Evidence that DNA is the Genetic Material

Evidence that DNA is the Genetic Material

Griffiths (1920's) and Avery (1940's) experimented with two strains of Pneumococcus bacteria. Griffiths showed that mice died if injected with the virulent non-capsulated strain but lived if injected either with the harmless capsulated strain or dead virulent bacteria.

If the harmless bacteria were allowed to grow in the presence of the dead virulent cells, they became virulent themselves and killed the mice. Some part of the dead virulent strain had transformed the previously harmless strain. This phenomenon is transformation.

Avery later (1940's) showed that it was almost certainly the DNA from the dead virulent bacteria that caused this effect.

Experiments on transformation of bacteria

Hershey and Chase grew cultures of T2 bacteriophage viruses in the presence of radioisotopes. A virus only contains protein and nucleic acid.

One culture was given 32P so that all the virus DNA became radioactive (proteins do not contain P).

The second culture was given 35S so that all the virus protein became radioactive (DNA does not contain S).

It is known that when bacteriophages attack bacteria, only their DNA enters the host, while the protein is always left outside the parasitised cell.

 the parasitised cell

Separate bacterial cultures were infected, one by each labelled virus. A blender was then used to dislodge the empty virus protein coats from the infected bacterial cells. A centrifuge was then used to separate the heavier bacteria from the light virus coats.

Bacteria infected with 32P viruses became radioactive, but bacteria infected with 35S viruses did not.

Only the 32P ever entered the bacteria. Thus it was shown that the DNA is the molecule used by viruses to pass their genetic information.

Other evidence includes:.

  1. The mass of DNA in somatic cells is constant, while gametes have half this mass.
  2. DNA is associated with the chromosomes (but so is protein).
  3. Mutagens that affect DNA molecules cause increased phenotypic mutation.
  4. The wavelengths of UV radiation which cause the most mutation are also those which pure DNA absorbs the strongest.
  5. All cells in one organism have the same type of DNA, as shown by Chargaff's work, but the DNA varies between different species.
  6. In non-dividing tissue such as nervous tissue, the DNA is not broken down, but other cellular molecules are constantly being recycled and replaced.
  7. DNA is copied exactly during each cell cycle and is therefore consistent and self-perpetuating (Meselson and Stahl)