Avery, McCarty and MacLeod’s Experiment

Aim:

To biochemically identify and characterise the "transforming principle" discovered by Griffith - specifically, to determine whether it was DNA, RNA, or protein.

Scientists and Setting:

Materials Used:

• Streptococcus pneumoniae Type III-S (virulent) and Type II-R (non-virulent) bacteria
• Cell-free extract of heat-killed S-strain bacteria (purified biomolecules: DNA, RNA, proteins, carbohydrates, lipids)
• Enzymes: DNase (degrades DNA), RNase (degrades RNA), Protease (degrades proteins)
• Culture medium with antibodies against R-strain cells

Preparation: Heat-killed Type III-S bacteria were used to prepare a cell-free extract; lipids and carbohydrates were removed from the solution.​

Purification: Biomolecules (DNA, RNA, proteins) were separately purified from the heat-killed S-strain extract.​

Enzyme Treatment: The purified extract was treated with specific hydrolytic enzymes in separate experimental tubes:

• Tube 1 — treated with Protease (destroys proteins)
• Tube 2 — treated with RNase (destroys RNA)
• Tube 3 — treated with DNase (destroys DNA)

Transformation Test: Each enzyme-treated extract was mixed with live R-strain bacteria and introduced onto the culture medium.

Observation: The culture medium was observed for the appearance of Type III-S colonies (smooth, virulent) - confirming or denying transformation.

• The purified transforming substance tested negative for proteins by chemical tests.
• It gave a strongly positive result in DNA-specific chemical tests.
• The elemental composition (nitrogen-to-phosphorus ratio) of the purified substance was consistent with that of DNA.
• The molecular weight and physical properties also matched DNA, not protein or RNA.

Primary Conclusion: DNA alone is responsible for transforming R-strain bacteria into virulent S-strain bacteria. Therefore, DNA is the hereditary (genetic) material.

Secondary Conclusions:

• Proteins and RNA do not carry genetic information (at least not in this context).
• The "transforming principle" identified by Griffith in 1928 is biochemically identified as DNA.
• Genetic information can be transferred from one organism to another via pure DNA.