Question

What is Griffith transforming principle? Explain Griffith experiments on Streptococcus pneumoniae while searching for genetic material.

Answer 1

Hint: Griffith performed this experiment to find genetic material. During the experiment, the bacteria Streptococcus pneumoniae was cultured. Streptococcus pneumoniae has two types of streptococcal strains – smooth strain which is called the S strain and rough strain which is called the R strain. Cells of the smooth strain have a protective layer known as the capsule which protects the cells against the immune system. This makes the smooth strain highly pathogenic. The rough strain lacks capsule. Hence, the immune system can easily kill the rough strain.

Complete answer:
In Griffith’s experiment, when the cells of the smooth strain are injected in mice, the mice are killed. Whereas when the cells of the rough strain are injected in mice, the mice survive.
In the next experiment, the cells of the smooth strain were killed by heat, when these cells were injected into mice, the mice survived. However, when the cells of the heat-killed smooth strains are mixed with the rough strain and injected in mice, the mice are killed. This result was surprising. He observed that something which he called “transforming principle” was getting transferred from heat killed S strain to live R strain which allowed the live R strain to transform into S strain which was highly pathogenic. This phenomenon is known as the transforming principle.

Note:
Griffith did not know what was getting transferred from the heat-killed S strain to the live R strain. This experiment was a turning point towards the discovery of genetic material. However, it couldn’t explain the biochemistry of genetic material. Further, experiments to find the transforming principle were done by Avery Macleod and McCarty. Their discovery revised the concept of genetic material from protein to DNA as genetic material. They showed that the transforming principle was due to DNA.