Final answer:
The transformation principle, coined by Griffith, is the concept that external DNA can be taken up by a bacterial cell, changing its characteristics. This was demonstrated when nonpathogenic R strain bacteria of Streptococcus pneumoniae became pathogenic after exposure to heat-killed S strain bacteria. Later studies identified DNA as this transforming principle.
Step-by-step explanation:
The transformation principle first proposed by Griffith through his studies on Streptococcus pneumoniae represents a groundbreaking discovery in the field of molecular biology. Frederick Griffith's experiments in 1928 demonstrated that nonpathogenic bacteria could become pathogenic by incorporating genetic material from a dead, pathogenic strain. This process was later understood to involve the uptake of external DNA by a bacterial cell, which changed its phenotype and functionality.
Griffith worked with two strains of Streptococcus pneumoniae: the nonpathogenic rough strain (R) and the pathogenic smooth strain (S). He discovered that when mice were injected with a mixture of live R strain and heat-killed S strain, the mice died, and only live S strain bacteria were recovered from their bodies. This suggested that the R strain bacteria had been transformed into the S strain by some agent from the dead S strain cells, which Griffith termed the transforming principle.
It was later shown by Avery, MacLeod, and McCarty that the transforming principle was, in fact, DNA, determining that DNA is the molecule responsible for heredity. Griffith's experiments provided the first step towards understanding genetic inheritance and the function of DNA in heredity.