Evidence That DNA Is Genetic Material

Before the mid-20th century, most scientists believed protein was the hereditary molecule: proteins come in thousands of varieties, while DNA's four bases looked too monotonous to encode life. A series of experiments progressively narrowed the candidates until only DNA remained. The MCAT tests these experiments at the level of their logic and controls — what each one ruled in or out.

Priority labels: Must know = cold; Know the logic = mechanism not names; Passage-level = recognize, don't memorize; Optional = skippable.

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Why the Question Mattered

Must know

A genetic material must carry information stably, be faithfully copied, and direct an organism's traits. By the 1920s chromosomes were known to contain both protein and DNA (deoxyribonucleic acid); the debate was which one did the hereditary work. DNA's four bases seemed too simple, while proteins' 20 amino acids seemed far more capable — a misconception worth remembering, because the MCAT may ask you to evaluate these experiments from first principles.

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Griffith's Transformation Experiment

Must know

Frederick Griffith worked with two strains of Streptococcus pneumoniae:

• Smooth (S): has a polysaccharide capsule, virulent — kills mice.
• Rough (R): lacks the capsule, avirulent — mice survive.

Injection	Result
Live S	Mouse dies
Live R	Mouse lives
Heat-killed S	Mouse lives
Heat-killed S + Live R	Mouse dies

The fourth result was the surprise: live S bacteria were recovered from the dead mice. Something from the dead S cells had transformed harmless R cells into virulent S cells. Griffith called this the "transforming principle" but did not identify the molecule. The change was heritable — daughter cells of the converted R cells were also S — showing that a discrete chemical substance carries genetic instructions.

Quick check: Why was it important that the transformation was permanent (heritable) rather than temporary?
Answer: A temporary change could be explained as a physiological response. Permanence means the R cells acquired new hereditary information — a chemical substance carries genetic instructions.

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Avery, MacLeod, and McCarty

Must know

Avery and colleagues asked which molecule in the S-strain extract does the transforming. Their approach: selectively destroy each macromolecule class and see which treatment abolishes transformation.

Treatment of S extract	Transformation of R cells?
Whole extract	Yes
Protease (destroys protein)	Yes — protein not responsible
RNase (destroys RNA)	Yes — RNA not responsible
DNase (destroys DNA)	No — DNA is required

Conclusion: DNA is the transforming principle. Many scientists still doubted this, suspecting protein contamination in the DNA extract — which is why Hershey-Chase became so decisive.

Quick check: If protease treatment had eliminated transformation, what would that have meant?
Answer: It would have supported the protein hypothesis. Because protease had no effect, protein was ruled out.

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Hershey-Chase Experiment

Must know

Hershey and Chase used bacteriophage T2 to separate protein from DNA. When a phage infects a bacterium, something is injected that directs new phage production — protein coat or DNA? They exploited one biochemical fact: protein contains sulfur but no phosphorus; DNA contains phosphorus but no sulfur.

• Phages labeled with $^{35}$S → labels the protein coat
• Phages labeled with $^{32}$P → labels the DNA

After infection, the mixture was blended (to shear coats off the bacterial surface) and centrifuged. Bacteria pellet; lighter phage coats stay in the supernatant.

Label	Radioactivity found
$^{35}$S (protein)	Mostly supernatant (outside cells)
$^{32}$P (DNA)	Mostly pellet (inside cells)

$^{32}$P-labeled DNA entered the bacteria and directed new phage; the $^{35}$S protein coat stayed outside. Using a natural delivery system (viral infection) and clean physical separation, this experiment ended the protein hypothesis: DNA is the genetic material.

Quick check: What was the purpose of the blender step?
Answer: It sheared phage protein coats off the bacterial surface, so centrifugation could cleanly separate what was injected (inside the pellet) from the coats (in the supernatant).

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Chargaff's Rules: Chemical Support for DNA's Role

Must know

Erwin Chargaff analyzed DNA base composition across organisms:

1. Base pairing: $[A] = [T]$ and $[G] = [C]$.
2. Species specificity: the $\frac{[A]+[T]}{[G]+[C]}$ ratio varies between species but is constant within a species.

Rule 1 was a critical clue for the Watson-Crick model; rule 2 supported DNA as a carrier of species-specific information.

Worked example. A double-stranded DNA sample is 30% adenine. Then $[T]$ = 30%; the remaining $100 - 30 - 30 = 40\%$ splits equally, so $[G] = [C] =$ 20%. This calculation appears regularly on the MCAT. It applies only to double-stranded DNA — in single-stranded DNA or RNA there is no complementary strand to enforce $A = T$.

Quick check: A single-stranded RNA has 25% A, 35% U, 20% G, 20% C. Does this violate Chargaff's rules?
Answer: No. Chargaff's rules apply to double-stranded DNA only.

