Transforming Principle By Frederick Griffith

Many scientists contributed to prove that DNA is a genetic material. Fredrick Griffith was the first scientist in this race and he had given the transformation principle in bacteria.

Fredrick Griffith (1928) conducted a series of experiments with Streptococcus pneumoniae, the bacterium causing pneumonia. He observed two strain of this bacteria:

• One forming smooth colonies with a polysaccharide mucus coated capsule that is S-strain.
• Other forming rough colonies without a capsule that is To strain.

Experiment:

• Mice infected with S- strain (virulent) die from pneumonia infection.
• Mice infected with the R- strain was alive.
• Griffith was able to kill bacteria by heating them. When heat killed S-strain cells were injected into mice. The disease did not appear and the mice was alive.
• When mice was infected with a mixture of heat killed S- strain and live R-strain strain, the mice died.
• He could isolate the live R-strain cells from the body of dead mice.

He concluded that R-strain bacteria somehow being transformed by heat killed S-strain bacteria and were able to synthesize a smooth polysaccharide coat and become virulent. This must be due to transfer of genetic material. This is called the Transformation principle.

Biochemical characterization of transforming principle

Oswald Avery, Colin MacLeod and Maclyn McCarty (1933-44) discovered that the heat killed S- strain bacteria caused the transformation of R- strain bacteria.

Their findings were:

• DNA alone from S- strain bacteria caused R- strain bacteria to become transformed.
• They found that proteases (protein digesting enzyme) and RNAses (RNA digesting enzymes) did not affect transformation.
• Digestion with DNAses inhibits transformation, so they finally concluded that DNA is a hereditary material.

Question and Answers

Question 1

What was Griffith’s experiment and why was it important?

Answer.

Griffith gave the bacterial transformation principle. He had worked on Streptococcus pneumoniae bacteria. He proved that due to transfer of genetic material the non- virulent bacteria transformed into virulent bacteria.

Question 2

What is the conclusion of the Griffith experiment?

Answer.

Griffith concluded that the R-strain bacteria which is non- violent had been transformed by heat-killed S- strain bacteria so there is something in S- strain  bacteria which transfer from S- strain to R-strain  bacteria and make it virulent. This must be due to transfer of genetic material.