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, beginning in the early 20th century, gradually revealed that it is, in fact, deoxyribonucleic acid (DNA) that carries the hereditary information. These experiments weren’t isolated events but a progressive series of investigations, each refining our understanding and building upon the findings of its predecessors. This answer will detail the experimental evidence that conclusively demonstrated DNA as the genetic material.
Griffith’s Transformation Experiment (1928)
Frederick Griffith, a British bacteriologist, conducted a groundbreaking experiment using Streptococcus pneumoniae bacteria. He observed two strains: a virulent (S) strain that caused pneumonia and a non-virulent (R) strain that did not. His experiment involved the following:
- Injection of S strain: Mice died.
- Injection of R strain: Mice lived.
- Injection of heat-killed S strain: Mice lived.
- Injection of a mixture of heat-killed S strain and live R strain: Mice died, and live S strain bacteria were recovered from their bodies.
Griffith concluded 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 transforming principle, but his experiment demonstrated that genetic information could be transferred between bacteria.
Avery-MacLeod-McCarty Experiment (1944)
Oswald Avery, Colin MacLeod, and Maclyn McCarty aimed to identify the “transforming principle” discovered by Griffith. They systematically eliminated different components from heat-killed S strain extracts and tested their ability to transform R strain bacteria.
Their experimental setup involved three sets of experiments:
- Treatment with protease (destroys proteins): Transformation occurred.
- Treatment with RNase (destroys RNA): Transformation occurred.
- Treatment with DNase (destroys DNA): Transformation did not occur.
This experiment conclusively demonstrated that DNA, not protein or RNA, was the transforming principle responsible for the genetic change in bacteria. However, some scientists remained skeptical, arguing that the DNase might have been contaminated.
Hershey-Chase Experiment (1952)
Alfred Hershey and Martha Chase provided the most convincing evidence that DNA is the genetic material. They used bacteriophages (viruses that infect bacteria) to demonstrate this. Bacteriophages consist of only DNA and protein.
Their experiment involved two batches of bacteriophages:
- Batch 1: Phages were labeled with radioactive phosphorus (32P), which is found in DNA but not in protein.
- Batch 2: Phages were labeled with radioactive sulfur (35S), which is found in protein but not in DNA.
These labeled phages were used to infect bacteria. After infection, the phage particles were separated from the bacterial cells using a blender. It was observed that:
- 32P labeled DNA was found inside the bacterial cells.
- 35S labeled protein remained outside the bacterial cells.
This experiment demonstrated that DNA, not protein, enters the bacterial cells during infection and is therefore the genetic material responsible for directing the production of new phage particles. This experiment effectively silenced the remaining skepticism and solidified DNA’s role as the carrier of genetic information.
Further Evidence
Later advancements, such as Chargaff’s rules (1950) regarding the base composition of DNA and the elucidation of DNA’s double helix structure by Watson and Crick (1953), further reinforced the understanding of DNA as the genetic material. These discoveries built upon the foundation laid by the earlier experiments.
Conclusion
The experimental evidence, starting with Griffith’s transformation experiment and culminating in the definitive Hershey-Chase experiment, unequivocally established DNA as the genetic material. Each experiment progressively refined our understanding, eliminating alternative possibilities and providing compelling evidence for DNA’s central role in heredity. These findings revolutionized the field of biology, paving the way for advancements in molecular biology, genetics, and biotechnology. The identification of DNA as the genetic material remains a cornerstone of modern biological science.
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.