Chargaff Mystery Slides The DNA Files
Lesson 1: Analyzing Chargaff's Ratios
The Universal Code?
"If DNA is the universal code for life, why do different species have different amounts of chemical bases?"
Universal
A, T, C, G are in everything.
Unique
Species look different.
Variable
The proportions change.
Erwin Chargaff
In 1950, biochemist Erwin Chargaff published data on the chemical composition of DNA from various organisms.
His Big Question:
Is the amount of Adenine (A), Thymine (T), Guanine (G), and Cytosine (C) the same in every species?
EVIDENCE FILE #1
SPECIMEN ANALYSIS DATA
The Chemical Players
A
Adenine
Double-ring Purine
G
Guanine
Double-ring Purine
T
Thymine
Single-ring Pyrimidine
C
Cytosine
Single-ring Pyrimidine
Crack the Code
You are now a researcher in Chargaff's lab. Use the data in your worksheet to find the pattern that defines the structure of life.
Analyze the Data
Find the Ratios
Formulate a Rule
Chargaff Code Tracker Worksheet Chargaff's Code Tracker
Lab Investigation: DNA Base Ratios
Scientist:
Date:
The Mission: Below is a subset of Erwin Chargaff’s 1950 data. Your task is to analyze the percentages of the four nitrogenous bases in different organisms and discover the hidden pattern that governs DNA structure.
1 Primary Data Set
Organism % Adenine (A) % Thymine (T) % Guanine (G) % Cytosine (C) Homo sapiens (Human) 30.9 29.4 19.9 19.8 Zea mays (Corn) 26.8 27.2 22.8 23.2 Drosophila (Fruit Fly) 27.3 27.6 22.5 22.5 E. coli (Bacteria) 24.7 23.6 26.0 25.7 Yeast 31.3 32.9 18.7 17.1
1. Look at the data for Humans. Which two bases are closest in percentage?
2. Look at the data for Yeast. Which two bases are closest in percentage?
3. Does this pattern hold true across all species in the table? Explain.
2 Chargaff's Rule
Based on your observations, write a "Rule" for DNA base pairing:
4. Why do you think the percentages aren't exactly identical (e.g., 27.3% and 27.6%)? Consider human error or measurement limits.
5. If a new organism has 15% Guanine, use Chargaff's rule to predict the percentages of the other three bases.
C:
A:
T:
Structural Prediction
Chargaff discovered the "WHAT" (the ratios), but he didn't know the "WHY" (the structure). If A always pairs with T, and G always pairs with C, what does that suggest about how the two strands of a DNA molecule are held together?
Chargaff Teacher Guide Teacher Guide: Chargaff's Ratios
Lesson 1: The Chemistry of Unity
Target Level
10th Grade Biology
Instructional Strategy
This lesson is designed as a **Productive Struggle** inquiry activity. Instead of telling students that "A pairs with T," they will arrive at this conclusion through data analysis. This builds scientific literacy and mirrors the actual historical process of discovery.
The Facilitation Loop
1
The Hook:
Present the variety of life (plants, bacteria, humans) and ask how a "Universal Molecule" could allow for such differences.
2
Data Diving:
Pass out the tracker. Let students work in pairs. Do NOT give hints for the first 5 minutes.
3
Guided Synthesis:
As you walk around, ask: "Are any numbers suspicious close? Why aren't they perfect?"
Teacher Tips
COMMON MISCONCEPTION
Students often think the % of all four bases should be 25%. Emphasize that species have different amounts of G+C vs A+T, but the ratios within those pairs are fixed.
ANSWER KEY: Q5
If G=15%, then C=15% (30% total). Remaining 70% is A+T. A=35%, T=35%.
VOCABULARY
Purines (A, G)
Pyrimidines (T, C)
Complementary
Suggested Pacing (50 min)
0-10 min
Slide Presentation & Hook discussion.
10-30 min
Independent work on the Code Tracker worksheet.
30-40 min
Group sharing and derivation of "Chargaff's Rule."
40-50 min
Exit ticket: predicting ratios for a mystery organism.
Shadows of Life Slides Shadows of Life
Lesson 2: Interpreting Photo 51
The Hidden Molecule
"How can a blurry X-ray photograph reveal the shape of a molecule too small to see with a microscope?"
Think about a shadow puppet . You don't see the hand itself, but the shape of the shadow tells you what the hand is doing. X-ray diffraction works exactly the same way.
Rosalind Franklin
"Science and everyday life cannot and should not be separated."
Expert in X-ray Crystallography .
Produced the famous Photo 51 in 1952.
Determined the density and helical structure of DNA.
X
FILE: PHOTO_51
The most important photo in biological history.
