Experiments Proving DNA as Genetic Material — Revision Notes

⚡ 30-Second Revision

Griffith (1928): — Discovered 'transformation' in *S. pneumoniae*. Live R-strain + Heat-killed S-strain $ightarrow$ Live S-strain. Identified 'transforming principle' but not its chemical nature.
Avery, MacLeod, McCarty (1944): — Identified DNA as the transforming principle. Used enzymes: Protease (no effect), RNase (no effect), DNase (abolished transformation).
Hershey-Chase (1952): — Definitive proof. Used bacteriophages.

- DNA labeled with $^{32}\text{P}$ (in phosphate backbone). - Protein labeled with $^{35}\text{S}$ (in sulfur-containing amino acids). - $^{32}\text{P}$ found in bacterial pellet (entered cell). - $^{35}\text{S}$ found in supernatant (remained outside). - Conclusion: DNA is the genetic material.

Properties of Genetic Material: — Replication, Storage, Expression, Variation.

2-Minute Revision

The journey to prove DNA as the genetic material involved three landmark experiments. Frederick Griffith (1928) observed bacterial transformation, where a 'transforming principle' from heat-killed virulent bacteria could make non-virulent bacteria virulent.

He didn't identify the chemical nature, but showed genetic information transfer. Avery, MacLeod, and McCarty (1944) then biochemically identified this principle. By treating bacterial extracts with enzymes, they found that only DNase (which degrades DNA) abolished the transforming ability, strongly indicating DNA was the genetic material.

The final, unequivocal proof came from Hershey and Chase (1952). They used bacteriophages, labeling DNA with radioactive phosphorus ($^{32}\text{P}$) and protein with radioactive sulfur ($^{35}\text{S}$).

Their experiment showed that only the $^{32}\text{P}$-labeled DNA entered the host bacterial cells during infection, directing the synthesis of new viruses, while the $^{35}\text{S}$-labeled protein remained outside.

This conclusively established DNA as the genetic material, fulfilling the criteria of replication, information storage, expression, and variation.

5-Minute Revision

The identification of DNA as the genetic material is a cornerstone of molecular biology, established through a series of elegant experiments. Initially, proteins were considered strong candidates due to their complexity.

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Griffith's Experiment (1928): — Working with *Streptococcus pneumoniae*, Griffith observed that injecting mice with a mixture of live non-virulent R-strain bacteria and heat-killed virulent S-strain bacteria resulted in the mice dying. He recovered live S-strain bacteria from the dead mice, concluding that a 'transforming principle' from the dead S-strain had genetically altered the R-strain. This demonstrated the transfer of heritable material but didn't identify its chemical nature.

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Avery, MacLeod, and McCarty's Experiment (1944): — These scientists aimed to identify Griffith's transforming principle. They prepared extracts from heat-killed S-strain bacteria and treated separate samples with enzymes: proteases (degrade proteins), RNases (degrade RNA), and DNases (degrade DNA). They found that only the sample treated with DNase lost its ability to transform R-strain bacteria into S-strain. This provided strong biochemical evidence that DNA was the transforming principle and thus the genetic material.

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Hershey-Chase Experiment (1952): — This experiment provided the definitive proof. They used bacteriophages (viruses that infect bacteria), which consist only of DNA and protein. They selectively labeled the phage DNA with radioactive phosphorus ($^{32}\text{P}$, as DNA contains P but not S) and the phage protein with radioactive sulfur ($^{35}\text{S}$, as protein contains S but not P). They allowed these labeled phages to infect *E. coli* bacteria. After infection, they agitated the mixture in a blender to detach viral coats from the bacterial surface and then centrifuged it to separate the heavier bacteria (pellet) from the lighter viral components (supernatant). They found that most of the $^{32}\text{P}$ (DNA) was in the bacterial pellet, while most of the $^{35}\text{S}$ (protein) remained in the supernatant. Crucially, the bacteria with $^{32}\text{P}$ produced new phages. This conclusively showed that DNA, not protein, entered the host cell and carried the genetic information for viral replication.

These experiments collectively established that DNA possesses the key properties of genetic material: the ability to replicate, store information, express information, and undergo variation.

Prelims Revision Notes

Experiments Proving DNA as Genetic Material (NEET Revision)

I. Griffith's Transforming Principle (1928)

Organism: — *Streptococcus pneumoniae* (causes pneumonia).
Strains:

* S-strain (Smooth): Virulent, has polysaccharide capsule. * R-strain (Rough): Non-virulent, lacks capsule.

Experiments & Observations:

1. Live S-strain $ightarrow$ Mouse dies. 2. Live R-strain $ightarrow$ Mouse lives. 3. Heat-killed S-strain $ightarrow$ Mouse lives. 4. Live R-strain + Heat-killed S-strain $ightarrow$ Mouse dies. Live S-strain recovered from dead mouse.

Conclusion: — A 'transforming principle' from heat-killed S-strain transferred to live R-strain, making it virulent. Did NOT identify the chemical nature.

II. Avery, MacLeod, and McCarty's Biochemical Characterization (1944)

Aim: — To identify the chemical nature of Griffith's 'transforming principle'.
Methodology: — Isolated biochemicals (DNA, RNA, protein) from heat-killed S-strain. Treated samples with specific enzymes.
Enzymes Used:

* Protease: Degrades proteins. * RNase: Degrades RNA. * DNase: Degrades DNA.

Observations:

* Transformation occurred after protease treatment. * Transformation occurred after RNase treatment. * Transformation did NOT occur after DNase treatment.

Conclusion: — DNA is the transforming principle and thus the genetic material. Strong biochemical evidence.

III. Hershey-Chase Experiment (1952) - The Definitive Proof

Organism: — Bacteriophages (viruses) and *E. coli* bacteria.
Principle: — Phages inject genetic material into bacteria to replicate. DNA contains P, protein contains S.
Radioactive Labels:

* **$^{32}\text{P}$:** Labels DNA (in phosphate backbone). * **$^{35}\text{S}$:** Labels protein (in sulfur-containing amino acids like methionine, cysteine).

Methodology:

1. Infection: Labeled phages infect *E. coli*. 2. Blending: Agitation to separate viral coats from bacterial cells. 3. Centrifugation: Separates heavier bacteria (pellet) from lighter viral coats (supernatant).

Observations:

* $^{32}\text{P}$ (DNA) found predominantly in the bacterial pellet. * $^{35}\text{S}$ (Protein) found predominantly in the supernatant. * Bacteria with $^{32}\text{P}$ produced new phages.

Conclusion: — DNA, not protein, is the genetic material that enters the host cell and directs viral replication.

IV. Properties of Genetic Material:

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Replication: — Must be able to make copies of itself.
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Storage of Information: — Must store complex genetic information.
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Expression of Information: — Must express information to produce traits.
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Variation (Mutation): — Must be capable of change for evolution.

Vyyuha Quick Recall

Griffith Altered Heredity: Griffith found transformation, Avery identified DNA, Hershey-Chase confirmed DNA.