The Transforming Principle

Griffith's experiment (1928)

Frederick Griffith, working with Streptococcus pneumoniae (the bacterium causing pneumonia), used two strains:

  • the S (smooth) strain - it has a mucous (polysaccharide) capsule and is virulent (kills mice), and
  • the R (rough) strain - it has no capsule and is non-virulent.

His results in mice: live S -> mice die; live R -> mice live; heat-killed S -> mice live; but heat-killed S + live R -> mice die, and living S bacteria were recovered from them. Griffith concluded that some 'transforming principle' from the heat-killed S bacteria had transformed the living R strain into the virulent S strain.

Avery, MacLeod and McCarty

Oswald Avery, Colin MacLeod and Maclyn McCarty set out to identify the transforming principle biochemically. They treated the heat-killed S extract with different enzymes: digestion with proteases and RNases did not stop transformation, but digestion with DNase abolished it. This showed that the transforming principle was DNA - though not everyone was convinced that DNA alone was the genetic material.

One-liners: Griffith - transformation in Streptococcus pneumoniae (S = virulent/capsule, R = non-virulent); heat-killed S + live R -> live S recovered; Avery-MacLeod-McCarty - DNase stops transformation -> transforming principle = DNA.

The Hershey-Chase Experiment (1952)

Alfred Hershey and Martha Chase gave the unambiguous proof that DNA is the genetic material, using bacteriophages (viruses that infect bacteria). A phage is essentially DNA inside a protein coat.

  • They grew one batch of phages in a medium with radioactive phosphorus (32P), which labels DNA (protein has no phosphorus), and another batch with radioactive sulphur (35S), which labels protein (DNA has no sulphur).
  • The labelled phages were allowed to infect E. coli; the cultures were then agitated in a blender to shake off the phage coats and centrifuged.
  • The 32P (DNA) was found inside the bacteria, whereas the 35S (protein) stayed outside in the supernatant. Since only the DNA entered and directed the formation of new phages, DNA is the genetic material.

Properties of a Genetic Material - Why DNA?

To serve as the genetic material, a molecule must be able to (i) replicate, (ii) be stable chemically and structurally, (iii) provide scope for slow mutations (for evolution) and (iv) express itself as characters. On these criteria:

  • DNA is more stable than RNA (its sugar lacks the reactive 2'-OH group and it uses thymine), which suits it as the store of genetic information.
  • RNA can also be the genetic material (as in TMV and some bacteriophages), but it is more reactive and mutates faster; RNA is also catalytic.

One-liners: Hershey-Chase - phage + 32P labels DNA, 35S labels protein; DNA (32P) enters the bacterium -> DNA is genetic material; DNA more stable than RNA; RNA can be genetic material (TMV) but mutates faster.

The RNA World and the Central Dogma

The RNA world

Most evidence suggests that RNA was the first genetic material. RNA can act both as genetic information and as a catalyst (ribozyme), so it could have carried information and run reactions in the earliest life. Because RNA is reactive and unstable, it later evolved into the more stable DNA for information storage, with proteins taking over most catalysis. Many essential processes (transcription, translation, RNA splicing) are still RNA-based or RNA-catalysed, supporting this idea.

The Central Dogma

Francis Crick proposed the Central Dogma of molecular biology - the direction in which genetic information flows:

DNA -> (transcription) -> RNA -> (translation) -> Protein

In some viruses (retroviruses) the flow can be reversed (RNA -> DNA) by reverse transcriptase - the phenomenon of reverse transcription.

One-liners: RNA world - RNA first genetic material and catalyst (ribozyme); DNA (more stable) evolved from RNA; Central Dogma: DNA -> RNA -> Protein (Crick); reverse transcription (RNA -> DNA) in retroviruses.