Avery, MacLeod, and McCarty (1944) demonstrated that DNA was the molecule of her

Question

Avery, MacLeod, and McCarty (1944) demonstrated that DNA was the molecule of heredity by treating heat-killed Streptococcus pneumoniae S (smooth) strain with DNase, thereby preventing transfer of the "transforming substance" from the killed S strain to the live R (rough) strain when the two were mixed together and injected into mice. Another way that one could demonstrate transformation in bacteria would be to extract DNA from: A.) an R strain and mix with cells of an S strain. B.) both S and R strains and mix to allow recombination to take place. C.) an S strain and mix with cells of an R strain. D.) an ampicillin-sensitive strain and mix with cells of an ampicillin-resistant strain. E.) none of the above

Explanation / Answer

Basics: R strain is non-virulent while S- strain is virulent.

In the experiment, they showed heat killed S-strain mixed with R-strain = mouse dies. This means a piece od DNA of S-strain has been picked up and recombine with S-strain DNA which resulted in conversion of R-Strain to S-strain.

Lets check out the options:

1. A.) an R strain and mix with cells of an S strain : It means R strain are dead ones and S-strain cells are alive. Doesn't matter much as R-strain are dead and even if S-strain did pick up some pieces of R-strain DNA.. there is no way to confirm that which piece of DNA has been transformed.

B. both S and R strains and mix to allow recombination to take place (Not viable option as if they are dead no recombination and if alive how do we know what piece of DNA has undergo recombination)

C. an S strain and mix with cells of an R strain - This seems to be viable option. Cells of R-strain can pick up the virulent gene of S-strain as a result these R-strain will now behave as S-strain but the problem is we are extracting DNA. So how can we make sure that the DNA which has been extracted is from only R-strain? No DNA from S-strain has been extracted?

D)an ampicillin-sensitive strain and mix with cells of an ampicillin-resistant strain - this opton doesn't make any sense

E.none of the above So, answer shoould be none of the above.

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