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X-Ray Crystallography and the Double Helix

Must know

Rosalind Franklin and Maurice Wilkins used X-ray crystallography; Franklin's diffraction image (Photo 51) revealed DNA as a helix with two antiparallel strands (one 5'→3', the other 3'→5'). The crystallographic mechanics are beyond MCAT scope — what matters is the conclusion: a regular, antiparallel double helix.

Watson and Crick combined Franklin's data with Chargaff's rules to propose the double helix (1953). The model explained:

• Replication: complementary base pairing lets each strand template a new one.
• Information storage: sequence of bases along the strand.
• Chargaff's rules: $A$-$T$ and $G$-$C$ pair via specific hydrogen bonds.

Quick check: How does the double helix support faithful copying?
Answer: Each strand is complementary to the other, so either can template a new partner with a predictable sequence — suggesting semiconservative replication.

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Meselson-Stahl Experiment: Semiconservative Replication

Know the logic

This experiment addresses how DNA replicates, not whether it is genetic, but it is canonically grouped here and validates the Watson-Crick model. E. coli was grown in heavy $^{15}$N until all DNA was heavy, then switched to light $^{14}$N; DNA density was tracked by $\ce{CsCl}$ density-gradient centrifugation.

Model	After 1 generation	After 2 generations
Conservative	Heavy + light bands	Heavy + light (more light)
Semiconservative	One intermediate band	Intermediate + light
Dispersive	One intermediate band	One band, lighter than intermediate

• Gen 1: a single intermediate band → rules out conservative.
• Gen 2: intermediate + light bands → rules out dispersive, confirming semiconservative replication (each daughter molecule keeps one parental strand and one new strand).

Quick check: Why doesn't the intermediate band at gen 1 alone distinguish semiconservative from dispersive?
Answer: Both predict a single intermediate band at gen 1. Gen 2 is the discriminator: semiconservative yields two distinct bands (some pure-light molecules appear), while dispersive yields one band that keeps shifting lighter.

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Why It Matters — and One Exception

Must know

Establishing DNA as the genetic material founds the central dogma (DNA → RNA → protein) and all of molecular genetics and biotech.

One exception, concept-level: DNA is not universal. Some viruses (retroviruses and other RNA viruses) use RNA as their genetic material. The classic experiments established DNA as the hereditary molecule in cellular organisms and DNA phages; the truly universal principle is that nucleic acids carry heritable information.

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Common Confusions & Tricks

Griffith ≠ identifying DNA. Griffith only showed a "transforming principle" existed; Avery, MacLeod, and McCarty identified it as DNA.

Hershey-Chase labels. DNA has a phosphate backbone → $^{32}$P; protein has sulfur-containing amino acids (cysteine, methionine) → $^{35}$S.

Chargaff's rules apply only to double-stranded DNA. For a single-stranded RNA or DNA sequence, $A \neq T$ (or $U$) is fine, not a violation.

Semiconservative vs. conservative vs. dispersive. Conservative = parental strands stay together; semiconservative = one old + one new per daughter; dispersive = old and new interspersed. Meselson-Stahl ruled out conservative and dispersive.

Photo 51 was Franklin's work. Franklin's crystallography gave structural data; Watson and Crick built the model; Chargaff provided base ratios.

Bacterial "transformation" ≠ oncogenic transformation. Here it means uptake of exogenous DNA, not a cell becoming malignant.

DNase destroys transforming ability; proteases do not. If a passage digests DNA and asks about transformation frequency, it drops to zero.

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Key Takeaways

• Griffith: heat-killed S + live R → virulent S in mice. A "transforming principle" exists; molecule unidentified.

• Avery, MacLeod, McCarty: selective enzyme digestion showed DNase alone eliminates transformation → DNA is the transforming principle.

• Hershey-Chase: $^{35}$S labels phage protein, $^{32}$P labels phage DNA. Only $^{32}$P (DNA) enters bacteria and directs new phage → DNA, not protein, carries genetic instructions.

• Chargaff's rules (double-stranded DNA only): $[A] = [T]$, $[G] = [C]$; base composition is species-specific. If %A is known, %T = same and %G = %C = $(100 - 2 \times \%A)/2$.

• Franklin / Watson & Crick: crystallography (Photo 51) revealed the helix; Watson and Crick proposed the antiparallel double helix with complementary base pairing.

• Meselson-Stahl: $^{15}$N/$^{14}$N labeling and $\ce{CsCl}$ density-gradient centrifugation demonstrated semiconservative replication.

• Logical progression: transformation (Griffith) → chemical identity (Avery) → viral injection confirms DNA (Hershey-Chase) → structure explains mechanism (Watson-Crick).