Model Answer
0 min readIntroduction
The quest to identify the genetic material has been a cornerstone of biological research. For a long time, proteins were considered the prime candidates due to their structural diversity. However, a series of elegant experiments in the mid-20th century conclusively demonstrated that it is, in fact, deoxyribonucleic acid (DNA) that carries the hereditary information. This realization revolutionized our understanding of life and paved the way for modern molecular biology. This answer will detail the experiments that proved DNA is the genetic material, using the Hershey-Chase experiment as a detailed example.
Early Evidence & Griffith’s Transformation Experiment
The initial hints towards a ‘transforming principle’ came from Frederick Griffith’s experiment in 1928. He worked with two strains of Streptococcus pneumoniae: a virulent (S) strain with a polysaccharide capsule, and a non-virulent (R) strain lacking the capsule.
- Observation: When mice were injected with the S strain, they died. When injected with the R strain, they survived.
- Crucially: When mice were injected with heat-killed S strain, they survived. However, injecting a mixture of heat-killed S strain and live R strain resulted in the death of the mice, and live S strain bacteria were recovered.
- Conclusion: Griffith proposed that some ‘transforming principle’ from the heat-killed S strain had converted the R strain into the virulent S strain. He didn’t identify the principle itself.
Avery-MacLeod-McCarty Experiment (1944)
Oswald Avery, Colin MacLeod, and Maclyn McCarty aimed to identify the ‘transforming principle’. They systematically isolated different components from heat-killed S strain bacteria and tested their ability to transform R strain bacteria.
- Methodology: They treated heat-killed S strain extracts with enzymes that degraded proteins, RNA, and DNA, separately.
- Observation: Only when DNA was degraded, transformation did not occur.
- Conclusion: This strongly suggested that DNA, not protein or RNA, was the transforming principle and therefore the genetic material.
However, this experiment faced skepticism, as some scientists believed that the DNA preparation might have been contaminated with protein.
Hershey-Chase Experiment (1952) – A Detailed Example
Alfred Hershey and Martha Chase conducted a landmark experiment using bacteriophages (viruses that infect bacteria) to definitively prove that DNA is the genetic material. Bacteriophages consist of only DNA and protein.
Methodology
They used two batches of T2 bacteriophages:
- Batch 1: Phages were grown in a medium containing radioactive 32P, which selectively labels DNA (phosphorus is present in DNA but not in protein).
- Batch 2: Phages were grown in a medium containing radioactive 35S, which selectively labels protein (sulfur is present in protein but not in DNA).
These labeled phages were then used to infect E. coli bacteria. After infection, the phage particles were separated from the bacterial cells using a blender. The mixture was then centrifuged, separating the heavier bacterial cells (pellet) from the lighter phage particles (supernatant).
Observations
Radioactivity was measured in both the pellet and the supernatant for both batches of phages.
- 32P-labeled phages: Most of the radioactivity was found in the bacterial pellet, indicating that the DNA of the phage had entered the bacterial cells.
- 35S-labeled phages: Most of the radioactivity remained in the supernatant, indicating that the protein coat of the phage did not enter the bacterial cells.
Conclusion
Hershey and Chase concluded that DNA, not protein, is the genetic material because it is the DNA that enters the bacterial cell during infection and directs the production of new phage particles. This experiment provided compelling evidence that resolved the long-standing debate.
Further Confirmation
Later, Chargaff’s rules (1950) regarding the base composition of DNA (A=T and G=C) and the X-ray diffraction studies by Rosalind Franklin and Maurice Wilkins (1952) provided further insights into the structure of DNA, culminating in Watson and Crick’s double helix model in 1953, solidifying DNA’s role as the genetic material.
Conclusion
The experiments, starting with Griffith’s transformation and culminating in Hershey-Chase’s elegant demonstration, provided irrefutable evidence that DNA, and not protein, is the carrier of genetic information. These discoveries were pivotal in the development of molecular biology, genetics, and biotechnology, fundamentally changing our understanding of life and heredity. The identification of DNA as the genetic material opened doors to advancements like gene cloning, genetic engineering, and genome sequencing, continuing to shape biological research today.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.