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SEGMENT ONE: NUCLEIN = DNA
This segment introduces students to the first vestiges of DNA extraction, thanks to the work of Swiss biochemist Friedrich Miescher (1844–1895). Working in a German laboratory, Miescher was the first scientist documented to have extracted DNA from white blood cells. Meischer called the material he found, rich in nitrogen and phosphorus, “nuclein”; it was, in fact, DNA. Students are also given an introduction to the basic molecular structure of nucleotides.
Keywords
adenine nucleotide cytosine ribonucleic acid (RNA) deoxyribonucleic acid (DNA) thymine guanine uracil nuclein
Learning Objectives
Students will:
- Identify Swiss biochemist Friedrich Miescher’s contribution to the history of genetics.
- Define Miescher’s gummy substance, which he called “nuclein,” by its proper name, deoxyribonucleic acid.
- Identify the five nitrogenous bases (A, C, G, T, U).
Pre-Viewing Activity
Day One: Distribute to students the KWL: What I Know, What I Want to Know, What I Learned, Where I Learned It activity sheet (SL-1a). Using the handout as their recording page (SL-1b), students will first examine their own knowledge base about
The DNA Obsession^1
The DNA
Obsession
This episode focuses on both the biological and chemical processes central to the transfer of genetic material. The story starts in the middle of the nineteenth century and turns into a mad race in the late 1940s. As in a crime novel, the main characters—scientists in Europe and America — zero in on their prime suspect, the DNA molecule. They are sure that the structure of DNA is the key to understanding the transfer. The rivalries and collaborations among a handful of scientists heat up as they compete to be the first to determine the actual structure of the molecule. In the end, James Watson, a brash young biologist, and Francis Crick, a post-graduate physicist, discover the now famous double-helix structure of the DNA molecule.
Lesson Planner
Day 1: Pre-viewing activity: DNAKWL View Segment 1 Post-viewing activity: Word Splash! Day 2: View Segment 2 Post-viewing activity: DNA Extraction Lab (or DNA White- Slimy Lab) Day 3: View Segments 3 & 4 Home or group work: The Name of the Game
National Science
Education Standards
Content Standard B, Scientific Inquiry New ideas in science usually grow from the contributions of many investigators.
Content Standard C, The Molecular Basis of Heredity Mutations, cell differentiation.
deoxyribonucleic acid, and then will work together as a full class to augment that knowl- edge. This activity introduces the topic of study by accessing prior knowledge and gaining attention to the topic. The final aspect of this activity has the students themselves offering ideas on how to garner the answers to ques- tions they have about DNA. Teachers may keep the KWL charts to use at a later date as an assessment tool.
Viewing Activity
PLAY through the first segment of the episode, directly from the musical opening through to the conclusion of the animated ses- sion depicting the computer keyboard and monitor showing the nitrogenous bases.
PAUSE when the molecule is depicted full screen and the words “Acid, Sugar, and Base” are visible—at the point where the narrator says, “... with a sugar in the middle, and a phosphorus-containing acid or phosphate on one side, and nitrogen containing base on the other.”
Discussion Point This is an ideal time to elaborate on the differences between DNA and RNA.
PLAY until the graphic shows both DNA and RNA.
PAUSE here for review and to check for stu- dent comprehension.
STOP tape.
Discussion Point Review the five nitrogenous bases.
Post-Viewing Activities
Word Splash! “Splash” refers to a haphazard arrangement of key words relating to the lesson content. Students scrutinize words posted in a disordered form and then try to make a rela- tionship from all the words. This could be done on paper, on a blackboard, on the wall, or in presentation software such as PowerPoint™.
The Word Splash! Tactic can be employed at any time within a learning experience: before- hand, to determine pre-existing knowledge; during a lesson, to reinforce new or revisited material; or afterward, to assess information retention.
A variety of “report-out” techniques may be employed, such as pair-share, a graphic repre- sentation, general class discussion, or, as sug- gested in this lesson, writing the story. Students will compose a paragraph or two using the list of keywords provided at the beginning of Segment Two, below. Teachers should add key- words that best fit their students’ level of study. The end result of student-written paragraphs represents a tangible assessment of the stu- dents’ comprehension. Timing is important in this task; the activity is usually limited to three to four minutes.
