translated (RNA→ polypeptide). The protein products determine the metabolism and thus the cellular activities and phenotypes upon which evolution operates. Although all cells of an organism contain the same complement of DNA, some genes are continually expressed, whereas expression of others is regulated to allow more efficient energy utilization and increased metabolic fitness for the organism.
Gene expression is controlled by environmental signals and developmental cascades that involve both regulatory and structural genes. But how do we know what we know? What scientific evidence supports the claim that DNA is the molecule of heredity? What key experiments allowed scientists to conclude that DNA is able to store, retrieve, and transmit information necessary for living systems? Why do changes in genotype result in changes in phenotype? Why do gene regulatory mechanisms in bacteria provide useful tools for modeling control systems in eukaryotes? This Curriculum Module asks students to explore these questions and many more as they trace the pathway from gene to protein from a historical perspective. 1 A Curriculum Module for AP Biology This Curriculum Module begins with students drawing their own conclusions from the experiments of Frederick Griffith and those of Alfred Hershey and Martha Chase that identify the source of genetic information. Next, students read an original article describing the proposed model of the structure of DNA by James Watson and Francis Crick, which was published in Nature; the students then construct their own model through a formative assessment.
Students extract DNA from living cells and make observations about its chemical makeup. Using data gleaned from experiments by Mathew Meselson and Franklin Stahl, students will create visual representations that describe the process of DNA replication. They build on the “one gene–one polypeptide” concept to model transcription and translation and also use biotechnology to induce a new phenotype in bacteria. Finally, students will examine the lac and trp models of regulation of gene expression in bacteria to model control systems in eukaryotes.
The authors chose the information and approach based on the organizing principles of Big Ideas and Enduring Understandings that provide depth of study. Peppered among the instructional strategies are activities designed to help students answer the question, “How do we know what we know about DNA?” with “This is why we know what we know.” Each learning environment is different. Each school has its own mix of students with different abilities and interests. Each classroom is different, even among AP® Biology courses with common and required elements of content and skill.
In all cases, students learn best by doing. The lessons presented in this module are intended to be used as strategies or guides, and each teacher should tap into his or her own expertise to make the content rich, engaging, challenging, relevant, and unique within the curriculum and cognitive frameworks. The instructional activities are examples of how teachers can engage students by accommodating their different learning styles, knowledge bases, and abilities and, at the same time, provide depth of content and skills. All activities are inquiry based and serve to make the course less teacher focused and more student driven.
Sample AP Biology Exam questions pertaining to the module are also included. 2 Prerequisite Knowledge Biochemistry Understanding biological processes at the molecular level allows students to study biology at a deeper, more conceptual level. The relationship between structure and function (a key theme in biology) begins at the molecular level. Carbon-based molecules—carbohydrates, lipids, proteins, and nucleic acid—make up the bulk of organic matter essential to living systems.
The structure and function of polypeptides, especially enzymes, should be reviewed in some inquiry-based manner (e., teacher-provided questions or student- generated visual representations) because the processes of DNA replication, transcription, and translation are enzyme catalyzed. Additionally, enzymes and proteins also play important roles in gene regulation. In this Curriculum Module, the study of the chemical makeup of DNA precedes the study of how it works. Energy and Metabolism The nature of the transfer of energy in living systems is a fundamental theme in Advanced Placement® study.
For example, the concept of energy coupling of catabolic (exothermic) and anabolic (endothermic) processes—as well as the role of nucleoside triphosphates, such as ATP and GTP—allows students to explore DNA beyond Watson and Crick. The role that these processes play should be relatively familiar to students if teachers follow a sequence put forth by information in their textbook, but the processes should be reviewed through an examination of diagrams such as ATP recycling and coupling of reactions. This foundation will allow students to appreciate the anabolic processes of DNA synthesis (replication), transcription, and translation. It is strongly suggested that students perform AP LAB 2: Enzyme Catalysis in the AP Lab Manual.
