Lecture notes on Avery MacLeod and McCarthy experiments

Griffith could not understand the cause of bacterial transformation and that is first of all identified by Oswald Avery, Colin MacLeod and Maclyn McCarty (1944). They partially purified the transforming principle from the cell extract (i.e., cell free extract of S-III bacteria) and demonstrated that it was DNA. These workers modified the known schemes for isolating DNA and prepared samples of DNA from S-III bacteria. They added this DNA to a live R bacterial culture; after a period of time they placed a sample of S-III containing R-II bacterial culture on an agar surface and allowed it to grow to form colonies. Some of the colonies (about 1 in 104) that grew were S-III typeTo show that this was a permanent genetic change, they dispersed many of the newly formed S-III colonies and placed them on a second agar surface. The resulting colonies were again S-III type. If an R-II colony arising from the original mixture was dispersed, only R-II bacteria grew in subsequent generations. Hence, the R-II colonies retained the R-II character, whereas the transformed S-III colonies bred true as S-III.Further, because S-III and R-II colonies differed by a polysaccharide coat around each S-III
bacterium, the ability of purified polysaccharide to transform was also tested, but no transformation
was observed.
Avery, MacLeod and McCarthy repeated Griffith's expts in vitro in a much refined way. Culture of live IIR cells produced typical IIR cell colonies, while a culture of heat-killed III s cell or a a culture of the DNA isolated from IIIS cells produced no colony. At the same time. of IIR cells, mixed either with heat-killed IIIS cells or with the DNA isolated from lllS on a medium containing antibodies for IIR cells (Ab IIR), produced some colonies of III s cells. AbIlR was used for inactivating some llR cells so that the number of IIR cells may not exceed the number of IIIS cells. These findings reveal that DNA can be the transforming Since DNA preparations often contain traces of RNA and proteins. this conclusion is not beyond doubt. In order to establish beyond doubt that DNA alone is the transforming principle, Avery and associates conducted two separate experiments, using the DNA isolated from lllS cells.In one of them, the DNA isolated from lllS cells was treated with the enzyme RNAse (to digest RNA if any), and in the other with the enzyme protease (to digest protein), before it was mixed with live llR cells. ln both these expts, some IllS cell colonies were fomied. This clearly shows  that RNA and Proteins are not responsible for the transformation of III R cells to lllS cells.In another expt DNA was treated with the enzyme DNAase before it was mixed with live IIR cells.This did not yield a III S colony.This confirms that DNA is the transforming principle.