Discovery of DNA’s Structure

Essential Question

How did evidence from several scientists reveal the shape of DNA?

By the early 1900s scientists knew that traits are inherited, but not which molecule carries the instructions. The answer came in steps, from several scientists working in different places and years. In this lesson you follow the evidence in order: bacteria and mice, viruses, base measurements, X-ray patterns, and finally a model of DNA that fit all of it.

Clue 1: A Substance Can Change a Trait

• In 1928, Frederick Griffith injected mice with two strains of one kind of bacteria: harmless R bacteria and disease-causing S bacteria.
• Mice given live R bacteria lived. Mice given heat-killed S bacteria lived. Mice given heat-killed S bacteria mixed with live R bacteria died, and live S bacteria were found in them.
• Griffith inferred that some substance passed from the dead S bacteria into the R bacteria and changed them.
• In 1944, Oswald Avery, Colin MacLeod, and Maclyn McCarty tested which molecule did this. Destroying DNA stopped the change. Destroying protein did not.
• Their results pointed to DNA, but many scientists at the time were not yet convinced.

A substance could carry a trait from one bacterium to another. Avery, MacLeod, and McCarty’s tests pointed to DNA as that substance.

Clue 2: Viruses Inject DNA

• In 1952, Alfred Hershey and Martha Chase studied a bacteriophage, a virus that infects bacteria. The virus is made of only DNA and protein.
• They labeled the virus protein with radioactive sulfur (S-35), because protein contains sulfur and DNA does not. They labeled the virus DNA with radioactive phosphorus (P-32), because DNA contains phosphorus.
• After the viruses infected bacteria, a blender shook the virus parts off the outside of the cells. A centrifuge then separated the virus parts (supernatant) from the bacteria (pellet).
• Most of the S-35 label stayed outside the bacteria. Most of the P-32 label was inside them.
• The viral DNA, not the protein, had entered the bacteria and directed the making of new viruses.

Together with Avery, MacLeod, and McCarty’s results, this was strong evidence that DNA is the genetic material.

Clue 3: Chargaff’s Base Ratios

• Erwin Chargaff measured the amounts of the four bases (A, T, G, and C) in DNA from many different species in the late 1940s.
• In the DNA he measured, the amount of A was close to the amount of T, and the amount of G was close to the amount of C.
• The overall base composition was different in different species. The pattern A is about T and G is about C stayed the same.
• Any model of DNA had to explain this pattern.
• The bar graph shows sample values made for practice, in the same pattern as Chargaff’s results.

A matches T and G matches C in each sample, even though the samples differ in how much A and G they contain.

Clue 4: X-ray Patterns

• Maurice Wilkins and Rosalind Franklin, at King’s College London, studied DNA fibers with X-ray diffraction.
• Franklin and her student Raymond Gosling produced very clear X-ray patterns of DNA in 1951 and 1952. One of them is known as Photo 51.
• The X shape in a pattern like this is what a helix produces. Measurements from the patterns gave the width of the molecule and the length of one full turn.
• The diagram is a new, simplified schematic. It is not a copy of a real X-ray image.

X-ray evidence showed that DNA is a helix and gave its size.

The Double Helix Model

• In 1953, James Watson and Francis Crick built a model of DNA that fit the X-ray measurements and Chargaff’s base ratios.
• In their model, two strands wind around each other in a double helix. The sugar-phosphate backbones are on the outside and the bases are on the inside.
• A always pairs with T, and G always pairs with C. This pairing explains why the amounts of A and T match, and the amounts of G and C match.
• One full turn is 3.4 nm long and holds about 10 base pairs.
• Watson, Crick, and Wilkins shared the 1962 Nobel Prize in Physiology or Medicine. Franklin had died in 1958. Historians still discuss how the X-ray results reached Watson and Crick and how credit should be shared.

The model worked because it matched every kind of evidence: DNA carries the genetic information, it is a helix of a known size, and its bases pair A with T and G with C.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Which Part of the Virus Entered the Bacteria?

In a Hershey-Chase style experiment, a virus was labeled in two ways. After infection, a blender and a centrifuge separated the supernatant (virus parts outside the cells) from the pellet (bacteria). The table gives the percent of each label found in each part. These are illustrative values. Write a Claim, Evidence, Reasoning response: which molecule is the genetic material?

