Biology · Molecular Basis of Inheritance · NEET
They proved the chemical (biochemical) nature of Griffith's transforming principle. Griffith only showed that some 'principle' moved from dead S bacteria to live R bacteria, but he did not know what it was made of. Avery and his team purified proteins, RNA and DNA from heat-killed S cells and tested each one. They found that DNA alone from S bacteria caused R bacteria to change into S bacteria. So the transforming principle is DNA.
Griffith (1928) used live mice and showed that heat-killed S bacteria could transform live R bacteria into virulent S bacteria, but he did NOT know the chemical nature of the transforming material. Avery, MacLeod and McCarty (1933-44) worked in a test tube (in vitro) with purified biochemicals and enzymes to identify that the material is DNA. In short: Griffith discovered transformation; Avery identified the molecule (DNA).
DNase (an enzyme that digests DNA) stopped transformation. This is the most important result for NEET. Proteases (digest protein) and RNases (digest RNA) did NOT stop transformation, so protein and RNA were ruled out. Since only DNase blocked it, DNA must be the transforming principle. Do not confuse DNA with DNase, they are different: DNA is the material, DNase is the enzyme that destroys it.
Because not all biologists were convinced by it. Some still thought a small amount of protein contamination might be responsible. The unequivocal (final) proof that DNA is the genetic material came later from the Hershey-Chase experiment (1952) using bacteriophages and radioactive labels. NEET often traps students here: Avery gave strong evidence, but Hershey and Chase gave the final proof.
They used the same bacterium as Griffith, Streptococcus pneumoniae (the pneumococcus). They took the heat-killed virulent S (smooth) cells, purified different biochemicals from them, and tested each on live R (rough) cells to see which one caused transformation.
The final proof for DNA as the genetic material came from the experiments of
Unequivocal proof that DNA is the genetic material was first proposed by
Match List I with List II. A. Frederick Griffith; B. Francois Jacob & Jacque Monod; C. Har Gobind Khorana; D. Meselson & Stahl :: I. Genetic code; II. Semi-conservative mode of DNA replication; III. Transformation; IV. Lac operon
Try the real previous-year questions from this chapter — each with the answer and a full solution.
They worked from 1933 to 1944 (often written 1933-44). This is after Griffith's 1928 experiment and before the Hershey-Chase experiment of 1952.
To test which biochemical is the transforming principle. Proteases destroy protein, RNases destroy RNA and DNase destroys DNA. Only when DNA was destroyed (by DNase) did transformation stop, so DNA is the transforming principle. Protein and RNA were ruled out.
No. DNA is the genetic material (the transforming principle). DNase is an enzyme that cuts and digests DNA. NCERT even asks you to note the difference between DNA and DNase, so read the spelling carefully in exam options.
Protein. Before Avery, MacLeod and McCarty, most scientists believed the genetic material was a protein. Their experiment shifted the evidence toward DNA.
Griffith is matched with 'transformation' (discovery of the transforming principle). Avery, MacLeod and McCarty are the ones who found this principle is chemically DNA. Keep the two separate.