3 A Curriculum Module for AP Biology Mitosis In eukaryotic organisms, heritable information is packaged into chromosomes that are passed from one generation of cells to the next. Mitosis provides a mechanism that ensures each daughter cell receives an identical and complete set of chromosomes; thus, mitosis ensures fidelity in the transmission of heritable information. Additionally, mitosis allows for asexual reproduction of organisms in which progeny are genetically identical to the parental cell. Since chromosomes duplicate in mitosis, their chemical constituent, DNA, also must be able to replicate.
Students will ultimately link DNA replication to the behavior of chromosomes during mitosis. Meiosis Sexual reproduction of diploid organisms involves the recombination of heritable information from both parents through fusion of gametes during fertilization. Meiosis produces haploid gametes and increases genetic variation through random assortment of maternal and paternal chromosomes and random exchanges between homologous chromosomes. Meiosis followed by fertilization provides a spectrum of possible phenotypes for natural selection and evolution.
DNA provides the genotype, while translation of its information into polypeptides provides the phenotype. Ultimately, students will link DNA replication to the behavior of chromosomes (duplication) observed in the first stage of meiosis. Mendelian Genetics Some traits (phenotypes) are products of action from single genes. Single gene traits provided the experimental system through which Mendel described a model of inheritance from parent to progeny.
Mendel’s “factors” have been identified as genes— or discrete sequences of DNA with information to produce polypeptides. Students must make connections between Mendel, mitosis and meiosis, DNA, and phenotype. Furthermore, the principles of Mendelian genetics can be applied to many observable phenotypes, including human genetic disorders and the ethical, social, and medical issues surrounding them. 4 Lesson 1: Protein vs.
DNA Plan the Lesson Connections to Course Outline The content of this lesson addresses the following areas of the AP Biology course outline: • Big Idea: Living systems store, retrieve, transmit, and respond to information critical to living systems. • Enduring Understanding: DNA, not protein, is the source of heritable information. Objectives • Students investigate two key historical experiments to identify the source of genetic information: protein or DNA. • Students are able to analyze data provided by Frederick Griffith and draw conclusions about the discovery of an unknown “transforming factor” in bacteria.
• Students are able to analyze data provided by the Hershey–Chase experiments with bacteriophage T2 and draw conclusions supporting the identification of nucleic acid, not protein, as the source of genetic information. • Students extract DNA from living cells and make observations about its chemical makeup. Common Student Misconceptions Students, like scientists of the early twentieth century, have difficulty distinguishing between the “language of DNA” and the “language of proteins.” Both contain information, but only the information contained in DNA sequences of nucleotides is passed from parent to offspring. Additionally, students often confuse the double helix of DNA with the alpha helix secondary structure of protein.
With respect to the chemical components of DNA and protein, students might want to include both sulfur and phosphorous; sulfur is a component of proteins containing the amino acid cysteine, while phosphorous is a component of nucleic acids. 5 A Curriculum Module for AP Biology Protein or DNA? How did scientists discover the source of heritable genetic information? What evidence supported the theory that nucleic acid enables living systems to store, retrieve, and transmit information critical to life processes from one generation to the next? By asking questions, teachers can engage students to seek answers. In the first two instructional activities, students examine the work of Griffith and the Hershey–Chase experiments in the quest to identify the source of genetic information. In the third activity, students extract DNA from living cells and make observations about its chemical makeup.
Teach the Lesson Instructional activity I: Griffith’s experiments This first activity asks students to examine the work of Frederick Griffith and draw conclusions about how his research helped identify the source of genetic material. Students are asked to address the inquiry question below and answer the questions provided in the experimental analysis. Students may work in pairs or small groups. Instructional time is approximately 20 minutes.
Background Information Once T. Morgan and his co-researchers showed that Mendel’s traits (genes) are located on chromosomes, the two chemical components of chromosomes—DNA and protein—became the candidates for the genetic material. Until the 1940s, the case for protein seemed more likely (Campbell and Reece 2005, 293). Why? Because scientists had previously discovered that polypeptides contain a “language” based on a 20-letter amino acid alphabet from which myriad proteins could be synthesized.
Furthermore, little was known about the structure and function of nucleic acids.