SEGMENT TWO: PRIME SUSPECT,
DNA
Protein was first considered to be the possible source of heritable (genetic), trait transfer because of its complexity of 20 amino acids, as compared to the simplicity of only the four nitrogenous bases of DNA. But, the various events depicted in this segment brought DNA into the spotlight instead as the important stuff of the gene: Frederick Griffith’s studies of the strains of the bacteria, pneumococcus, and a transformation factor; Oswald Avery’s discovery of DNA as that transformation factor; and Joshua Lederberg’s experiments in the ability of bacteria to exchange genetic information as they reproduce. This segment epitomizes the synergism often occurring in scientific endeavor and discovery—scientific exploration does not happen in a vacuum.
Keywords
amino acids genetic material inheritance protein transformation factor
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2 Cracking the Code: The Continuing Saga of Genetics
ble helix. It closes with Moxy Früvous’s musical review of the fundamental material.
It is recommended to view the complete seg- ment once, without pausing. The flow of the time line is essential to student understanding. To reinforce the information gleaned in the ini- tial viewing, utilize the strategy of REWIND and REPLAY. After viewing the entire segment without pausing, view it a second time, this time with pauses.
PLAY until the animated sequence depict- ing the DNA molecule in x-ray crystallography appears, and the X shape fills the screen. PAUSE the tape here.
Discussion Point Point out to your students that the visual arts as well as pure science played a role in this discovery. Discuss the importance of patterns and the art form of photography; it becomes apparent that the humanities can play a strong role in science. Challenge stu- dents to engage in a discussion of other ways in which the arts or humanities interact, such as the double role of DaVinci as an artist and scientist or the use of computer graphics to investigate forensic evidence.
PLAY until the end of the animation depict- ing the nitrogenous base pairs and their shapes; PAUSE the tape.
Discussion Point Take this opportunity to make sure that stu- dents fully comprehend the differences between purines (adenine and guanine) and pyrimidines (cytosine, thymine, and uracil).
PLAY until you hear Witkovski say: “They were the right people in the right place at the right time.” PAUSE.
Discussion Point This quote offers a tremendous opportunity for teachers to foster discussion of the nature of discovery and scientific endeavor. Ask stu- dents how many scientific breakthroughs
they think may have been the result of serendipity.
SEGMENT 4: SPLIT AND COPY
This short segment discusses how James Watson and Francis Crick publicized their dis- covery; also, how the structure of DNA revealed the “copying mechanism” of genetics, and how large a role obsession, rivalry, and collaboration can play in advancing human knowledge.
Viewing Activity
PLAY this segment through to the end. This segment is brief and affords no specific pause points within it. The conclusion offers a good opportunity for class discussion. STOP tape.
Discussion Point Can you think of other discoveries or inno- vations that were made because of a person or group’s obsession or rivalry? (Possible answers: The space race between the United States and the Soviet Union; the Bill of Rights made to address the issues between Federalists and states-rights advocates; the Firefox Internet browser made to compete with rival Microsoft Internet Explorer; wire- less WiFi Internet connections made to com- pete with DSL, ISDN, and cable Internet; digital satellite TV made to compete with cable TV.)
Post-Viewing Activity
The Name of the Game. Two supplementary handouts are included at the end of this lesson: a Teacher Lab Packet describing the activity The Name of the Game (TL-4a), and the student handout for the same activity giving clear instructions and a rubric (SL-4a, 4b, and 4c). This activity pulls together all the threads pre- sented in this episode, woven into the tapestry that is the story of the double helix. Refer to the DNA time line and biographies of key sci- entists found at www.geneticstv.org/ dna_obsession/activities.htm. Alternate activi- ties on the Web site are good preparation for students to complete the Name of the Game assignment.