How to Read an X-Ray Pattern
1
The 'X' Pattern
When waves hit a helix (corkscrew), they diffract into an 'X' shape. This proved DNA was a **Helix**.
2
The Spacing
The distance between the spots shows how often the helix turns. It's a precise **Mathematical Signature**.
3
The Missing Spots
Certain empty areas suggested the strands were **Doubled** and offset from each other.
Analysis Mode: ACTIVE
Analyze the Evidence
Open your "Shadows of Life" analysis packet. Use your ruler to measure the patterns and deduce the dimensions of the DNA molecule.
WARNING
This data led to the Nobel Prize. Accuracy is everything.
Shadows of Life Worksheet Shadows of Life
Primary Source Analysis: Photo 51
FILE NO: DNA-1952
Researcher: __________________________
Date: __________________________
DIFFRACTION_MODEL_V1
Simplified Photo 51
Technique: X-Ray Diffraction
"X-rays are shot through a crystallized DNA fiber. The atoms in the molecule deflect the rays, creating a pattern on film."
Step 1: Identifying the Helix
1. Look at the central "X" shape of the dots. In crystallography, an X-shaped pattern is a mathematical signature of which shape?
2. Notice the "empty spaces" in the diamond-shaped regions. Franklin noted these "missing" spots suggested that there are two strands intertwined. Why might two strands interfere with each other's diffraction spots?
3. The distance from the center to the outermost dots represents the width of the molecule. If this distance corresponds to 2.0 nanometers (nm), would you describe DNA as a wide molecule or a thin one relative to a cell (approx 10,000 nm)?
Step 2: Mathematical Clues
Franklin used the distance between the horizontal bars (dots) to calculate the pitch of the helix (the height of one full turn). She found it was 3.4 nm.
Deduction: If one full turn is 3.4 nm, and there are 10 base pairs per turn, what is the distance between individual base pairs?
4. Franklin's data also proved the "Backbone" (sugar and phosphate) was on the outside. Why is it chemically important for the water-loving phosphate groups to be on the outside in a watery cell environment?
The Legacy Question
James Watson and Francis Crick used Photo 51 (without Franklin’s direct permission) to build their famous model. Looking at this data, could they have discovered the double helix structure without Franklin’s photograph? Justify your answer based on the "clues" you found today.
Helix Blueprint Slides Helix Blueprint
Lesson 3: Constructing the Double Helix
The Builder's Challenge
"Can you build a structure that fits all the chemical rules we've discovered so far?"
Chargaff's Rule
A pairs with T; G pairs with C.
Franklin's Data
Double Helix; 2nm wide; backbone outside.
The Fundamental Unit
P
Phosphate
S
Sugar (Deoxyribose)
BASE
Nitrogenous Base
Together, these form a Nucleotide .
The "Upside Down" Rule
DNA strands run in opposite directions. This is called being Antiparallel .
Think of a highway : cars move in opposite directions, but they are parallel to each other.
5' → 3'
3' ← 5'
Construction Phase
1 Assemble Bases
Join your sugar and phosphate into two long backbone strands.
2 Pair Up
Connect the bases using Chargaff's rules (A-T, G-C).
3 The Twist
Twist your ladder into a right-handed helix. Does it hold together?
STATION WORK: BEGIN CONSTRUCTION
Helix Assembly Manual Worksheet Helix Assembly Manual
Engineering the Molecular Structure of Life
Blueprint ID: DNAMOD-03
Lead Engineer:
Lab Station:
Phase 1: Component Audit
Before building, verify you understand the "bonds" that hold the molecule together. In your model (using candy, paper, or plastic kits), you must represent three distinct types of connections.
1. Sugar-Phosphate Bond
Strong covalent bonds forming the "sides" of the ladder.
Audit Check:
Does your backbone feel sturdy?
2. Hydrogen Bonds (A-T)
Weak bonds between Adenine and Thymine. (2 bonds each).
Audit Check:
Represent these with 2 connectors.
3. Hydrogen Bonds (G-C)
Weak bonds between Guanine and Cytosine. (3 bonds each).
Audit Check:
Represent these with 3 connectors.
Critical Engineering Check: Antiparallel Orientation
In the box below, draw a simple "skeleton" of a 4-base-pair DNA strand. Use arrows to show the direction of the two sugar-phosphate backbones (5' → 3' and 3' ← 5').
Phase 2: Post-Construction Analysis
1. The "Stability" Question: Hydrogen bonds are weak. Why is it beneficial for the bases in the middle of DNA to be held together by weak bonds instead of strong covalent ones?
2. The "Storage" Question: How does the specific pairing of bases (A-T, G-C) allow DNA to act as a reliable "blueprint" that can be copied exactly during cell division?
