Molecular Genetics

Identification of DNA as the transforming principle of pneumococci was a major factor in the realization that hereditary material consisted of nucleic acids.

Facts

Fred Griffith found that pneumococcal strains R and S differ in colony morphology and virulence.
	MacLeod added heat-killed S cells to live R cells. He found the production of S colonies on subsequent plating. These S colonies were virulent.
DNA isolated from heat-killed S cells can also cause virulent S colonies to appear after mixing with R cells and plating. Enzymes vary in their ability to prevent S colony formation.  The conversion of organisms of one phenotype to another by incubation with non-living material is known as transformation. The ability to transform organisms with extracted DNA is important to the molecular analysis of chromosomes.

Interpretations

• S cells contain a 'transforming principle' (2).
• This transforming principle copurifies with DNA (3).
• The experiments of Avery, MacLeod & MacCarty (3) demonstrated that the transforming principle was not protein or RNA and strongly suggested that it was DNA.
• These experiments are recognized as the first in a series that lead to the conclusion that DNA is the carrier of genetic information, the genetic material.

Further information

• Genetic transformation is one of several tools in bacterial genetics.
• The experiments of Griffith and of Avery, MacLeod & MacCarty were important to the history of molecular genetics.
• Pneumococcal cells are only competent to take up DNA for a brief time after they reach a certain density of cell growth. A small peptide acts as the signal for transformation competence (review).