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4 Cracking the Code: The Continuing Saga of Genetics
Suggested Reading
The Double Helix: A Personal Account of the Discovery of the Structure of DNA by James D. Watson (London: Weidenfeld & Nicolson, 1968)
This is Watson’s personal account of the race against time, professional rivalries, bureaucratic red tape, and ethics which all played a role in the momentous discovery of the double helix. ISBN: 074321630X
Gregor Mendel and the Roots of Genetics (Oxford Portraits in Science) by Edward Edelson (New York: Oxford University Press, 2001)
When Gregor Mendel died in 1884, not a sin- gle scholar recognized his epochal contribu- tions to biology. Twentieth-century scientists were stunned to learn that their findings about inheritance had already been made by Mendel three decades earlier. In an informed narrative, Edelson provides an inspired account of what a modest man can accomplish with dedication and ingenuity. ISBN: 0195150201 (paper)
Rosalind Franklin: The Dark Lady of DNA by Brenda Maddox (New York: HarperCollins,
This is a powerful story of a remarkably single- minded, forthright, and tempestuous young woman who, at the age of 15, decided she was going to be a scientist, but who was airbrushed out of the greatest scientific discovery of the 20th century because she died before the Nobel Prize was awarded. ISBN: 0060184078
National Science Education
Standards
http://nap.edu/readingroom/books/nses/html
Content Standard B: Scientific Inquiry There are different traditions in science about what is investigated and how, but they all have in common certain basic beliefs about the
value of evidence, logic, and good arguments. And there is agreement that progress in all fields of science depends on intelligence, hard work, imagination, and even chance.
New ideas in science are limited by the context in which they are conceived; are often rejected by the scientific establishment; sometimes spring from unexpected findings; and usually grow slowly, through contributions from many investigators.
Content Standard C: The Molecular Basis of Heredity In all organisms the instructions for specifying the characteristics of the organism are carried in DNA, a large polymer formed from subunits of four kinds (A, G, C, and T). The chemical and structural properties of DNA explain how the genetic information that underlines hered- ity is both encoded in genes (as a string of molecular “letters”) and replicated (by a tem- plate mechanism). Each DNA molecule in a cell forms a single chromosome.
Links
BBC NEWS: Decoding Humanity http://news.bbc.co.uk/hi/english/static/ in_depth/sci_tech/2000/human_genome/ timeline/default.stm
Time line of the race for the Human Genome Project to decode human DNA. (BBCi)
DNA from the Beginning http://www.dnaftb.org
An interactive history of genetic science with biographies, animations, puzzles, and more.
DNA Interactive http://www.dnai.org/index.html
An animated journey through DNA history and science. (Dolan DNA Learning Center at Cold Spring Harbor Laboratory).
The DNA Obsession^5
The DNA Obsession^7
WATSON AND CRICK SONG
Adenine, thymine, cytosine, guanine, oh Adenine, thymine, cytosine, guanine, oh Watson and Crick, Pulled off a very slick trick, When they unlocked the secret Of the gene. The facts they Knew were quite few, Just the occasional clue, Pointing to DNA, Not protein. But what was its configuration. How did it store information, And allow for Duplication With only four nuc-le o-tides Only four nuc-le o-tides How they fit together must be where the secret hides. Adenine, thymine, cytosine, guanine, oh Then using new facts, Learned when an x-ray diffracts, The pieces started to fall Into places. (hey) They began to play, With models that could display, A way to form DNA, By linking bases. Then they had their inspiration. Let's give them a big ovation For proposing
DNA
Was built just like a spiral staircase. They proposed that All our genes Are built just like a spiral staircase, Built just like a spiral staircase, Each step made up of a double base. Adenine, thymine, cytosine, guanine, oh It’s called a double helix, Watson said to Crick.
PART I: DNA KWL
(What I K now, What I W ant to Know, What I L earned, Where I Learned It)
The purpose of this KWL is to activate your prior knowledge about deoxyribonucleic acid, DNA. You will first spend about five minutes on your own, writing down anything that comes to mind with the prompt, DNA. Think of this as personal brainstorming.
(1) 5-minute think time, writing about what you know You’ll find on SL-1b a page divided into four columns, asking you to list what you already know about DNA, what the class knows, what you should know, and how you can find the answers to the questions you have. In this five-minute opening activity, you will fill in Column I: “What do I know about DNA?”
(2) 10-minute, whole-class, chart writing Functioning as a whole class, you and your classmates will now provide a recorder, or scribe, with your ideas for Column II: “What does the class know about DNA?” Give yourselves about 10 minutes to contribute as much known information as possible.
(3) Instructor review and discussion of misconceptions... variable time allotment There may be some misconceptions, but your instructor will discuss these once the list is generated.
(4) 15-minute question generation Now a 15-minute block of time will be used to generate questions about DNA, providing the material for Column III: “What do we want to know about DNA?”
(5) Who, What, Where, When, Why, How? At this point there should be a class discussion of what steps need to be taken in order to find the answers to the class’s questions (the material to fill Column IV: “How can we find answers to our questions?” ).
Your chart and the class chart should resemble the figure below.
