In the 1940's, Avery, MacCleod, and McCarty transformed nonencapsulated bacteria into
encapsulated forms by growing the nonencapsulated cells in a culture containing an
extract made from dead encapsulated cells. The transformed cells produced colonies of
encapsulated bacteria. Three different procedures and their results are outlined below.
Procedure I
Exact made from dead encapsulated cells added to culture medium.
Nonencapsulated bacteria added to culture medium.
Results Both nonencapsulated and encapsulated bacteria grow.
Procedure II
Extract made from dead encapsulated cells treated with protein-degrading enzymes
before adding exact to culture medium.
Nonencapsulated bacteria added to culture medium.
Results Both nonencapsulated and encapsulated bacteria grow.
Procedure III
Extract made from dead encapsulated cells treated with DNAse (an enzyme that
selectively destroys DNA) before adding extract to culture medium.
Nonencapsulated bacteria added to culture medium.
Results Only nonencapsulated bacteria grow.
(A) To remove the encapsulated bacteria from the extract
(B) To serve as a positive control by demonstrating that a protein in the extract is the transforming factor
(C) To serve as a negative control by demonstrating that transformation does not occur without DNA
(D) To destroy any enzymes in the extract
(E) To destroy any capsules that might be in the extract
Answer: C
encapsulated forms by growing the nonencapsulated cells in a culture containing an
extract made from dead encapsulated cells. The transformed cells produced colonies of
encapsulated bacteria. Three different procedures and their results are outlined below.
Procedure I
Exact made from dead encapsulated cells added to culture medium.
Nonencapsulated bacteria added to culture medium.
Results Both nonencapsulated and encapsulated bacteria grow.
Procedure II
Extract made from dead encapsulated cells treated with protein-degrading enzymes
before adding exact to culture medium.
Nonencapsulated bacteria added to culture medium.
Results Both nonencapsulated and encapsulated bacteria grow.
Procedure III
Extract made from dead encapsulated cells treated with DNAse (an enzyme that
selectively destroys DNA) before adding extract to culture medium.
Nonencapsulated bacteria added to culture medium.
Results Only nonencapsulated bacteria grow.
What was the purpose of treating the extract with DNAse in Procedure III
(A) To remove the encapsulated bacteria from the extract
(B) To serve as a positive control by demonstrating that a protein in the extract is the transforming factor
(C) To serve as a negative control by demonstrating that transformation does not occur without DNA
(D) To destroy any enzymes in the extract
(E) To destroy any capsules that might be in the extract
Answer: C