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1/ There are many ways to insert #recombinantDNA into host cells. For example, chemical treatment or electrical shock can make membranes permeable to naked DNA. 
But some bacteria naturally take up genetic material. One experiment, in 1928, uncovered this "natural competence".
But some bacteria naturally take up genetic material. One experiment, in 1928, uncovered this "natural competence".
2/ Frederick Griffith was a British bacteriologist studying pneumonia, a serious cause of death during the 1920s. His landmark study has come to be known as "Griffith's experiment" — you will find it in #Biology textbooks because it laid the foundation for understanding DNA.
3/ Pneumococcus #bacteria of one strain (smooth) are virulent and kill laboratory mice, while those of another strain (rough), are harmless. In the experiment, Griffith mixed living pneumococci of the rough-type with heat-killed smooth ones and administered the mixture to mice.
4/ Astonishingly, the mice developed pneumonia and died. 
Upon examination of the mice's blood, Griffith also found living bacteria, which bore characteristics of the virulent smooth strain (which had been heat-killed)!
https://en.wikipedia.org/wiki/Griffith's_experiment
Upon examination of the mice's blood, Griffith also found living bacteria, which bore characteristics of the virulent smooth strain (which had been heat-killed)!
https://en.wikipedia.org/wiki/Griffith's_experiment
5/ Conclusion: living rough cells — normally harmless — had been "transformed" into lethal form by the presence of the dead smooth bacteria.
In hindisght, it's easy to see what happened...
!
In hindisght, it's easy to see what happened...
!
6/ We know that the "transforming principle" Griffith observed was the DNA of the smooth-strain bacteria. But remember this was 1928 — it wasn't until several years later that Avery, McLeod & McCarty, then Hershey & Chase, verified the nature of the transforming principle:
7/ Today, transforming bacteria with foreign DNA, for example through #vectors such as #SEVAplasmids, is second-nature to us! 
