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Cellular and Molecular Biology

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The Avery, MacLeod, and McCarty Experiment

CELLS-13ILMJ

In 1944, Oswald Avery, Colin MacLeod, and Maclyn McCarty published the results of an experiment which indicated that DNA is responsible for bacterial transformation (the conversion of one strain of bacteria to another strain by a chemical substance) and therefore is likely to be the genetic material of bacteria.

Their experiment followed the demonstration of bacterial transformation by Frederick Griffith, published in 1928. Griffith had shown that a combination of heat-killed type III-S Streptococcus pneumoniae bacteria and live type II-R bacteria produced live type III-S cells, and he concluded that a chemical component of the dead III-S cells had “transformed” the II-R cells.

Avery, MacLeod, and McCarty set out to identify the chemical responsible using enzymes that destroy specific polymers: proteases to digest proteins, an RNase to digest RNA, and a DNase to digest DNA. They found that only the DNase prevented transformation of cells and concluded that DNA, but not RNA or protein, was Griffith’s “transforming principle.”

Now imagine that Avery and his colleagues had found that the protease treatment prevented bacterial transformation.

What would their conclusion have been?

A

Protein is the bacterial genetic material.

B

DNA is the bacterial genetic material.

C

RNA is the bacterial genetic material.

D

Either protein or DNA is the bacterial genetic material.