Search for the genetic material

The essentials

In 1928, Frederick Griffith worked with Streptococcus pneumoniae. The S strain has a smooth polysaccharide coat and is virulent; the R strain lacks it and is harmless. Injecting heat-killed S together with live R killed mice, and live S bacteria were recovered. Griffith concluded that some "transforming principle" from the dead S cells had changed the R strain, but he did not identify it.

Oswald Avery, Colin MacLeod and Maclyn McCarty purified biochemicals from heat-killed S cells. Proteases and RNases did not affect transformation, but DNase stopped it, showing that DNA caused transformation.

Alfred Hershey and Martha Chase (1952) gave unequivocal proof using bacteriophages. Phages grown with radioactive phosphorus (³²P) had labelled DNA; those grown with radioactive sulphur (³⁵S) had labelled protein. After infection, blending and centrifugation, the bacteria infected with ³²P phages were radioactive, while ³⁵S stayed in the supernatant. So DNA, not protein, entered the bacteria.

Key points

• Criteria of genetic material: able to replicate, chemically and structurally stable, able to mutate slowly (for evolution), able to express itself as Mendelian characters.
• DNA is more stable than RNA because RNA's 2′-OH group makes it reactive and uracil is replaced by thymine in DNA.
• RNA was probably the first genetic material (RNA world); it can act as a catalyst. DNA evolved later from RNA as a more stable store.
• Proteins contain sulphur but no phosphorus; DNA contains phosphorus but no sulphur.

Common mistakes

• Griffith did not prove that DNA is the genetic material; Avery and colleagues did the biochemical identification.