The DNA Obsession SL-1a
The DNA
Obsession
STUDENT LAB PACKET
Figure 1.2. A typical KWL chart used as a learning strategy and graphic organizer.
What do I know What does the class What do we want to How can we find about DNA? know about DNA? know about DNA? answers to our questions?
The DNA Obsession SL-2a
DNA EXTRACTION LAB
For this laboratory experience to be successful, a working biology lab is a prerequisite.
All cells contain DNA. Prokaryotic cells (bacteria) have no nucleus, so their DNA is not bound by a nuclear membrane. DNA is not capable of crossing the nuclear membrane, because it is too big to penetrate the nuclear pores. To free the DNA for observation requires destruction of membranes and, in the case of some organisms, cell walls.
Recall that cell membranes are composed of lipids and proteins, primarily lipids. What will break down or emulsify lipids? Those of you who have washed dishes should easily recall … right: deter- gent!
But there is yet another dilemma. Once the DNA is exposed to the cytoplasm, which contains all kinds of enzymes (proteins) capable of chewing up DNA, a second problem must be overcome. These proteins, DNA “shredders,” must be rendered inactive. The tool of choice is called a protease (PRO-tee-aze), a chemical capable of destroying protein enzymes.
Consider Fredrich Miescher’s use of the enzymes in the stomach of a pig. Miescher knew that stomach juices contain chemicals that break down proteins. In this laboratory learning experience, you will not be using pig stomach juices but an easily obtainable substitute for such. In other words, you will use a protein to destroy a protein.
How do bacteria prevent destruction of their DNA? Bacteria have one circular chromosome attached to their cell membranes. The ends of their DNA are not exposed to chemical onslaught, and special chemicals called methyl groups protect the other portions.
As you work through this laboratory experience, please be mindful of safety procedures. Wear gog- gles and handle all materials and glassware appropriately.
Materials per group (2–3 students)
- 250 mL beaker
- hot plate
- 1.5 grams of non-roasted (raw) wheat germ
- thermometer
- pH paper (range 5–9) or a pH meter
- 5 mL of detergent (Palmolive®) or any other clear variety
- test-tube rack (beaker or some container to hold a test tube at a 45˚ angle)
- baking soda
- 3 grams of Adolph’s®^ natural meat tenderizer
- 6 mL ice-cold 95% ethanol
STUDENT LAB PACKET
The DNA
Obsession
- 2 15-mL (small) test tubes
- glass stirring rod, or wooden skewer, or Pasteur pipette
- 100 mL of distilled or tap H 2 O
- graduated cylinders (10 mL and 100 mL)
- 9 mL of a 4% sodium chloride solution
- boiling water bath
- 9 mL diphenylamine solution
- 3 mL DNA standard solution
Protocol
- Place 100 mL of H 2 O in a beaker and heat to 50˚ to 60˚ C.
- Add 1.5 grams of raw wheat germ and stir until dissolved.
- Add 5 mL of detergent, maintaining the temperature at 50˚ to 60˚ C and stir constantly.
- Add 3 grams of meat tenderizer.
- Prepare a baking soda solution of 50 mL H 2 O and a teaspoon of baking soda in a separate beaker. Use this solution to bring the wheat germ mixture to a pH of ~8.
- Maintain the temperature at 50˚ to 60˚ C, stirring for another 10 minutes.
- Remove solution from heat and place 6 mL of wheat germ mixture into a test tube.
- Slowly and carefully pour 6 mL of the ice-cold ethanol down the inner edge of the test tube so it is layered over the wheat germ suspension.
- Allow the mixture to stand (undisturbed) for approximately five minutes. Observe the interface region for the appearance of DNA strands, and record your observations.
- Put your initials on and mass a small piece of filter paper. Record the mass. Using an eyedrop- per or Pasteur pipette, draw up the DNA from the alcohol layering and place on filter paper. Allow this to dry overnight. Reweigh the filter paper the next day. Calculate the DNA extracted per gram of wheat germ and record your calculation.
- Remove the DNA from the filter paper and place in a test tube, which contains 3 mL of 4% salt solution. Add 3 mL of diphenylamine solution. Label tube.
- Place 3 mL of the standard DNA solution into another test tube and add 3 mL of dipheny- lamine solution. Label tube.
- In a third test tube place 3 mL of 4% salt solution and 3 mL of diphenylamine solution. Label tube.