Label	% found in supernatant (virus parts)	% found in pellet (bacteria)
Protein (S-35)	80	20
DNA (P-32)	30	70

Show the solution

1. Claim: DNA is the genetic material, because it is the molecule that entered the bacteria.
2. Evidence: 70% of the DNA label (P-32) was in the pellet, compared with only 20% of the protein label (S-35). 80% of the protein label stayed in the supernatant.
3. Reasoning: The pellet holds the bacteria, so material found there entered the cells. The molecule that enters and directs the making of new viruses must carry the instructions. That molecule was DNA.
4. Limit: The separation was not perfect, since some of each label ended up in the other part. The data show a strong trend, not an all-or-nothing result.
5. Answer: DNA is the genetic material.

Example 2: Base Percentages in Three Samples

The table shows base percentages for three DNA samples (sample values for practice). Compare %A with %T and %G with %C in each sample, and compare the samples with each other. Write a Claim, Evidence, Reasoning response about the pattern and what it suggests about DNA structure.

Sample	% A	% T	% G	% C
Sample 1	31	30	19	20
Sample 2	24	25	26	25
Sample 3	28	29	22	21

Show the solution

1. Claim: In every sample, the amount of A is about equal to the amount of T, and the amount of G is about equal to the amount of C. This suggests that A is paired with T and G is paired with C.
2. Evidence: Sample 1: A 31 and T 30, G 19 and C 20. Sample 2: A 24 and T 25, G 26 and C 25. Sample 3: A 28 and T 29, G 22 and C 21. The samples differ from each other, for example A is 31% in Sample 1 and 24% in Sample 2.
3. Reasoning: If every A sits across from a T and every G sits across from a C in a double strand, then the amounts of A and T must match, and the amounts of G and C must match, even when the overall composition differs between samples. The small differences (1 percentage point) are what we expect from rounded measurements.
4. Check: in each sample A + G = 50 and T + C = 50.
5. Answer: A is about T and G is about C in each sample, which supports base pairing.

Example 3: Predicting the Fourth Mouse

Use the diagram of Griffith’s experiment. Looking at the first three treatments only, predict what a scientist would expect when a mouse is injected with heat-killed S bacteria mixed with live R bacteria. Then explain how the actual result changes the conclusion.

Show the solution

1. Live R bacteria alone are harmless, and heat-killed S bacteria alone are also harmless (the mouse lives in both cases).
2. If nothing passed between the bacteria, a mixture of two harmless things should also be harmless. Prediction: the mouse lives.
3. Actual result: the mouse died, and live S bacteria were found in it. Live S bacteria can only come from R bacteria that were changed.
4. Conclusion: a substance from the dead S bacteria changed the R bacteria, and the new trait was passed on to their offspring. Griffith’s experiment did not show which molecule it was.
5. Answer: Prediction: the mouse lives. Actual: it died, so a substance passed from the dead S bacteria to the R bacteria and changed them.

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Practice

Use the practice worksheet to match each scientist to their evidence, read data like a scientist, and explain how the evidence led to the double helix model. The answer key shows a full Claim, Evidence, Reasoning model.

• Discovery of DNA’s Structure Practice Worksheet (PDF)
• Discovery of DNA’s Structure Practice Answer Key (PDF)

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Lesson Slides

• 9.1 Discovery of DNA’s Structure Slides (PDF)

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Big Ideas

• Experiments by Griffith, Avery, MacLeod, McCarty, Hershey, and Chase showed that DNA carries genetic information.
• Chargaff found that A is about T and G is about C in DNA, and that base composition differs between species.
• X-ray patterns from Franklin, Gosling, and Wilkins showed that DNA is a helix and gave its dimensions.
• Watson and Crick built a double helix model, with A paired to T and G paired to C, that fit all of the evidence.
• Scientific knowledge builds on evidence from many people over many years.

Back to the essential question: Evidence from several directions came together. Transfer experiments showed DNA carries the information, base ratios showed a pairing pattern, and X-ray patterns showed a helix of a known size. The double helix model explained all three.

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