Detailed Explanation

For a significant period in the early 20th century, the chemical nature of the hereditary material remained elusive. Scientists knew that chromosomes, composed of both protein and nucleic acids, were responsible for inheritance, but the specific molecule carrying the genetic blueprint was a mystery. Proteins, with their complex and diverse structures arising from 20 different amino acids, seemed to be the more logical candidates for encoding vast amounts of information, compared to nucleic acids, which only contained four types of nucleotide bases. However, a series of elegant experiments ultimately and definitively established DNA as the molecule of heredity.

Pivotal Experiments Confirming DNA as the Genetic Material

1. Griffith's Transformation Experiment (1928): The "Transforming Principle"
   • Experiment Setup: Frederick Griffith studied two strains of Streptococcus pneumoniae (a bacterium causing pneumonia in mammals):
     • S strain (Smooth): Possesses a polysaccharide capsule, making colonies smooth. This strain is virulent (pathogenic) and causes disease.
     • R strain (Rough): Lacks the capsule, making colonies rough. This strain is non-virulent (non-pathogenic).
   • Procedure:
     • Injected live S strain into mice: Mice died. (Control)
     • Injected live R strain into mice: Mice lived. (Control)
     • Injected heat-killed S strain into mice: Mice lived. (Control)
     • Injected a mixture of heat-killed S strain and live R strain into mice: Mice died. Crucially, live S strain bacteria were recovered from the dead mice.
   • Observation: The non-virulent R strain had been "transformed" into the virulent S strain by something from the dead S strain.
   • Conclusion: Griffith proposed that a "transforming principle" from the heat-killed S bacteria had been transferred to the live R bacteria, causing a heritable change. The chemical identity of this principle was unknown, but it demonstrated the possibility of transferring genetic information.
2. Avery-MacLeod-McCarty Experiment (1944): Isolating the Principle
   • Hypothesis: Building directly on Griffith's work, Oswald Avery, Colin MacLeod, and Maclyn McCarty meticulously aimed to identify the chemical nature of the "transforming principle."
   • Procedure: They prepared extracts from heat-killed S strain bacteria and systematically treated these extracts with enzymes that specifically degrade different classes of macromolecules:
     • Treated with proteases (degrade proteins).
     • Treated with RNase (degrade RNA).
     • Treated with DNase (degrade DNA).
     • Each treated extract was then mixed with live R strain bacteria and tested for its ability to cause transformation.
   • Results:
     • Extracts treated with proteases or RNase still caused transformation.
     • However, extracts treated with DNase lost their ability to transform the R strain.
   • Conclusion: This provided compelling evidence that DNA was the chemical substance responsible for genetic transformation, thus strongly suggesting that DNA is the genetic material.
3. Hershey-Chase Experiment (1952): Definitive Confirmation with Viruses
   • Goal: To provide final, unambiguous proof, Alfred Hershey and Martha Chase designed an experiment using bacteriophages (viruses that infect bacteria), which are composed only of DNA and protein.
   • Strategy (Differential Labeling): They used radioactive isotopes to label either the protein or the DNA of the phages:
     • Radioactive Sulfur (35S): Incorporated into amino acids (e.g., methionine, cysteine), thus labeling phage proteins. Sulfur is absent from DNA.
     • Radioactive Phosphorus (32P): Incorporated into the phosphate backbone of nucleotides, thus labeling phage DNA. Phosphorus is absent from typical proteins.
   • Procedure:
     • Phages labeled with 35S were allowed to infect one batch of bacteria.
     • Phages labeled with 32P were allowed to infect another batch of bacteria.
     • After a short infection period, the cultures were agitated in a blender. This agitation sheared off the viral coats (containing most of the protein) from the bacterial cells.
     • The mixture was then centrifuged. Bacteria, being heavier, formed a pellet at the bottom, while the lighter viral coats remained in the supernatant (liquid above the pellet).
     • The radioactivity in the pellet (inside bacteria) and supernatant (outside bacteria) was measured.
   • Results:
     • In the 35S experiment, most of the radioactivity remained in the supernatant (outside the bacteria, associated with the viral coats). The infected bacteria themselves showed very little radioactivity.
     • In the 32P experiment, most of the radioactivity was found in the bacterial pellet (inside the bacteria). This radioactive material was passed on to the next generation of phages produced by the infected bacteria.
   • Conclusion: This unequivocally demonstrated that it was the DNA (and not the protein) that entered the bacterial cells to direct the synthesis of new viruses. Therefore, DNA is the genetic material responsible for carrying and transmitting hereditary information.

Fundamental Properties Required of Genetic Material

These groundbreaking experiments, along with subsequent discoveries, solidified the understanding that DNA possesses the essential characteristics required for a molecule to serve as the genetic material:

1. Information Storage: DNA's linear sequence of four bases (A, T, C, G) can encode vast amounts of complex information required for building and operating an organism. The precise order of these bases acts as a digital code.
2. Accurate Replication: The double helix structure with its complementary base pairing (A with T, G with C) provides a perfect mechanism for accurate replication. Each strand can serve as a template for synthesizing a new complementary strand, ensuring faithful transmission of genetic information.
3. Information Expression: DNA contains the instructions for synthesizing RNA and proteins, which are the functional molecules of the cell. This "expression" allows the stored information to be put into action.
4. Capacity for Variation (Mutation): While replication is highly accurate, occasional changes (mutations) in the DNA sequence can occur. These heritable changes are the raw material for evolution, allowing populations to adapt over time.

DNA's chemical stability, ability to self-replicate with high fidelity, and its capacity to encode and express information make it the ideal molecule for heredity, underpinning the continuity and diversity of life.