- Place all three tubes in a boiling water bath for 5 minutes. Record color changes over the time period. Diphenylamine reacts with deoxyribose (found in DNA) and produces a color in the blue range. It is a positive indicator for DNA presence.
SL-2b Cracking the Code: The Continuing Saga of Genetics
DNA—WHITE AND SLIMY
When discussing DNA, it is a good idea to have some familiarity with this important molecular stuff. Science is based on questioning and on observations, so let’s do a bit of both …
We will use some kitchen chemistry and a few easily obtainable materials to make observations and reflect on questions.
As you perform the steps in this learning experience, you need to record your observations and thoughts. Try to answer these questions:
The DNA Obsession SL-3a
STUDENT LAB PACKET
The DNA
Obsession
What’s in the wheat germ?
Why use warm or hot water?
Why use dish detergent?
Why use meat tenderizer?
Why do we need ice-cold ethanol?
Why do we tilt the tube?
What do the solutions look like?
What is a supernatant—or, how did it get its name?
Why did we need to use unroasted wheat germ?
Materials
- unroasted wheat germ (this can be purchased at a health-food store or co-op)
- dish detergent (clear is best)
- one wooden skewer (these are the wooden sticks used for barbecues)
- one clear plastic cup
- a glass or plastic test tube
- something to measure mL amounts of liquid
- Adolph’s®^ meat tenderizer (this brand works the best)
- a plastic pipette or eye dropper
- teaspoon
- ethanol (an alcohol) works best—you might try the drug-store alcohol used for a disinfectant
- warm (or hot) water
- a stirring implement
Procedure
- Place one to two teaspoons of unroasted wheat germ into a cup.
- Fill the cup 1 / 3 full of warm water (or you may try hot water) and stir for 10 minutes.
- Place 3–5 mL of dish detergent into the cup and stir for five minutes.
- Add 1 tsp of meat tenderizer and stir for five minutes.
- Let the slurry settle.
- Pour the supernatant into a test tube (about 1 / 3 full).
- Tip the tube at a 45˚ angle and slowly pour about 1 to 1^1 / 2 inches (remembering that 2.54 cm = 1 inch) of ice-cold ethanol down the side of the tube.
- Keep the test tube at an angle for about five minutes and make careful observations.
- Use the wooden skewer and attempt to wind the DNA.
SL-3b Cracking the Code: The Continuing Saga of Genetics
- Presentation of your game counts. It has to LOOK like it is fun to play. This involves wise use of color, graphics, and artistic design. Perhaps your board is three-dimensional, or physical movement is part of the game.
- Get it right. Make sure your data is correct.
- Get it balanced. A game with too much information in one area, while neglecting another, is not going to score well. For instance, providing too much data on Watson and Crick and not enough about the contributions of Friedrich Miescher is not a proper balance. The historical reality of correlation in research throughout history is not remarkable to the history of genet- ics; it represents history and science overall. Make use of these connections in your game. One example might be to have a “bridge” on the board: If a player lands on one specific spot—per- haps a spot depicting Miescher’s German laboratory where the discovery of nuclein occurred— he can cut short a long path of the game by jumping directly to a spot further along on the path/time line (such as the English lab of Griffth and his discovery of the transformation factor within the pneumococcus bacteria).
- Have fun!
SL-4b Cracking the Code: The Continuing Saga of Genetics
Excellent Very Good Fair Poor Failure Good
Content
Accuracy of Material 5 4 3 2 1 0
Depth of Individual Information 5 4 3 2 1 0
Depth of Overall Time line 5 4 3 2 1 0
Game Awards and Penalties reflect synergisms (real-life associations) in history of genetics 5 4 3 2 1 0
Grammatically/ Structurally Error Free 5 4 3 2 1 0
Creativity
Game Design 5 4 3 2 1 0
Presentation 5 4 3 2 1 0
Attention to Detail 5 4 3 2 1 0
Marriage of Fact and Design 5 4 3 2 1 0
Meeting Specified Parameters 5 4 3 2 1 0
Best Possible Scores in each Category 50 40 30 20 10 0
Corresponding A+ (50–45) A (44–42) B (35–32) C (26–24) D (16–15) F (9–0) Letter Grade A- (41–39) B- (31–29) C- (23–20) D- (14–10) B+ (38–36) C+ (28–27) D+ (19–17)
The DNA Obsession SL-4c
“The Name of the Game” Rubric
