Understanding Griffith's Transformation Experiment Steps

Frederick Griffith's experiment in 1928 was a landmark study in genetics. He worked with two strains of the bacterium Streptococcus pneumoniae: the smooth (S) strain, which is pathogenic and causes pneumonia, and the rough (R) strain, which is non-pathogenic.

The question asks to arrange the steps of Griffith's experiments in the correct sequence. Let's analyze the provided statements and their outcomes:

• (A) ‘S’ strain → injected into mice → Mice died
• (B) ‘S’ strain (Heat killed) → injected into mice → Mice lived
• (C) ‘R’ strain → injected into mice → Mice lived
• (D) ‘S’ strain (Heat killed) + ‘R’ strain (Live) → injected into mice → Mice died

To determine the correct series of steps in Griffith's experiment, we consider the logical progression Griffith might have followed to understand the disease and the nature of the genetic material. A common sequence for presenting these steps highlights the controls first, then the key experiment that led to the discovery of transformation.

Analyzing the Steps of Griffith's Experiment

Let's arrange the steps according to a plausible experimental flow, keeping in mind the provided correct order:

1. Step (B): Testing Heat-Killed S Strain

   In this step, Griffith injected mice with the S strain bacteria that had been killed by heat. The S strain is normally lethal, but heating kills the bacteria.

   Result: The mice lived. This showed that the dead S strain alone did not cause the disease.

2. Step (C): Testing Live R Strain

   Here, Griffith injected mice with the live R strain bacteria. The R strain is known to be non-pathogenic.

   Result: The mice lived. This confirmed that the live R strain alone does not cause pneumonia.

3. Step (D): Testing Heat-Killed S Strain with Live R Strain

   This was the crucial part of the experiment. Griffith mixed the heat-killed S strain bacteria (which did not kill mice) with the live R strain bacteria (which also did not kill mice) and injected the mixture into mice.

   Result: The mice died. Surprisingly, live S strain bacteria were recovered from the dead mice. This indicated that something from the heat-killed S strain had transformed the live R strain into pathogenic S strain.

4. Step (A): Testing Live S Strain (Control)

   As a fundamental control, Griffith injected mice with the live S strain bacteria.

   Result: The mice died. This confirmed that the live S strain is indeed pathogenic and lethal.

Following this sequence, the steps are performed in the order (B), (C), (D), then (A). This progression makes sense as it moves from showing that the individual components (heat-killed S and live R) are harmless, to showing that combining them results in lethality due to transformation, and finally confirming the original lethality of the live S strain as a baseline.

Summary of Griffith's Experiment Steps

The steps of Griffith's experiment can be summarized in the order determined above:

Therefore, the correct arrangement of the steps of Griffith's experiments is (B), (C), (D), (A).

Revision Table: Key Concepts of Griffith's Experiment

Additional Information: The Transforming Principle

Griffith's experiment showed that a 'transforming principle' from the heat-killed S strain could permanently change the R strain. However, Griffith himself did not identify the chemical nature of this transforming principle. It wasn't until later experiments by Avery, MacLeod, and McCarty (1944) that DNA was conclusively identified as the genetic material responsible for this transformation. They purified the transforming principle and showed that it was DNA, not protein or RNA, that caused the R strain to become S strain.

Griffith's work laid the foundation for the discovery of DNA as the carrier of genetic information, which was a major milestone in molecular biology.