3. Width Consistency: Purines (A, G) are 2-ring molecules. Pyrimidines (T, C) are 1-ring molecules. Based on your model, why must a Purine always pair with a Pyrimidine?
Structural Fact:
If two Purines (A+G) paired together, the DNA molecule would "bulge" out because they are too wide (4 rings across). If two Pyrimidines (T+C) paired, the molecule would "pinch" in.
Conclusion: Photo 51 showed a perfectly uniform 2nm width. Does your model match this?
Teacher Signature Required
Call your teacher over to demonstrate your model. Show them the antiparallel strands and explain one base-pairing rule.
Nucleic Acid Showdown Slides Nucleic Acid Showdown
Lesson 4: Comparing RNA and DNA
DNA
vs
RNA
The "Hard Drive" and the "Email"
"Why does life need two different types of nucleic acids to function?"
DNA
Long-term storage of genetic information. Permanent and stable.
RNA
Short-term transmission of messages. Temporary and flexible.
The Chemical Breakdown
1
Sugar Type
Deoxyribose
vs
Ribose
2
Strands
Double Helix
vs
Single Strand
3
The Bases
Thymine (T)
vs
Uracil (U)
The Mystery of Uracil
RNA uses Uracil (U) instead of Thymine (T).
Uracil is energetically cheaper to produce than Thymine. Since RNA is constantly being made and broken down, it's a cost-saving measure for the cell!
RNA Base Pairing
A
---
U
G
---
C
Stability vs. Speed
DNA: The Vault
The double helix and deoxyribose sugar are chemically stable. They can last for decades or even centuries in the right conditions.
RNA: The Sticky Note
The single strand and extra oxygen atom in ribose make RNA reactive and short-lived. This allows cells to turn off signals quickly.
Nucleic Acid Profile Worksheet Nucleic Acid Profile
Subject
DNA vs RNA Comparison
Version
1.0 Analysis
Investigator:
Date:
The Structural Map
Sketch DNA Double Helix Here
Sugar: ____________________
Shape: ____________________
Sketch RNA Single Strand Here
Sugar: ____________________
Shape: ____________________
Base-Pairing Comparison
Base DNA Partner RNA Partner Adenine (A) Guanine (G) Cytosine (C) Thymine (T) N/A Uracil (U) N/A
Synthesis Challenge
Explain the "Life Span" trade-off. If RNA is so unstable that it breaks down within minutes or hours, why doesn't the cell just use DNA for everything (including sending messages to the ribosomes)?
Helix on Trial Slides Helix on Trial
Lesson 5: Synthesizing Structural Evidence
Final Defense
The 1953 Competition
The Triple Helix
Proposed by Linus Pauling. Bases on the outside, phosphates in the middle.
Status: REJECTED
The Double Helix
Proposed by Watson & Crick. Bases inside, antiparallel phosphate backbones outside.
Status: ACCEPTED
Chemical Logic
Phosphates are negatively charged .
If you put them in the center (Pauling's model), they would repel each other and the molecule would explode!
(-) (-) (-)
Like charges repel.
Geometric Perfection
Photo 51 Evidence
The uniform width of 2nm only works if a Purine (wide) always pairs with a Pyrimidine (narrow).
Chargaff's Evidence
The 1:1 ratios of A to T and G to C only make sense if they are Hydrogen Bonded in the middle.
Conclusion: The Double Helix is the only structure that fits ALL the data.
Write the Defense
Using your "Helix Defense" packet, write a formal scientific argument (CER) explaining why the Double Helix model is the definitive structure of life.
CLAIM
EVIDENCE
REASONING
Helix Defense Paper Assessment The Helix Defense
Summative Assessment: Scientific Argumentation
Final Submission
Biological Sciences Unit 4: DNA Structure
Scientist Name:
Date:
The Writing Prompt
"In 1953, several models were proposed for the structure of DNA, including a triple helix with nitrogenous bases facing outward. Defend the Watson-Crick Double Helix model. Why is it the only structure that satisfies the chemical and physical constraints discovered by Chargaff and Franklin?"
CLAIM
What is your answer to the prompt?
Ex: The double helix structure is the only valid model because...
EVIDENCE
Cite data from Chargaff (ratios) and Franklin (Photo 51).
EVIDENCE PIECE #1 (Chemical Ratios):
EVIDENCE PIECE #2 (X-Ray Diffraction):
REASONING
Connect your evidence to your claim. Why does the data prove the structure?
Explain the chemical "Why": Why must A pair with T? Why can't phosphates be in the middle? Why is the antiparallel orientation necessary?
Mastery Checklist
Mentioned Base-Pairing Rules (A-T, G-C)
Mentioned Photo 51 (Helix Shape)
Explained Purine-Pyrimidine Width (2nm)
Explained Phosphate Repulsion in Triple Helix
